Proceedings Volume 3492

Third International Conference on Solid State Lasers for Application to Inertial Confinement Fusion

W. Howard Lowdermilk
cover
Proceedings Volume 3492

Third International Conference on Solid State Lasers for Application to Inertial Confinement Fusion

W. Howard Lowdermilk
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 23 July 1999
Contents: 26 Sessions, 118 Papers, 0 Presentations
Conference: Third International Conference on Solid State Lasers for Application to Inertial Confinement Fusion 1998
Volume Number: 3492

Table of Contents

icon_mobile_dropdown

Table of Contents

All links to SPIE Proceedings will open in the SPIE Digital Library. external link icon
View Session icon_mobile_dropdown
  • Future Facilities I
  • Future Facilities II
  • Smoothing and Deflection
  • Advanced ICF Laser Technology
  • Poster Session I
  • Pulse Generation I
  • Pulse Generation II
  • E/O Components
  • Optical Damage
  • Thin Film Coatings
  • Systems Experiments
  • Poster Session II
  • Optical Design of Large Laser Systems I
  • Optical Design of Large Laser Systems II
  • Frequency Conversion Materials
  • Frequency Conversion Technology
  • Diffractive Optics
  • Nonlinear Optics
  • Spatial Filter Pinholes
  • Laser Systems Controls Diagnostics and Integration
  • Amplifier Technology
  • Amplifier Wavefront Characterization and Thermal Recovery
  • Adaptive Optics and Wavefront Controls
  • Target Chambers and Vacuum Barriers
  • Laser Glass Manufacturing and Performance
  • Large-Optics Fabrication and Characterization
Future Facilities I
icon_mobile_dropdown
Baseline design of proposed AWE 100-TW solid state laser system
John A. McMordie, Thomas H. Bett, P. C. Morris, et al.
To meet the future requirements of the AWE plasma physics program, design studies are being conducted for a 100 TW Nd doped glass laser system converted to the third harmonic. The baseline design of the proposed system will be presented. The AWE laser is based on technology developments for the US National Ignition Facility (NIF). The proposal is for a 32 beam laser system which is a sub-set of the 192 beam NIF.
Future Facilities II
icon_mobile_dropdown
High-power phosphate-glass laser system Luch: a prototype of the Iskra-6 facility module
Stanislav A. Sukharev
The results of the development of project of 4 channel Nd phosphate glass laser facility 'Luch' are presented. The principle of 4-pass amplification of short pulse with profiled temporary from and pulse duration of (tau) 0.5 approximately equals 3-5 ns in two amplifying cascades with beam aperture of 20 X 20 cm2 is used as the basis of the facility design. The facility includes the system of reference radiation formation SRF; the adaptive system of laser beam wavefront correction; transport and cavity angular spatial filters; main amplifying cascades paced in four-pass cavity; system of power conditioning, and non-linear frequency converters.
Status of the SG-III solid state laser project
Hansheng Peng, Xiao Min Zhang, XiaoFeng Wei, et al.
High power solid state laser technologies for application to inertial confinement fusion have been developed over the past three decades in China. The XG-1 laser facility was built in 1984 and upgraded into XG-II in 1993. The SG-1 was completed in 1985 and the upgrade into SG-II will be finished in a few months. As the next step, the SG-III laser facility has been proposed to produce 60-kJ blue light for ICF target physics experiments and is one being conceptually designed. A preliminary baseline design suggest that he SG- III be a 64-beam facility with an output beam size of 25 cm X 25cm. The main amplifier column of 4 high by 2 wide has been chosen as a module. New laser technologies, including multipass amplification, large aperture plasma electrode switches, fast growth of KDP, laser glass with fewer platinum grains, Ce-doped quartz long flash lamps, capacitors with higher energy density, Ce-doped quartz long flash lamps, capacitors with higher energy density and precision manufacturing technique of large optical components have been developed to meet the requirements of the SG-III Project. In addition, numerical simulations are being conducted to optimize the optical design of the facility. The technical integration line with a 4 X 2 segmented aperture array of the amplifiers as a prototype beamline of the SG-III has been scheduled for the next few years.
Progress of direct drive laser fusion research at ILE, Osaka
Kunioki Mima, Hiroshi Azechi, Hisanori Fujita, et al.
Reviewed are the progress in direct drive implosion researches with Gekko XII laser system. Precise observation of the growth rate of Rayleigh-Taylor instability and the suppression of imprints using indirect-direct hybrid implosion have been investigated. Theoretical and experimental researchers on the fast ignition scheme are also studied. Relativistic laser plasma interaction experiments with Peta-Watt Module and Gekko XII are also described. Finally, the future direction of the research including the development of solid state laser for fusion reactor is discussed.
Smoothing and Deflection
icon_mobile_dropdown
Final optics design for the megajoule laser
Alain Adolf, Alain C. L. Boscheron, Arnaud Dulac, et al.
We present the final optics design of the Megajoule Laser using high efficiency transmission gratings at 1053 nm and 351 nm for broadband tripling, wavelength filter, laser diagnostics sampling and focusing on target. We also present a 1:3 scale prototype of our optical system and we summarize the experimental results obtained during the year of 1998.
Issue of FM to AM conversion on the National Ignition Facility
The National Ignition Facility (NIF) baseline configuration for inertial confinement fusion requires phase modulation for two purposes. First, approximately 12 angstrom of frequency modulation (FM) bandwidth at low modulation frequency is required to suppress buildup of Stimulated Brioullin scattering in the large aperture laser optics. Also, approximately 3 angstrom or more bandwidth at high modulation frequency is required for smoothing of the speckle pattern illuminating the target by the smoothing by spectral dispersion method. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. However, as a result of a large number of effects, the FM converts to amplitude modulation (AM). In general this adversely affects the laser performance, e.g. by reducing the margin against damage to the optics. In particular, very large conversion of FM to AM has been observed in the NIF all-fiber master oscillator and distribution systems. The various mechanisms leading to AM are analyzed and approaches to minimizing their effects are discussed.
Beam Wiggler operating in high frequency and single-pulse modes
A. Yu. Goltsov, Arkadiy N. Kolomiysky, N. G. Kovalsky, et al.
The possibility to control an intensity distribution in the far field of a powerful laser system by rapid motion of a focal spot is considered. Quadruple electro optic deflector on the base of LiNgO3 crystal installed in resonance capacity with 1 cm clear aperture has been developed, constructed and tested both in high frequency and single pulse operation modes. The main parameters of the device are as follows: amplitude of the angular deflection +/- 4 dif. limits at 6.5 GHz operation frequency, total angular deflection 12 dif. limits in the single ns-pulse operation mode. Results of the Beam Wiggler dynamic testing are presented and discussed.
Advanced ICF Laser Technology
icon_mobile_dropdown
Focused intensities of 1020 Wcm-2 with the upgraded Vulcan CPA interaction facility
Colin N. Danson, Ric M. Allott, S. Angood, et al.
There has been considerable interest in the last 10 years in the physics of ultra-high power laser interactions. With all high power lasers such as Vulcan there is a limit to the energy that can be extracted from laser amplifiers at short pulse-lengths due to the intensity dependent non-linear refractive index. The technique of Chirped Pulse Amplification has overcome the classic limit and has resulted in massive increases in focused intensity. The large increase in on target intensity is achieved by a substantial, usually orders of magnitude, reduction in pulse duration while at the same time maintaining comparable pulse energy and focusability.
LULI 100-TW Ti:sapphire/Nd:glass laser: a first step toward a high-performance petawatt facility
Jiping Zou, Denise Descamps, Patrick Audebert, et al.
We have implemented a Ti:sapphire/mixed Nd:glass laser syste at LULI producing sub-picosecond pulses in the 100 TW power range. Focusing to a 1.5-times diffraction-limited spot results in a peak intensity on target over 1019 W/cm2. Significant experiments in particle acceleration, X-ray laser and other basic plasma physics researchers have been carried out since this implementation. This paper details the characteristics of the present set-up and the main path of progress towards a high performance petawatt facility.
Poster Session I
icon_mobile_dropdown
Development of LD-pumped 10 J x 10 Hz Nd:glass slab laser
Hiroki Matsui, Takeyoshi Egushi, Tadashi Kanabe, et al.
We have proposed and designed a diode-pumped solid-state laser (DPSSL) driver for inertial fusion energy (IFE) which consists of water-cooled zig-zag path Nd:glass slab amplifiers. A driver module has 10 kJ total output energy at 351 nm and operates at 12 Hz with 10.4 percent overall efficiency. The laser driver producing 4 MJ blue output for IFE will consists of 400 modules. To confirm the design, we are developing a small scale DPSSL module of 10J X 10Hz laser output at 1053 nm.
Modeling of the effects of KDP crystal gravity sag on third-harmonic generation
Olivier Lubin, Claude Gouedard
The deformation and the stresses of KDP frequency converters due to the gravitational sag can affect the frequency conversion by causing a departure from the phase-matching condition: the deformation changes the phase-matching direction and the stresses generate spatial variations in the index of refraction through the stress-optic effect. In order to predict the influence of these effects on the third harmonic conversion efficiency of the future Laser MegaJoule, we have carried out coupled mechanical-optical calculations. This paper presents a description of the modeling and the main results we obtained.
Bulk laser damage in KDP crystal as a function of laser irradiation direction, polarization, and wavelength
Hidetsugu Yoshida, Takahisa Jitsuno, Hisanori Fujita, et al.
Bulk laser induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c-axis is about two times higher than that in the a(b)-axis at 0.532 and 1.064-micrometers wavelength. This result consistent with the experimental results that (001) direction is mechanically weaker than (100) direction with the result of mechanical strength tests.
Using a design of experiment method to improve KDP crystal machining process
Philippe Lahaye, Christian Chomont, Pierre Dumont, et al.
Surface quality is an important issue for KDP crystal implemented in high power lasers. Single Point Diamond Turning is the only way to get the required transmitted wavefront and damage threshold, specially in the UV band of the laser Megajoule. The aim of the process development was to reach an rms roughness lower than 3 nm using a vertical fly-cutter, an appropriate cleaning equipment and an interferometric microscope for measurements. A Taguchi Design of Experiment method has been sued to optimize the fly-cutting process parameters on 100 X 100 mm2 KDP crystals.
Design of a new technique for alleviating thermal dephasing of nonlinear optical crystal
Y. K. Yap, Kyoichi Deki, Y. Kagebayashi, et al.
A novel mechanism to alleviate the thermal dephasing of nonlinear optical crystal was developed. We named it temperature-profile compensation as defined from the in-situ temperature-profile measurement. Based on the concept of this mechanism, design on a technique for 3D temperature- profile compensation was demonstrated. By applying this technique on a CsLi B6O10 crystal, 10.6-W of 266-nm output power was obtained at 100-Hz. This result is 2.3-fold of that without alleviation. The conversion efficiency of 52 percent from the green is also more effective than the best result optimized at room temperature. Based on the similar concept, 1D temperature-profile compensation is also discussed for cases when the laser beam diameter reduced below approximately 2 mm. This is appropriate for frequency conversion of cw and diode-pumped kHz lasers. The design for 1D temperature-profile compensation can be very simple without involving any gas cooling.
Nonlinear optical crystal for near-ultraviolet generation: gadolinium yttrium calcium oxyborate GdxY1-xCa4O(BO3)3
Masashi Yoshimura, Hiroyuki Furuya, Takeshi Kobayashi, et al.
In this study, yttrium calcium oxyborate, YCa4O(BO3)3 (YCOB) was grown and investigated as a nonlinear optical material. This crystal is phase-matchable for the third harmonic generation (THG) of a Nd:YAG of a Nd:YAG laser by type-I mixing between 1.064 and 0.532 micrometers . By partial substitution of Gd for Y in YCOB, a solid solution GdxY1-xCa4O(BO3)3 gradually changes the phase-matching angles of THG to ((Theta) , (phi) ) equals (90 degrees, 90 degrees). In this paper we present the properties of noncritically phase-matched THG in GdxY1-xCa4O(BO3)3 (x equals 0.24).
Broad-band computations using the Miro software
O. Morice, Xavier Ribeyre, V. Rivoire
A new calculation mode has been developed inside the Miro software to treat the broad-band effects involved in optical smoothing. Bandwidth effects such as gain narrowing in amplifiers or group velocity difference in frequency converters can be computed. This program has been used to simulate the Megajoules laser with smoothing by spectral dispersion.
Design of kinoform phase plate for irradiating spherical target
Kouji Tsubakimoto, Noriaki Miyanaga, Motoyuki Okuo, et al.
We estimate the adaptation of the kinoform phase plate (KPP) for direct irradiation of the spherical target. The KPP has the focusing depth of about 200 micrometers in Gekko XII irradiation system. This corresponds to the radius of typical target. The optimum KPP is designed by using the direct-search method. The KPP removes the nonuniformity of 6th spherical mode.
Temporal response diagnostic for the megajoule laser
Pascal Leclerc, Virginie Allouche
The laser Megajoule requires very precise beam-to-beam power balance. Moreover, each laser pulse will have a particular temporal response to generate specific effects such as mechanical compression, plasma heating, and ignition. To control the proper operation of the facility, several kinds of measurement systems have to be developed for beam diagnostics. The temporal shape diagnostic is a critical one.
LMJ cladding industrialization
C. Remontet, J. M. Bouchut, V. Licchesi, et al.
ANGENIEUX/GIAT Industries consortium agreement (GME) has been selected by CEA to industrialize the cladding process on amplifier slabs used on LIL and MEGAJOULE (LMJ) lasers. Each amplifier slab is clad with Ci-doped glass to absorb 1 053 nm parasitic amplified spontaneous emission light, and avoid gain depletion.
High-energy laser system for HIF research at GSI
M. Roth, R. Bock, U. N. Funk, et al.
For the development of a heavy ion driven inertial confinement fusion scenario a detailed knowledge of the interaction processes of the ions with the converter material is crucial. As this converter will be predominantly in the plasma state one of the main topics of the plasma physics group at Gesellschaft fuer Schwerionenforschung (GSI) is the interaction of heavy ions with dense hot plasma. Based on the latest result on interaction experiments with laser generated plasma targets presented here and concerning the high current upgrade of GSI a new high energy laser system is proposed. It will serve as a driver for interaction experiments with heavy ions as well as a diagnostic tool for heavy ion generated plasmas. In addition, with the combination of high current heavy ion beams and intense lasers innovative, fundamental research in the field of high energy density physics will be accessible for the first time.
Geometric arrangement of laser beams in target area
Jichun Tan, XiaoFeng Wei, Qin Zhong, et al.
A ray tracing method for design of geometrical arrangement of laser beams in target area is described in this paper. A mathematically mode for path's design are suggested to minimize the objective functions fj(Ri, Li, (alpha) i, (beta) i), which are the optical length difference between the real path length from laser amplifier to the target center and the design objective path length. And the constraint conditions for beam's path are as follows: (1) Beams illuminate the cylindrical target with the incident angles required for the physical experiments. (2) 60 beams propagate without overlapping each other, and each of them with equal optical length from the laser amplifier to the target center. (3) Bema incident to the reflection mirror Pi with the decided angle (phi) i, for example, value of (phi) i is less than or equal to 45 degrees for all paths, and etc. Mathematical methods, such as 'Depth First Search technique', or searching optimum point for objective function fj(Ri, Li, (alpha) i, (beta) i) with constraint conditions are stated. Design examples are reported, in which the 60-Gaussian beam are symmetrically distributed around the target chamber.
Preliminary design of target area optical layout for the SG-III facility
XiaoFeng Wei, Z. Q. Shi, Xiaodong Yuan, et al.
A computer code has been developed and simulations have been conducted to design the target area optical layout according to the requirements of both the physical and the SG-III laser facility itself for both direct and indirect drive configurations, respectively. The number and location of the required 240 turning mirrors from the output lens of the transport spatial filter to the target chamber have been initially decided, and the mechanical interventions of optical paths have also been solved.
Preliminary design of Technical Integration Line (TIL) for the SG-III laser facility
Xiao Min Zhang, Feng Jing, Z. Shui, et al.
In this paper, we present the preliminary design of Technical Integration Line (TIL). TIL is a full scale 4 X 2 module of Shenguang-III (SG-III). laser facility with a two-aperture output of 3.0kJ at 3 (omega) in a temporally shaped pulse of 1.0-3.0 ns. The goal of TIl is to demonstrate the laser technology of the proposed SG-III. TIL consists of front-end, pre-amplifier stage, main amplifier stage, diagnostic target systems and control system and the average fluency is designed to operate at 5.0J/cm2 in a 1.0 ns output pulse. The optical scheme of a four-pass main amplifier and a booster amplifier have been chosen. The clear aperture of amplifier is 30 X 30cm2, and the numbers of Nd:glass disks in the two amplifiers are optimized in system design. Two spatial filters are inserted in the system to remove high spatial frequencies from the beam, and SF1 is the multi-pass spatial filter and SF2 is the transport spatial filter. In order to correct the output wavefront for static and dynamic wavefront aberrations of disk amplifiers, a deformable mirror system is used in the main amplifier stage of TIL.
Multipass slab Nd:glass amplifier
Zhan Sui, Yue Liang, Qiao Liu, et al.
The aim of this research is to develop a repetition multi- pass Nd:glass slab amplifier with output energy range of joules. A Nd: glass slab amplifier was developed with a high efficient repetition. Some analyze and experimental researchers, such as the gain, gain distribution, thermal effect, store energy and efficiency, etc. have been done. The maximum storage efficiency of the amplifier is up to 2.1 percent. A multi-pass amplification system by the Nd:glass slab amplifier was set up. Further analyze and experimental researches were carried out on some characters, such as the structure energy fluent distribution, amplification character, beam quality, ASE and self-Las character, etc. The experimental result shows that the amplifier produce the output energy of 1J with beam quality of six times diffraction limit.
SG-II solid state laser ICF system
Gangyao Xiao, Dianyuan Fan, Shiji Wang, et al.
The 8 beam SG-II laser facility with beam size of 245 mm is reported. Single-longitudinal mode oscillator including the temporal-spatial-transform pulse shaping, and a switch free coaxial double pass disk amplifier technology will be described. CCD beam quality diagnostic system is discussed. The two-beam co-line focus and two series coupling targets are in x-ray experiment is also introduced. The goal of the facility is to produce total pulse energy of 2.4 KJ at tripling frequency.
High-damage-threshold pinhole for glass fusion laser applications
Norman A. Kurnit, S. A. Letzring, Randall P. Johnson
We are investigating methods to fabricate high-damage- threshold spatial-filter pinholes that might not be susceptible to plasma closure for relatively high energies and long pulses. These are based on the observation that grazing-incidence reflection from glass can withstand in excess of 5kJ/cm2 without plasma formation. The high damage threshold result from both the cos(theta) spreading of the energy across the surface and the reflection of large fraction of the energy from the surface, thereby greatly reducing the field strength within the medium.
Long-lifetime low-contamination metal beam dumps for NIF spatial filters
Mary A. Norton, Charles D. Boley, James E. Murray, et al.
The management of back reflections and leakage through the Pockels cell and polarizer switch in the NIF beamlines requires robust beam dumps that can operate unattended for thousands of shots within the confines of the vacuum spatial filters. Off-line experiments and modeling have been used to investigate the use of metal for these beam dumps. We conclude that a stainless steel beam dump will have insufficient lifetime and will safely absorb fluences in the required range, from 1 J/cm2 to 4 kJ/cm2, at 1 micrometers .
High-laser-damage-threshold HfO2/SiO2 mirrors manufactured by sputtering process
Anne Fornier, D. Bernardino, Odile Lam, et al.
A major preoccupation for the design of the LMJ laser is the mirrors laser damage threshold. SAGEM SA, in collaboration with the CEA, has conducted a study in order to improve the laser induced damage threshold under operational conditions.
Optics damage inspection for the NIF
Calvin E. Thompson, Carl F. Knopp, Derek E. Decker
Two optics damage inspection system will be implemented on the NIF, one to inspect optics within the laser and transport sections and the other to inspect the final optics at the target chamber. Both system use dark-field imaging technology to enhance defect contrast. Details of each system design will be provided. A functional optics damage inspection system prototype, using dark-field imaging technology, is currently in operation on the Beamlet laser. This system provides us with the opportunity to measure non- ideal optical surfaces expected to be present on NIF. Prototpye details and performance will be presented.
Vendor-based laser damage metrology equipment supporting the National Ignition Facility
Sheldon Schwartz, Richard T. Jennings, James F. Kimmons, et al.
A sizeable laser damage metrology effort is required as part of optics production and installation for the 192 beam NIF laser. The large quantities, high damage thresholds, and large apertures of polished and coated optics necessitates vendor-based metrology equipment to assure component quality during production. This equipment must be optimized to provide the required information as rapidly as possible with limited operator experience. This equipment must be optimized to provide the required information as rapidly as possible with limited operator experience. The damage metrology tools include: 1) platinum inclusion damage test system for laser amplifier slabs, 2) laser conditioning stations for mirrors and polarizers, and 3) mapping and damage testing stations for UV transmissive optics. Each system includes a commercial Nd:YAG laser, a translation stage of the optics, and diagnostics to evaluate damage. The scanning parameters, optical layout, and diagnostics vary with the test fluence required and the damage morphologies expected. This paper describes the technical objectives and milestones involved in fulfilling these metrology requirements at multiple vendors.
Compact Nd3+-based laser system with gain G < 1013 and output energy of 20 J
Andrei Babushkin, John H. Kelly, Christopher T. Cotton, et al.
We have developed a compact laser system capable of amplifying nanosecond-scale pulses form a few picojoules to 20J. The system has a 40-mm clear aperture and a 37-mm working aperture for high-energy output. We measured less than 1 wave phase distortion over full 37-mm aperture for a pulse with 18-J output energy at a shot repetition rate of one shot every 10 min. In experiments with a 30-mm diam beam, a flat-top spatial profile with 4 percent rms over the entire beam diameter was demonstrated for a 1-ns pulse with 20_j output energy. The amplifier has a net gain up to 1013 and fits easily on a 5-ft X 14-ft optical table.
Pulse Generation I
icon_mobile_dropdown
Integrated operations of the National Ignition Facility (NIF) optical pulse generation development system
John K. Crane, Russell B. Wilcox, N. W. Hopps, et al.
We describe the Optical Pulse Generation (OPG) testbed, which is the integration of the MOD and Preamplifier Development Laboratories. We use this OPG testbed to develop and demonstrates the overall capabilities of the NIF laser system front end. We will present the measured energy and power output, temporal and spatial pulse shaping capability, FM bandwidth and dispersion for beam smoothing, and measurements of the pulse-to-pulse power variation o the OPG system and compare these results with the required system performance specifications. We will discus the models that are used to predict the system performance and how the OPG output requirements flowdown to the subordinate subsystems within the OPG system.
Microchip lasers for the LMJ front end
Laurent R. Fulbert, Philippe Kramer, Marc Rabarot, et al.
Microchip lasers are used as pulse generators in the LMJ front end. These lasers are made with Nd:YLF using collective fabrication processes. A linearly polarized, single-frequency, 1053 nm pulsed emission has been achieved with very good efficiency. Prototypes of fiber-coupled microchip lasers have been fabricated.
Pulse Generation II
icon_mobile_dropdown
Multipurpose diode-pumped Nd:YLF laser for OMEGA pulse shaping and diagnostics applications
Andrey V. Okishev, Mark D. Skeldon, Wolf D. Seka
We have developed a diode-pumped, single-mode laser that provides stable amplitude output over many hours. The laser is capable of operating with three types of output: (1) as a single-mode, Q-switched laser, (2) as a single-mode, cw laser; and (3) as a Q-switched laser operating with two adjacent, equal-amplitude cavity modes to provide sinusoidally modulated output. No laser realignment is required to change the laser to these various outputs.
Regenerative amplifier for the OMEGA laser system
Andrei Babushkin, W. Bittle, S. A. Letzring, et al.
We present the requirements, design, and experimental results for a negative feedback-controlled Nd:YLF regenerative amplifier for the OMEGA laser system. This externally synchronizable region boosts the energy of temporally shaped optical pulses from the subnanojoule to the submillijoule energy level with a measured long-term output energy stability of 0.2 percent rms. To our knowledge this represents the highest energy stability ever demonstrated for a millijoule-level laser system, either flashlamp pumped or diode pumped. In addition to the excellent stability and reproducibility, the regen output is very insensitive to the injected pulse energy and the temporal distortions due to the negative feedback are immeasurable. Four regenerative amplifiers equipped with this negative feedback system have operated flawlessly on OMEGA over the past two year period.
Optical pulse shaping system based on aperture-coupled striplines for OMEGA pulse shaping applications
Mark D. Skeldon, Andrey V. Okishev, Robert L. Keck, et al.
An optical pulse shaping system based on an aperture-coupled stripline electrical waveform generator is discussed. Shaped electrical and optical pulses have been produced. The measured pulse shapes agree well with our models.
Using a chirp pulse amplification technique and temporal multiplexing for flexible shaping of broadband laser pulses
Vladimir E. Yashin, A. B. Van'kov, A. A. Kozlov
A pulse shaping and frequency modulation system based on chirped-pulse-amplification and pulse train multiplexing techniques in regenerative amplifier is proposed. It is shown that by using standard frequency modulators of MHz range one can obtain up to 100 GHz modulation frequencies. This system enables to generate arbitrary pulse-shaped pulses with temporal resolution determined by length of initial pulses from mode-locked master oscillator. Nd:glass CPA laser system has been used for demonstration of such type system operation.
E/O Components
icon_mobile_dropdown
Plasma electrode Pockels cell for the National Ignition Facility
Mark A. Rhodes, Scott N. Fochs, Peter J. Biltoft, et al.
The NIF, now under construction at LLNL, will be the largest laser fusion facility ever built. The NIF laser architecture is based on a multi-pass power amplifier to reduce cost and maximize performance. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm X 40 cm square and allows close horizontal and vertical beam spacing. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a Plasma-Electrode Pockels Cell (PEPC). In a PEPC, low-pressure helium discharges are formed on both sides of a thin slab of electro-optic material. Typically, we use KH2PO4 crystals (KDP). The discharges form highly conductive, transparent sheets that allow uniform application of a high-voltage pulse across the crystal. A 37 cm X 37 cm PEPC has been in routine operation for two years on the 6 kJ Beamlet laser at LLNL. For the NIF, a module four apertures high by one wide is required. However, this 4 X 1 mechanical module will be comprised electrically of a pair of 2 X 1 sub-modules. Last year, we demonstrated full operation of a prototype 2 X 1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2 X 1 PEPC over the Beamlet PEPC is the use of anodized aluminum construction that still provides sufficient insulation to allow formation of the planar plasmas. In this paper, we discuss full 4 X 1 NIF prototypes.
Plasma electrode Pockels cell of CEA's megajoule laser project
Rene Bailly-Salins, C. Sudres, Jean-Pierre Marret
The large plasma electrode Pockels cell (PEPC), which has originally been developed at LLNL for high energy 4 pass laser amplifier, has been taken up in the megajoule laser project (LMJ) at CEA. Nevertheless, a few alternative technologies and methods have been taken into account to build the first prototype. The frame of both, 2 cm thick, plasma chambers have been cast in one piece of polycarbonate. They are refilled with He + 1 percent O2 at pressure around 1 mbar. each chamber is equipped with 20 cathodes and 20 anodes. Each cathode is in series with a 10 (Omega) ballast resistor. Each anode is in series with a 10 (Omega) or 5 (Omega) ballast resistor. The electrodes are made of solid graphite plus carbon fiber material. According to the switching function of LMJ, the KDP's electrical driver has to produce 2 pulses of amplitude V(pi ) equals 17kV, 650ns apart, with a rise and fall time of 100 ns. After a general description of this PEPC, experimental results and theoretical considerations show how we got 2 plasma electrodes with a very good homogeneity and a low resistivity since the plasma current peaks at 1400 angstrom when the 5 (mu) F energy storage capacitor is charged to 4kV. Finally, the design of the plasma current supply and the 17kV pulser is presented. The device is not quite up to scratch but, computer simulations are given.
Programable generation of high-voltage pulses for Pockels cell modulators
Alain Jolly, M. Urbanik, P. Estraillier
An original design for electro-optical pulse shaping is presented, enabling the operation of programmable high voltage pulses with fast Pockels cell amplitude modulators. The basic idea is to make use of diode lasers together with photoconductors, inside a simple arrangement comprised of two resistivity coupled microstrip lines. We demonstrate a prototype with better than 200 ps a time resolution of more than 200 ps within 3.5 ns wide pulse, at apeak output voltage of 2 kilovolts and more than 50 dB amplitude dynamics.
Transient bandwidth analysis of photoconductive microwave switches implemented in the OMEGA pulse shaping system
Kenton Green, William R. Donaldson, Robert L. Keck, et al.
Previous measurements have demonstrated that the greatest limitation on the pulse-shape bandwidth of the Laboratory for Laser Energetics' OMEGA laser's driver line occurs within the electrical pulse-shaping subsystem during transmission through the photoconductive switches. A comparison of the spectral content of the shaped pulses before and after the switch indicated that the switch 3dB bandwidth was approximately 3 GHz. Since this was the limiting bandwidth of the system, it was necessary to measure and improve the microwave transmission through the switches. This challenge required a novel measurement scheme, described in this paper, to allow accurate optimization of the switch parameters that affect bandwidth. The switches were characterized and their 3dB transmission bandwidth was expanded to over 5 GHz, thereby improving them beyond the system bandwidth limits.
Superconductive transient digitizer for laser diagnostics
S. B. Kaplan
We have successfully designed, fabricated, and demonstrated a 6-bit superconductive transient digitizer using niobium tri-layer technology. For the first time, we have used a superconductive device to record transient waveforms with greater than 10 GHz instantaneous bandwidth, and to store the data on chip.
0.5-mA vacuum closing switch
S. Bower, K. Cook, R. Keyse, et al.
For several decades the mercury cathode ignitron has been the most common switch choice for high voltage very high current applications. Such devices were capable of operation at voltages of typically up to 20kV and currents of 200kA providing they were carefully operated and maintained. However, in recent years the emergence of a number of specific applications with current and voltage requirements in excess of these levels, and increasing opposition to the use of mercury on environmental grounds, have defined a requirement for a switch of enhanced capability which does not use a mercury cathode. This paper outlines a program ofwork which has been undertaken at EEV to develop such a switch for operation at 24kV, 500kA, 130C/shot. The development progressed by the design, construction and testing of devices in a sequential manner, with design changes being made at each stage following post mortem analysis ofthe preceding iteration. This process has resulted in a device which has recently demonstrated operation at 24kV, 54OkA, 170C/shotover a test of several hundred shots duration.
Optical Damage
icon_mobile_dropdown
Statistical description of laser damage initiation in NIF and LMJ Optics at 355 nm
Michael D. Feit, Francois Y. Genin, Alexander M. Rubenchik, et al.
Understanding the 'extreme statistics' of failure at a weak link allows extrapolation of the results of small area laser damage tests to predict damage levels for the large areas pertinent to NIF/LMJ. Conceptually, it is important to focus on the fluence dependence of the surface density of damage sites. Results of different types of damage tests can be reported in terms of this sample characteristic property.
Laser-induced damage of silica glass at 1064 nm and its higher harmonics
Kunio Yoshida, Nobu Kuzuu, Takahisa Jitsuno, et al.
Laser induced damage threshold (LIDT) of various types of vitreous silica at 1064, 532, 355 and 266 nm were investigated. At 1064 nm no difference of LIDT were observed for all samples. At 1064-355 nm, wavelength dependence of LIDT of synthetic fused silica (SFS) can be well described by a relation Ith equals 1.45 (lambda) 0.43 where Ith is LIDT in J/cm2 and (lambda) is wavelength in nm. At 266 nm, however, LIDTs were smaller than a half of the calculated value from above relation. This difference can be explained by the damage mechanism; at 266 nm, two-photon absorption-induced defects lowered the LIDT same as in the case of KrF-excimer-laser induced defects, whereas at longer wavelength two-photon processed is not occur. LIDTs of fused quartz (FQ) at 532 and 355 nm, and that of a SFS containing about 1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS. This may be related to the absorption of metallic impurities in FQ and dissolved Cl2 molecules in SFS. At 266 nm, on the other hand, LIDTs of FQs were lower than those of most SFSs.
Laser-induced damage of optical coatings grown with surface chemical reaction
Shin-ichi Zaitsu, Shinji Motokoshi, Takahisa Jitsuno, et al.
We prepared optical thin films grown with surface chemical reactions using TiCl4 and H2O for TiO2, Al(CH3)3 and H2O2 for Al2O3 as reactants. The uniformity of thickness distribution was good over 240 mm in diameter. The film thickness could be preciously controlled only by the number of cycles. Two-layer TiO2/Al2O3 anti-reflection coating with less than 0.2 percent reflection loss at 532nm was fabricated. Laser induced threshold strongly depend on the growth temperature and the introduced gas pressure. They increasing with decreasing the growth temperature and with increasing the introduced gas pressure.
Thin film contamination effects on laser-induced damage of fused silica surfaces at 355 nm
Francois Y. Genin, Alexander M. Rubenchik, Alan K. Burnham, et al.
Fused silica windows were artificially contaminated to estimate the resistance of target chamber debris shields against laser damage during NIF operation. Uniform contamination thin films were prepared by sputtering various materials. The loss of transmission of the samples was first measured. They were then tested at 355 nm in air with an 8- ns Nd:YAG laser. The damage morphologies were characterized by Nomarski optical microscopy and SEM. Both theory and experiments showed that metal contamination for films as thin as 1 nm leads to a substantial los of transmission. The laser damage resistance dropped very uniformly across the entire surface. The damage morphology characterization showed that contrary to clean silica, metal coated samples did not produce pits on the surface, B4C coated silica, on the other hand, led to a higher density of such damage pits. A model for light absorption in the thin film was coupled with a simple heat deposition and diffusion model to perform preliminary theoretical estimates of damage thresholds. The estimates of the loss due to light absorption and reflection pointed out significant differences between metals. The damage threshold predictions were in qualitative agreement with experimental measurements.
Thin Film Coatings
icon_mobile_dropdown
Method for reducing the effect of environmental contamination of sol-gel optical coatings
Ian M. Thomas, Alan K. Burnham, J. R. Ertel, et al.
Antireflection (AR) coatings prepared form colloidal suspensions of silica have a large surface area because of their porosity. The surface is quite polar and readily absorbs vapor contamination to the detriment of the optical performance and the laser damage threshold. This effect is particularly bad in 'dirty' vacuum systems, such as target chambers. The polar surface is due to residual Si-OH and Si- ethoxyl groups formed as a result of the method of preparation of the coating suspension. We have now found that these groups can be removed by further treatment of the coating after preparation. This involves two steps, the first being exposure to ammonia and water vapor, which hydrolyses the ethoxyl groups to hydroxyl groups with the formation of more Si-OH groups. Some of these react further by self condensation to Si-O-Si linkages. The remaining Si- OH groups are removed in the second step by reaction with hexamethyl-disilazane, which converts them to trimethylsilyl groups. The latter are completely nonpolar and substantially eliminate vapor absorption. We have carried out a series of tests involving exposure of trace and untreated coatings to various types of vapor contamination and followed the degree of contamination by the reduction in optical transmission. In all cases, the treated coating showed a significant reduction in transmission loss. These tests also provide guidance as to which materials are acceptable for use in the NIF.
Sol-gel broadband antireflective and scratch-resistant coating for megajoule-class laser amplifier blastshields
A novel optical coating developed to reduction of specular reflection has been developed using the sol-gel route. The sol-gel antireflective (AR) coating is made from tantalum and silicon oxide-based solutions. First layer is deposited from a solution based on polymeric tantalum oxide. Second layer is containing silica polymeric matrix in order to get a double-layer optical stack. Sol-gel synthesis have been carried out starting from cheap precursors in order to produce metallic alkoxide-based solution, each one suitable for liquid-deposition technique use such as dip-coating. After layer deposition, a curing step is required. Both thermal and UV-curing could induce layer densification and generate final coating properties. Thermal baking step does not exceed 150 degrees C temperature. This two-layer antireflective coating has been optimized to offer scratch- resistance allowing easy-cleaning and also broadband anti- reflection property onto various substrate. Experiments of AR-coating deposition onto large-area high-power laser glass plates is described. Based on calculations, the amplification yield using such a sol-gel coating onto LMJ- blastshields is evaluated to be ca. 7 percent.
Depth profiling of sol-gel multilayers on fused silica using dynamic SIMS and SNMS
Nicholas J. Bazin, James E. Andrew, Hazel A. McInnes, et al.
Depth profiling using Dynamic Secondary Ion Mass Spectroscopy through multilayer coatings on fused silica substrates has revealed the effect of increasing the number of layers in the stack. Results are presented for both spin and dip coated multilayers and a significant difference in the interfacial boundary is seen between the two processes. Individual layer thicknesses were estimated using this technique and compared to values gained from UV-Visible spectroscopy. Depth profiling using SNMS of a thick 2-layer system also revealed the thickness of the layers and an indication of the intermixing between them. These measurements agreed well with UV-Vis data. A comparison between these depth-profiling techniques and previous work using AES/XPS depth profiling is discussed.
Laser cavity mirror preparation using sol-gel chemistry and laminar-flow coating technique
Philippe F. Belleville, Philippe M. Pegon
The CEA/DAM megajoule-class pulsed Nd:glass laser devoted to Inertial Confinement Fusion (ICF) research will require 240 cavity-end mirrors. The approved laser design necessitates 44cm X 44cm X 6cm highly-reflective (HR)-coated substrates representing more than 50m2 of coated area. Prototypes of these dielectric mirrors were prepared with interference quarterwave stacks of SiO2 and ZrO2-PVP thin films starting from sol-gel colloidal suspensions. Low reflective index materials was based on nanosized silica particles and high refractive index coating solution was made of a composite system. The colloidal/polymeric ratio in the composite system has been optimized regarding refractive index value, laser damage threshold and chemical interactions have been studied using FTIR spectroscopy. A promising deposition technique so-called 'Laminar Flow Coating' has been associated to sol-gel chemistry for HR laser damage-resistant sol-gel coating development. This novel coating method confirmed its main advantages compared to dipping or spinning processes: coating large flat square substrates at room temperature with small solution consumption, good thickness uniformity, weak edge-effects, induced stress-free coating, good optical properties and laser damage resistance.
3-w damage threshold evaluation of final optics components using Beamlet Mule and off-line testing
Mark R. Kozlowski, Stephen M. Maricle, Ron P. Mouser, et al.
A statistics-based model is being develop to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. In order to provide data for the mode, laser damage experiments were performed on the Beamlet laser system at LLNL. An early protoype NIF focus lens was exposed to twenty 351 nm pulses at an average fluence of 5 J/cm2, 3ns. Using a high resolution optic inspection inspection system a total of 353 damage sites was detected within the 1160 cm2 beam aperture. Through inspections of the lens before, after and, in some cases, during the campaign, pulse to pulse damage growth rates were measured for damage initiating both on the surface and at bulk inclusions. Growth rates as high as 79 micrometers /pulse were observed for damage initiating at pre-existing scratches in the surface. For most damage sites on the optic, both on the surface and at bulk inclusions. Growth rates as high as 79 micrometers /pulse were observed for damage initiating at per- existing scratches in the surface. For most damage sites on the optic, both surface and bulk, the damage growth rate was approximately 10(Mu) m/pulse.
Systems Experiments
icon_mobile_dropdown
Active Fresnel Rhomb zig-zag slab amplifier: design and test
R. G. Bikmatov, V. M. Chernyak, John T. Hunt, et al.
Zig-Zag-Slab-Amplifier had active element 4.5 X 40 X 43 cm3 in dimensions, six-bounce geometry of beam pass, pump cavity with transverse lamp orientation, and diffuse reflectors. Tests showed: the gain is equal to 8 per single pass at 77 kJ of pumping; its non-uniformity over about half of amplifier aperture is less than +/- 2.5 percent; the depolarization distortion over operating area is less than 0.5 percent.
Control of the amplifications of large-band amplitude-modulated pulses in an Nd-glass amplifier chain
Laurent Videau, Emmanuel Bar, Claude Rouyer, et al.
We study nonlinear effects in amplification of partially coherent pulses in a high power laser chain. We compare statistical models with experimental results for temporal and spatial effects. First we show the interplay between self-phase modulation which broadens spectrum bandwidth and gain narrowing which reduces output spectrum. Theoretical results are presented for spectral broadening and energy limitation in case of time-incoherent pulses. In a second part, we introduce spatial incoherence with a multimode optical fiber which provides a smoothed beam. We show with experimental result that spatial filter pinholes are responsible for additive energy losses in the amplification. We develop a statistical model which takes into account the deformation of the focused beam as a function of B integral. We estimate the energy transmission of the spatial filter pinholes and compare this model with experimental data. We find a good agreement between theory and experiments. As a conclusion, we present an analogy between temporal and spatial effects with spectral broadening and spectral filter. Finally, we propose some solutions to control energy limitations in smoothed pulses amplification.
Design progress for the National Ignition Facility laser alignment and beam diagnostics
Erlan S. Bliss, Steven J. Boege, Robert D. Boyd, et al.
Earlier papers have described approaches to NIF alignment and laser diagnostics tasks. Now, detailed design of alignment and diagnostic systems for the National Ignition Facility (NIF) laser is in its last year. Specifications are more detailed, additional analyses have been completed, Pro- E models have been developed, and prototypes of specific items have been built. In this paper we update top level concepts, illustrate specific areas of progress, and show design implementations as represented by prototype hardware. The alignment light source network has been fully defined. It utilizes an optimized number of lasers combined with fiber optic distribution to provide the chain alignment beams, system centering references, final spatial filter pinhole references, target alignment beams, and wavefront reference beams. The input and output sensor are being prototyped. They are located respectively in the front end just before beam injection into the full aperture chain and at the transport spatial filter, where the full energy infrared beam leaves the laser. The modularity of the input sensor is improved, and each output sensor mechanical package now incorporates instrumentation for four beams.
Poster Session II
icon_mobile_dropdown
Stitching interferometer for large optics: recent developments of a system
We have developed a stitching interferometer for use with large optics, such as those used in Laser MegaJoule and NIF. The idea behind this technique is that, to keep resolution at its highest value, a 'small' aperture phase-shifting interferometer must be used. But this means that measurement of the complete component has to be performed by stitching the individual sub-apertures together. We have overcome most of the obstacles involved in stitching interferometry, and have been using one such system at our optical and laser production facility in Orleans for some years. We have previously reported transmission measurements performed on a Phebus laser slab. We present here measurements performed on Laser MegaJoule transport mirrors measured in a horizontal position, and on glass inhomogeneity without oil plates. This latter requires multiple takes on each sub-aperture, and additional computing, but the whole process has been automated. In addition, we contribute to the pretty picture department by showing graphical results perhaps never previously seen. This work was performed under contract from CEA-LV, as part of the Laser Megajoule development.
Amplitude modulation of a kilowatt laser pulse with LiNbO3 Pockels cells: experiments and results on the Phebus facility
Emmanuel Bar, Francois Boubal, Nathalie Boyer, et al.
New amplitude modulators using LiNbO3 (LNO) bulk crystals have been developed at CEA/LV in order to create short temporal pulse shaping with a monomode power laser pulse before amplifying and focusing on a target. A 4 ns profile can be controlled by a growing high voltage pulse applied on electrodes pasted to the crystal. After having computed the voltage capacities and the efficiency of the Pockels effect in terms of contrast, shape quality, pulse duration and rise time, we experimented shots on Phebus facility to validate the performances of the device. This baseline of pulse shaping can be adopted in case of smoothing a broadband laser beam with an optical fiber in the LMJ facility design.
Formation of Teflon AF polymer thin films as optical coatings in the high-peak-power laser field
Nicholas J. Bazin, James E. Andrew, Hazel A. McInnes
Teflon AF 1600 and 2400 have been used to make thin film coatings with excellent anti-reflective (AR) properties. Spin, dip, and meniscus coating techniques have all successfully formed single layer AT coatings. Multi-spin and dipping of the Teflon AF polymer revealed a close linear relationship between the number of layers and total thickness of the Teflon film. HFE7100 and Hexafluorobenzene were used as co-solvents to allow good control of the coating thickness by dilution allowing the dipping or spinning rate to remain at their optimum values. Significant differences were seen between these solvents with respect to coating quality. LIDT results are reported for the single layers AT coatings along wit surface properties and attempts to stack other materials onto the Teflon films.
High-gain preamplifier module (PAM) engineering prototype for the National Ignition Facility (NIF) laser system
Mikael D. Martinez, Kenneth M. Skulina, Fred J. Deadrick, et al.
We describe recent results and developments in the preamplifier module engineering prototype located in NIF's front end or Optical Pulse Generation system. This prototype uses the general laser design developed on a physics testbed and integrates NIF type packaging as well as controls and diagnostics. We will present laser, mechanical and electrical hardware designed and built to data as well as laser energetics measurements.
Implementation of smoothing by spectral dispersion on Beamlet and NIF
Joshua E. Rothenberg, Jerome M. Auerbach, Bryan D. Moran, et al.
The performance of the Beamlet laser with one dimensional smoothing by spectral dispersion implemented is investigated. Measurements of then ear field beam quality, nonlinear breakup, and transmission through spatial filter pinholes show a modest effect only at large SSD divergence. No measurable effect was found at the divergence level planned for indirect drive ignition experiments. The efficiency of conversion to the third harmonic was also measured with SSD present and found to be somewhat larger than expected form an ideal plane wave model.
Polarization smoothing for the National Ignition Facility
Polarization smoothing (PS) is the illumination of the target with two distinct and orthogonally polarized speckle patterns. Since these two polarizations do not interfere, the intensity patterns add incoherently and thus the contrast of the intensity nonuniformity can be reduced by a factor of (root) 2 in addition to any reduction achieved by temporal smoothing techniques. Smoothing by PS is completely effective on an instantaneous basis and is therefore of particular interest for the suppression of laser plasma instabilities, which have a very rapid response time. The various implementations of PS are considered, and their impact, in conjunction with temporal smoothing methods, on the spatial spectrum of the target illumination is analyzed.
Optimization of the alignment sensitivity and energy stability of the NIF regenerative amplifier cavity
N. W. Hopps, Russell B. Wilcox, Mark Robert Hermann, et al.
The work to improve the energy stability of the regenerative amplifier for the NIF is described. This includes a fast feed-forward system, designed to regulate the output energy of the regen by monitoring how quickly a pulse builds up over many round trips. Shot-to-shot energy fluctuations of all elements prior to the regen may be compensated for in this way, at the expense of a loss of approximately 50 percent. Also included is a detailed study into the alignment sensitivity of the regen cavity, with the goal of quantifying the effect of misalignment on the output energy. This is done by calculating the displacement of the eigenmode by augmenting the cavity ABCD matrix with the misalignment matrix elements, E, F. In this way, cavity misalignment issues due to thermal loading of the gain medium are investigated. Alternative cavity designs, which reduce the alignment sensitivity and therefore the energy drift over periods of continuous operation, are considered. Alterations to the amplifier head design are also considered.
Calculational modeling of the Luch facility high-power amplifier pumping and pulse amplification
V. A. Eroshenko, Sergey G. Garanin, Gennadi A. Kirillov, et al.
Results of computational modeling of 'Luch' facility output amplifier module pumping by Monte-Carlo method is described as well as the results of calculations of short pulse amplification in 4-pass amplifier of this facility. The aim of calculations was study of stored energy space distribution homogeneity, optimization of amplifier configuration and investigation of output pulse profile sensitivity to input pulse from variations and value of small signal gain.
Development of a kilojoule channel for Nd-glass laser systems with a 70-ns pulse SBS compressor
Vladmir S. Sirazetdinov, V. N. Alekseev, A. V. Charukhchev, et al.
An optical scheme for a kJ channel of a neodymium glass laser system with SBS-compressor for a 70 ns pulse has been described. The use of a pump pulse of long duration - 70 ns - makes it possible to operate a near-steady SBS mode in compressed gasses. An experimental study of the SBS oscillator-compressor stage with pump focusing and a two- pass pattern has been carried out and a 70 ns, 8 J pump pulse has been compressed down to 1 ns.
Transverse SRS in KDP and KD*P crystals
V. N. Novikov, S. A. Belkov, S. A. Buiko, et al.
Gain coefficients at transverse SRS in the particular samples of KDP and KD*P crystals, grown following the traditional and rapid-growth technology have been determined.
Wavefront and divergence of the Beamlet prototype laser
Paul J. Wegner, Mark A. Henesian, J. Thaddeus Salmon, et al.
We have measured the wavefront and the divergence of the Beamlet prototype laser under a variety of conditions. Emphasis of the tests was on quantifying best attainable divergence in the angular regime below 30 (mu) rad to benchmark propagation models that are used to set wavefront gradient specifications for NIF optical components. Performance with and without active wavefront correction was monitored with radial shearing interferometers that measured near-field wavefront at the input and output of the main amplifier with a spatial resolution of 1 cm, and cameras which measured the corresponding intensity distributions in the far field with an angular resolution of 0.3 (Mu) rad. Details of the measurements are discussed and related to NIF focal spot requirements and optics specifications.
Optical Design of Large Laser Systems I
icon_mobile_dropdown
Optical design of the National Ignition Facility main laser and switchyard/target area beam transport systems
John L. Miller, R. Edward English Jr., Ronald J. Korniski, et al.
The optical design of the main laser and transport mirror sections of the National Ignition Facility are described. For the main laser the configuration, layout constraints, multiple beam arrangement, pinhole layout and beam paths, clear aperture budget, ray trace models, alignment constraints, lens designs, wavefront performance, and pupil aberrations are discussed. For the transport mirror system the layout, alignment controls and clear aperture budget are described.
National Ignition Facility main laser stray light analysis and control
John L. Miller, R. Edward English Jr., John Charles Schweyen, et al.
Stray light analysis has been carried out for the main laser section of the National Ignition Facility main laser section using a comprehensive non-sequential ray trace model supplemented with additional ray trace and diffraction propagation modeling. This paper describes the analysis and control methodology, gives examples of ghost paths and required tilted lenses, baffles, absorbers, and beam dumps, and discusses analysis of stray light 'pencil beams' in the system.
Ghost analysis visualization techniques for complex systems: examples from the NIF final optics assembly
James L. Hendrix, John Charles Schweyen, Jeffrey Rowe, et al.
The stray light or 'ghost' analysis of the NIF Final Optics Assembly (FOA) has proved to be one of the most complex ghost analyses ever attempted. The NIF FOA consists of a bundle of four beam lines that: 1) provides the vacuum seal to the target chamber, 2) converts 1 (omega) to 3 (omega) light, 3) focuses the light on the target, 4) separates a fraction of the 3 (omega) beam for energy diagnostics, 5) separates the three wavelengths to diffract unwanted 1 (omega) and 2 (omega) light away from the target, 6) provides spatial beam smoothing, and 7) provides a debris barrier between the target chamber and the switchyard mirrors. The three wavelengths of light and even optical elements with three diffractive optic surfaces generate three million ghosts through 4th order. Approximately 24,000 of these ghosts have peak fluence exceeding 1 J/cm2. The shear number of ghost paths requires a visualization method that allows overlapping ghosts on optics and mechanical components to be summoned and then mapped to the optical and mechanical component surfaces in 3D space. This paper addresses the following aspects of the NIF Final Optics Ghost analysis: 1) materials issues for stray light mitigation, 2) limitations of current software tools, 3) computer resource limitations affecting automated coherent raytracing, 4) folding the stray light analysis into the opto-mechanical design process, 5) analysis and visualization tools from simple hand calculations to specialized stray light analysis computer codes, and 6) attempts at visualizing these ghosts using a CAD model and another using a high end data visualization software approach.
Simulation and analysis of ghost images for the megajoule laser
Herve Benard, Mathieu Germa, N. Ferriou, et al.
The amplification of the Megajoule Laser will be made with a multi-pass cavity. This technical principle can generate many ghost images. In fact, beam reflections occur on lens faces, propagate in the whole system and may focus in several points. Some of these reflections can also be amplified. Furthermore, focusing points may be located near or inside optical elements and their fluence may be over the glass damage threshold. So the amplification cavity can not be realized without studying beam reflections. In consequence, OPTIS developed a new and specific software with the help and scientifical advice of the CEA/DAM.
Design optimization for the main amplification stage of Technical Integration Line (TIL)
Feng Jing, Xiao Min Zhang, Qihua Zhu, et al.
A four pass amplifier system with a small aperture beam reverser has been designed as the main amplifier stage of Technical Integration Line (TIL). TIL is the full scale two- beam prototype for Shenguang-III laser facility which will produce 1 kJ of UV radiation on the target from each beam in 1-3 nanoseconds shaped pulses. The variables were optimized for a fixed output beam aperture of 25 X 25 cm2 and the given parameters of the optical components under the constraints of amplifier gain, fluency damage/filamentation and so on. As a result, the baseline design for TIL was set to a 9-5 configuration.
Optical Design of Large Laser Systems II
icon_mobile_dropdown
NIF optical specifications: the importance of the RMS gradient
Janice K. Lawson, Jerome M. Auerbach, R. Edward English Jr., et al.
The performance of the National Ignition Facility (NIF), especially in terms of laser focusability, will be determined by several key factors. One of these key factors is the optical specification of the thousands of large aperture optics that will comprise the 192 beamlines. We have previously reported on the importance of the specification of the power spectral density (PSD) on NIF performance. Recently, we have been studying the importance of long spatial wavelength phase errors on focusability. We have concluded that the preferred metric for determining the impact of these long spatial wavelength phase errors is the rms phase gradient. In this paper, we outline the overall approach to NIF optical specifications, detail the impact of the rms phase gradient on NIF focusability, discuss its trade-off with the PSD in determining the spot size, and review measurements of optics similar to those to be manufactured for NIF.
Application of adaptive optics for controlling the NIF laser performance and spot size
Richard A. Sacks, Jerome M. Auerbach, Erlan S. Bliss, et al.
The National Ignition Facility (NIF) laser will use a 192- beam multi-pass architecture capable of delivering several MJ of UV energy in temporal phase formats varying from sub- ns square to 20 ns precisely-defined high-contrast shapes. Each beam wavefront will be subjected to effects of optics inhomogeneities, figuring errors, mounting distortions, prompt and slow thermal effects from flashlamps, driven and passive air-path turbulence, and gravity-driven deformations. A 39-actuator intra-cavity deformable mirror, controlled by data from a 77-lenslet Hartman sensor will be used to correct these wavefront aberrations and thus to assure that stringent farfield spot requirements are met. We have developed numerical models for the expected distortions, the operation of the adaptive optics systems, and the anticipated effects on beam propagation, component damage, frequency conversion, and target-plane energy distribution. These models have been extensively validated against data from LLNL's Beamlet, and Amplab lasers. We review the expected beam wavefront aberrations and their potential for adverse effects on the laser performance, describe our model of the corrective system operation, and display our predictions for corrected-beam operation of the NIF laser.
Simulations of a phase corrector plate for the National Ignition Facility
Simulations are presented on the effect of placing a static phase corrector plate in each beamline of the NIF to assist the adaptive optic in correcting beam phase aberrations. Results indicate that such a plate could significantly improve the focal spot, reducing a 3 (omega) , 80 percent spot half-angle from 21 to 8 (mu) rad for poorer-quality optics, and 17 to 7 for better optics. Such a plate appears to be within the range of current fabrication technologies. It would have an alignment requirement of +/- 0.5 mm,l if placed in the front end. The NIF operation, the occasional replacement of laser slabs would slowly degrade the beam quality for a fixed corrector plate, with the spot size increasing from 8 to 15 (mu) rad after four new slabs for poorer optics, and 7 to 12 (mu) rad for better optics. The energy fraction clipped on the injection pinhole would be < 0.5 percent due to this pre-correction.
Stability of optical elements in the NIF target area building
David J. Trummer, Richard J. Foley, Gene S. Shaw
The target Area Building (TAB) of the NIF is 300 feet long, 100 feet wide, and 100 feet tall and is comprised of a cylindrical target building and two switchyard space frames. The reinforced concrete target building houses the target chamber, target positioner, turning mirrors, final optics assemblies, and diagnostics, while the steel switchyard space frames support turning mirrors and diagnostic equipment. Within the TAB, the 192 independent laser beams of the NIF laser system are required to be accurately positioned. In order to satisfy the engineering system requirement for optical system positioning, the TAB must provide a stable platform for optical elements before and during a shot. This paper summarizes the stability analyses that were performed in support of the TAB and optical system design. Sources that influence optic stability are structural excitations, such as ambient and wind induced vibrations, and thermal transients, such as diurnal and HVAC temperature changes. A positioning error budget has been developed for the NIF project for use in the design and evaluation of structures which support optical elements. To satisfy the error budget requirements, vibrational stability will be achieved through a combination of facility design, optical support structure design, and passive damping. Thermal stability will be accomplished by using thermal-mass concrete structures, conditioned air flow, and a reduction of heat sources. Finite element analysis has been used to evaluate the design of the TAB and optical support structures. A detailed structural model of the TAG that includes the target positioner, target chamber, turning mirrors, and diagnostics, has been used for stability evaluations. Finite element analyses covering ambient ground vibration, thermal loads, pressure fluctuations, and wind excitations have demonstrated that the current design of the TAB provides a stable platform for maintaining beam alignment.
Frequency Conversion Materials
icon_mobile_dropdown
Overview of recent KDP damage experiments and implications for NIF tripler performance
Michael J. Runkel, Richard T. Jennings, Jim J. DeYoreo, et al.
Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements of inertial confinement fusion lasers at LLNL. The maximum fluence requirement for KD*P triplers on the NIF is 14.3 J/cm2 at 351 nm in a 3 ns pulse. Currently KD*P cannot meet this requirement without laser (pre)conditioning. In this overview, recent experiments to understand laser conditioning and damage phenomena in KDP and KD*P are discussed. These experiments have led to a fundamental revision of damage test methods and test result interpretation. In particular, the concept of a damage threshold has given way to measuring performance by damage distributions using beams of millimeter size. Automated R/1 damage test have shown that the best rapidly grown KDP crystals exhibit the same damage distributions as the best conventionally grown KD*P. Continuous filtration of the growth solution and post-growth thermal annealing are shown to increase the damage performance as well.
Growth of large KDP crystals in the form of plates
Vitali Tatartchenko, Emmanuel Beriot
This paper suggests a ne technique of growth-oriented KDP crystal in the form of plates. The technique includes: using small oriented seeds spaced between two parallel platforms with a rapid growth of crystal between these two platforms, in a tank conditioning a KDP solution. As a result, crystals in the form of plates can be obtained. The thickness of the crystal plate depends on the distance between platforms. The horizontal dimensions of the plate depend on the volume of solution and the diameter of the platforms.the orientation of the plates are defined by the orientation of the seed. KDP crystals in the form of plates of two orientations are grown. The peculiarities of morphology and some characteristics of crystals are discussed.
Frequency Conversion Technology
icon_mobile_dropdown
Frequency converter development for the National Ignition Facility
Paul J. Wegner, Jerome M. Auerbach, Charles E. Barker, et al.
The design of the NIF incorporates a type I/type II third harmonic generator to convert the 1.053-micrometers fundamental wavelength of the laser amplifier to a wavelength of 0.351 micrometers for target irradiation. To understand and control the tolerances in the converter design, we have developed a comprehensive error budget that accounts for effects that are known to influence conversion efficiency, including variations in amplitude and phase of the incident laser pulse, temporal bandwidth of the incident laser pulse, crystal surface figure and bulk non-uniformities, angular alignment errors, Fresnel losses, polarization errors and crystal temperature variations. The error budget provides specifications for the detailed design of the NIF final optics assembly and the fabrication of optical components. Validation is accomplished through both modeling and measurement, including full-scale Beamlet tests of a 37-cm aperture frequency converter in a NIF prototype final optics cell. The prototype cell incorporates full-perimeter clamping to support the crystals, and resides in a vacuum environment as per the NIF design.
Demonstration of dual-tripler broadband third-harmonic generation and implications for OMEGA and the NIF
Andrei Babushkin, Robert Stephen Craxton, Stephen Oskoui, et al.
The dual-tripler scheme for enhancing the bandwidth of third-harmonic generation, proposed by Eimerl et al., has been experimentally demonstrated for the conversion of 1054- nm radiation to 351 nm. The results are in excellent agreement with theory and show that the spacing between the triplers must be carefully controlled. Designs are given for the addition of a second tripler to OMEGA and the NIF. An approximately threefold increase in bandwidth is predicted for both laser systems.
Multi-terawatt frequency doubling of picosecond pulses for plasma interactions
David Neely, Colin N. Danson, Ric M. Allott, et al.
Frequency doubling a large aperture sub ps, chirped pulse amplified (CPA) 1053 nm beam for laser matter interaction studies was investigated at the Central Laser Facility. Efficiencies > 50 percent were achieved using a 4 mm thick KDP crystal to convert a 140 X 89 mm 700 fs beam. Measurements of the 527 nm beam's focal spot quality when the doubling crystal was driven at high intensities 200 GWcm-2 are presented. Using data from 2 and 4 m thick 25 mm diameter test crystals, the optimum crystal thickness in terms of conversion efficiency is reviewed for 1053 nm CPA systems in the 0.3-3 ps region and options for fourth harmonic production discussed.
Diffractive Optics
icon_mobile_dropdown
Focusing transmission gratings for high-energy lasers
Bruno M. Touzet, Jean Flamand, Alain Thevenon, et al.
To focus the third harmonic of high energy neodymium glass laser, the usage of focusing transmissions diffractive gratings ion etched directly into fused silica blank brings major benefits in comparison with classical optical solutions using aspheric lenses. The damage threshold of such a diffractive component is the same as the damage threshold of a high grade, thin, fused silica plane window. The transmission efficiency reaches 94 percent for polarization s or p. The image quality is nearly perfect: this diffractive component acts as a stigmatic lens. It separates the remaining first and second harmonic from the third one and in addition it creates a focused reflective order with 0.5 percent efficiency which offers the possibility to get a well focused sampling beam.
Beam shaping diffractive optical elements for high-power solid state laser systems
Ian M. Barton, Paul Blair, Andrew J. Waddie, et al.
Diffractive optical elements to modify laser beam spatial intensity distributions are described. The elements have been applied to free-space Gaussian to flat-top beam conversion and customization of the modes of a laser resonator. The single pass free space elements demonstrate a high efficiency but result in to much high frequency noise on the beam. The intra-cavity elements significantly altered the TEM00 profile but practical limitations with the positioning of the element within the cavity prevented operation in the design mode.
Diffractive beam samplers for large-aperture beam diagnostics
Thomas H. Bett, I. C. Smith
Diffractive beam samplers have been employed for some time on the HELEN laser at AWE. The phase gratings combine two functions viz. sampling and focusing in one element. The weak phase modulation generates low power diagnostic beams from which energy measurements have been made. This paper reports on further beam characterization measurements of the imaging properties of these components by recording far field and near field intensity distributions of beams up to 200mm diameter.
Nonlinear Optics
icon_mobile_dropdown
Cross-phase modulation between two intense orthogonally polarized laser beams copropagating through a Kerr-like medium
J. A. Marozas
The Laboratory for Laser Energetics plans to install KDP wedges on each beam line of the OMEGA laser system in order to split each beam into two orthogonally polarized beams propagating gin slightly different directions. The wedges improve the on-target laser uniformity by decreasing the instantaneous speckle through spatial averaging of the two orthogonal beams. The proposed wedge-finishing method - diamond turning - procedures small residual scratch marks, causing each beam to acquire a pseudorandom phase perturbation. In addition, the orthogonally polarized beams interfere such that their combined polarization state continuously cycles through all elliptical states along any transverse plane. Since the nonlinear refractive index depends on the polarization state, intense beams accumulate a periodic phase perturbation that is greatest for linear polarization. Propagation of both types of phase perturbation yields an intensity modulation that tens to be larger in the neighborhood of linear polarization, through a combination of diffraction and self- and cross-phase modulation. However, 1D and 2D calculations demonstrate that diamond-turned KDP wedges are not a significant source of intensity modulation under OMEGA laser conditions. Installation of diamond-turned rather than polished wedges will reduce costs without adversely impacting the system performance.
Suppression of small-scale-intensity nonuniformity introduced by array focus systems
Baida Lu, Bin Zhang, Bangwei Cai
In this paper, the speckles introduced by multiple interference of the crossed segmented wedge array focus system, which can result in small-scale intensity non- uniformity, have been studied in detail. The methods for the reduction of small-scale non-uniformity have been illustrated.
Modeling of frequency doubling and tripling with converter refractive index spatial nonuniformities due to gravitational sag
Jerome M. Auerbach, Charles E. Barker, Scott A. Couture, et al.
Accurate predictions of the performance of frequency conversion requires knowledge of the spatial variation of departures from the phase-matching condition in the converter crystals. This variation is caused by processes such as crystal growth and crystal surface finishing. Gravitational sag and mounting configurations also lead to deformation and stresses which cause spatially varying departures from the phase-matching condition. We have modeled the effect of gravitational forces on conversion efficiency performance of horizontal converter crystals and have shown for the NIF mounting configurations that gravity has very little effect on conversion efficiency.
Localized wavefront deformations--propagation in nonlinear media: II. A simple but accurate computational method
The propagation of waves in non-linear media is perturbed by the fact that the index of refraction of such media is modified by the intensity of the wave which, in turn, modifies the shape of the wave which, in turn, modifies their intensity of the wave, and so on. The localized small concavity in the wavefront due, typically, to non-perfect components, also perturbs the wavefront by creating a localized intensity peak further down the line. The computation of such modulation is relatively easy in vacuum, by simple wave summation according to Fresnel, but rather less so in non-linear media. This is because the above- mentioned index variations modify the actual optical path of the rays as compared to vacuum. For shallow defects, a simple but effective method is to proceed via the Fourier transform of the defect shape. Individual spatial modes propagate with accurately-known amplitude variation. Modes are summed up after individual propagation, leading to accurate results. The amplification of such modes can be very great, leading to many-fold amplification of localized intensity peaks and, therefore, to potential damage to optical components. We explain the method, and compare numerical with 'real' simulations performed on MIRO. Additionally, as we show, insight into the behavior of defect amplification is gained, and accurate predictions can be made with little or no computational effort.
Spatial Filter Pinholes
icon_mobile_dropdown
Spatial filter issues
James E. Murray, David Milam, Charles D. Boley, et al.
Beamlet measurements show that cone pinholes outperform other types tested with regard to both closure and back reflections. A +/- 150 (mu) rad stainless steel cone remained open for a 15.5 kJ, 10:1 contrast shaped pulse with +/- 7.5 (mu) rad of SSD divergence, which more than meets the requirements for a NIF ignition pulse. Measurements also showed the maximum tolerable pressures in the NIF spatial filters to be a few milliTorr, leading to recommendations of 0.1 nd 0.6 mTorr for the NIF transport and cavity spatial filters, respectively.
Pinhole closure measurements
David Milam, James E. Murray, Kent G. Estabrook, et al.
Spatial-filter pinholes and knife-edge samples were irradiated in vacuum by 1053-nm, 5-20 ns pulses at intensities to 500 GW/cm2. The knife-edge samples were fabricated of plastic, carbon, aluminum, stainless steel, molybdenum, tantalum, gold, and an absorbing glass. Time- resolved two-beam interferometry with a 40-ns probe pulse was used to observe phase shifts in the expanding laser- induced plasma. For al of these materials, at any time during square-pulse irradiation, the phase shift fell exponentially with distance from the edge of the sample.. The expansion was characterized by the propagation velocity V2(pi ) of the contour for a 2(pi) phase shift. To within experimental error, V2(pi ) was constant during irradiation at a particular intensity, and it increased linearly with intensity for intensities < 300 GW/cm2. For metal samples V2(pi ) exhibited an approximate M-0.5 dependence where M is the atomic mass. Plasmas of plastic, carbon, and absorbing glass produced larger phase shifts, and expanded more rapidly, than plasmas of heavy metals. The probe beam and interferometer were also used to observe the closing of pinholes. With planar pinholes, accumulation of on-axis plasma was observed along with the advance of plasma away from the edge of the hole. On-axis closure was not observed in square, 4-leaf pinholes.
Pinhole closure in spatial filters of large-scale ICF laser systems
R. G. Bikmatov, Charles D. Boley, I. N. Burdonsky, et al.
Pinhole plasma effects on parameters of the laser beam passing through the spatial filter in conditions of interest for large scale ICF laser facilities were investigated. The experiments on pinhole irradiation were conducted at power density range 1010-1011 W/cm2 with approximately 15 ns laser pulses. Al, Fe, and Ta pinholes were used. The diagnostic approach was chosen based on probing the pinhole region with frequency doubled 3-ns-long laser pulse. Ablative-plasma dynamics was studied with shadowgraphy and interferometry. Also measured were the parameters of transmitted probing beam in the near- and far-fields. The rate of pinhole 'closure' is found to decrease with the increase in the atomic number of pinhole material. The rate o pinhole closure ranges from approximately 5*106 cm/s for aluminum pinhole down to approximately 2*106 cm/s for tantalum pinhole in experiments with power density at the pinhole edge of approximately 50 GW/cm2. For aluminum and steel pinholes the parameters of the transmitted probing beam deteriorate to unacceptable level for approximately 15-20 ns after the irradiation start. In the same experimental conditions the pinholes of tantalum exhibits acceptable performance till the end of the irradiation process. Fast plasma jets converging to the pinhole axis with velocities up to approximately 107 cm/s and significantly deteriorating transmitted probing beam quality are observed. Reasonable agreement was found between the data obtained in experiments with circular pinholes and linear edge experiments.
Modeling of laser knife-edge and pinhole experiments
Charles D. Boley, Kent G. Estabrook, Jerome M. Auerbach, et al.
We describe simulations of experiments invovling laser illumination of a metallic knife edge in the Optical Sciences Laboratory (OSL) at LLNL, and pinhole closure in the Beamlet experiment at LLNL. The plasma evolution is modeled via LASNEX. In OSL, the calculated phases of a probe beam are found to exhibit the same behavior as in experiment but to be consistently larger. The motion of a given phase contour tends to decelerate at high intensities. At fixed intensity, the speed decreases with atomic mass. We then calculate the plasma associated with 4-leaf pinholes on the Beamlet transport spatial filter. We employ a new propagation code to follow a realistic input beam through the entire spatial filter, including the plasmas. The detailed behavior of the output wavefronts is obtained. We show how closure depends on the orientation and material of the pinholes blades. As observed in experiment, a diamond orientation is preferable to a square orientation, and tantalum performs better than stainless steel.
Laser Systems Controls Diagnostics and Integration
icon_mobile_dropdown
National Ignition Facility integrated computer control system
Paul J. Van Arsdall, R. C. Bettenhausen, Frederick W. Holloway, et al.
The NIF design team is developing the Integrated Computer Control System (ICCS), which is based on an object-oriented software framework applicable to event-driven control system. The framework provides an open, extensive architecture that is sufficiently abstract to construct future mission-critical control systems. The ICCS will become operational when the first 8 out of 192 beams are activated in mid 2000. THe ICCS consists of 300 front-end processors attached to 60,000 control points coordinated by a supervisory system. Computers running either Solaris or VxWorks are networked over a hybrid configuration of switched fast Ethernet and asynchronous transfer mode (ATM). ATM carries digital motion video from sensor to operator consoles. Supervisory software is constructed by extending the reusable framework components for each specific application. The framework incorporates services for database persistence, system configuration, graphical user interface, status monitoring, event logging, scripting language, alert management, and access control. More than twenty collaborating software applications are derived from the common framework. The framework is interoperable among different kinds of computers and functions as a plug-in software bus by leveraging a common object request brokering architecture (CORBA). CORBA transparently distributes the software objects across the network. Because of the pivotal role played, CORBA was tested to ensure adequate performance.
Timing system of LMJ: application for the LIL
Dominique Monnier-Bourdin, Alain Herve, Olivier Lutz
The laser megajoule facility, and the Ligne d'Integration Laser (LIL), its prototype, will require a timing system. This timing system will be supplied by contractors. Nevertheless, to fulfill the needs of the LIL before the deliveries of these industrial components CEA has developed some specific systems.
Development of practical damage mapping and inspection systems
Frank Rainer, Richard K. Dickson, Richard T. Jennings, et al.
We have developed and are continuing to refine semi- automated technology for the detection and inspection of surface and bulk defects and damage in large laser optics. Different manifestations of the DAMOCLES system provide an effective and economical means of being able to detect, map and characterize surface and bulk defects which may become precursors of massive damage in optics when subjected to high-fluence laser irradiation. Subsequent morphology and evolution of damage due to laser irradiation can be tracked efficiently. The strength of the Damocles system is that it allows for immediate visual observation of defects in an entire optic, which can range up to 1-meter dimensions, while also being able to provide digital map and magnified images of the defects with resolutions better than 5 micrometers .
Combined phase, near field, and far field diagnostic for large-aperture laser systems
Andrew R. Barnes, I. C. Smith
A method is described for extracting near field, far field and phase information from an interferogram of a laser system output generated by a self referencing, radial shear interferometer. The interferogram may be used as an amplitude mask of a coherent beam. This beam is numerically propagated to the far field where the laser output and its complex conjugate can be separated. The near field and optical phase difference map may then be extracted from the complex representation of the laser output obtained by propagating back to the near field. The practical application of this method and its limitations are discussed and compared with images from equivalent plane far field and near field cameras on the HELEN 2TW Nd:glass laser system.
Start-up plan for the first NIF laser bundle: transition from single Beamlet prototype to 192-beamline laser facility
Bruno M. Van Wonterghem, David Ralph Speck, Detlev H. Tiszauer, et al.
The compact multi-pass laser design and the extensive use of optical component assemblies as line replaceable units are essential to achieve the cost efficiency of the NIF design. These design philosophies require a new approach to start-up operation of the NIF 192-beam-line high-energy laser compared to existing fusion laser facilities. The limited access on the beam-line optics and the limited on-line verification and maintenance capability require that extensive component verification and alignment takes place in off-line facilitates before the LRU's are installed in the laser structure.
Amplifier Technology
icon_mobile_dropdown
Development progress for the amplifier of the SG-III laser facility
Wanguo Zheng, S. B. He, Xiao Min Zhang, et al.
Development plan and some progress have been made for amplifier of SGIII laser facility. According to this plan, a single-segment amplifier has been designed and experimented to test key units and correct simulating code in National Laboratory of Laser Fusion of China. The preliminary design for 4 X 2 X 3 amplifier prototypes, including amplifier modules, pulsed power, assembly equipment, and optics for SGIII will be finished.
Lasers for high-focused intensity applications
John A. Nees, Subrat Biswal, Gerard A. Mourou
We present the theoretical limits for generation of high focused intensity laser. The important role of high- saturation-fluence materials is established and the use of regenerative chirped pulsed amplification to achieve high energy extraction from such materials is discussed. A regenerative chirped pulse amplification model and related experiments support the view that the surface damage threshold does not limit the range of useful laser materials to those with saturation fluence below the surface damage threshold. In addition the importance of phase measurement and control in providing well-defined conditions for experiments are noted.
Assembly and maintenance of full-scale NIF amplifiers in the Amplifier Module Prototype Laboratory (AMPLAB)
John A. Horvath
Mechanical assembly and maintenance of the prototype NIF amplifiers in the AMPLAB at LLNL requires specialized equipment designed to manipulate large and delicate component in a safe and clean manner. Observations made during the operation of this assembly and maintenance equipment in AMPLAB provide design guidance for similar tools being built for the NIF. Fixtures used for amplifier frame installation, laser slab and flashlamp cassette assembly, transport, and installation, and in-situ blastshield exchange are presented. Examples include a vacuum slab gripper, slab handling clean crane, slab cassette assembly fixture, sealed transport vehicle for slab cassette movement between the cleanroom and amplifier, slab cassette transfer fixture between the cleanroom and transport vehicle, and equipment needed for frame assembly unit, blastshield, and flashlamp cassette installation and removal. The use of these tools for amplifier assembly , system reconfiguration, reflector replacement, and recovery from an abnormal occurrence such as a flashlamp explosion is described. Observations are made on the design and operation of these tools and their contribution to the final design of their NIF counterparts.
Achieving and maintaining cleanliness in NIF amplifiers
Irving F. Stowers, John A. Horvath, Joseph A. Menapace, et al.
Cleanliness measurements made on AMPLAB prototype NIF laser amplifiers during assembly, cassette transfer, and amplifier operation are summarized. These measurements include particle counts from surface cleanliness assessments using filter swipe technique and from airborne particle monitoring. Results are compared with similar measurements made on the Beamlet and Nova lasers and in flashlamp test fixtures. Observations of Class 100,000 aerosols after flashlamp firings are discussed. Comparisons are made between typical damage densities on laser amplifier optics from Novette, NOVA, Beamlet, and AMPLAB.
From filter to all-film energy storage capacitors
Michel Bramoulle, Pierre Michalczyk, Jean-Pierre Marret
This paper describes the evolution of the high voltage d.c. capacitors, starting from filters to energy storage applications. This evolution was relatively slow, up to the beginning of the 90's. The new requirements, which later appeared, led to profound changes in the basic design, thanks to a combination of existing but seemingly incompatible raw materials. The successful experiences obtained with filter capacitors were followed by a technology transfer to the energy storage field, with some different constrains, which necessitated a better knowledge of the performance limits of this new technology. The final result was improved safety and a permanent control of the estate of the component.
Amplifier Wavefront Characterization and Thermal Recovery
icon_mobile_dropdown
3D gain modeling of LMJ and NIF amplifiers
Geoffroy LeTouze, Olivier Cabourdin, J. F. Mengue, et al.
A 3D ray-trace model has been developed to predict the performance of flashlamp pumped laser amplifiers. The computer program, written in C++, includes a graphical display option using the Open Inventor library, as well as a parser and a loader allowing the user to easily model complex multi-segment amplifier systems. It runs both on a workstation cluster at LLNL, and on the T3E Cray at CEA. We will discuss how we have reduce the required computation time without changing precision by optimizing the parameters which set the discretization level of the calculation. As an example, the sample of calculation points is chosen to fit the pumping profile through the thickness of amplifier slabs. We will show the difference in pump rates with our latest model as opposed to those produced by our earlier 2.5D code AmpModel. We will also present the results of calculations which model surfaces and other 3D effects such as top and bottom refelcotr positions and reflectivity which could not be included in the 2.5D model. This new computer model also includes a full 3D calculation of the amplified spontaneous emission rate in the laser slab, as opposed to the 2.5D model which tracked only the variation in the gain across the transverse dimensions of the slab. We will present the impact of this evolution of the model on the predicted stimulated decay rate and the resulting gain distribution. Comparison with most recent AmpLab experimental result will be presented, in the different typical NIF and LMJ configurations.
Pump-induced wavefront distortion in prototypical NIF/LMJ amplifiers: modeling and comparison with experiments
Mark D. Rotter, Kenneth S. Jancaitis, Christopher D. Marshall, et al.
In large-aperture laser amplifiers such as those envisioned for the NIF and LMJ lasers, the geometry is such that the front and back faces of the laser slab are heated unevenly by the pump process. This uneven heating result in a mechanical deformation of the laser slab and consequent internal stresses. The deformation and stresses, along with a temperature-dependent refractive index variation, result in phase variations across the laser beam. These phase variations lead to beam steering which may affect frequency conversion as well as energy-on-target. We have developed a model which allows us to estimate the pump-induced wavefront distortion for a given amplifier configuration as well as the spatially-resolved depolarization. The model is compared with experiments taken in our amplifier development laboratory, AMPLAB.
Thermal recovery of LMJ amplifiers
Ph. Arnoux, Geoffroy LeTouze, J. P. Caltagirone, et al.
LMJ/NIF shot rate is limited by thermal recovery of laser slabs and beamtubes. Most of the electrical energy input is dissipated thermally in the amplifiers. Wall-plug efficiency is around 1 percent. Wavefront distortion due to non uniform heat deposition or thermal gradient in gases can only be corrected in the limits of the adaptive optics. Laser focusing requirements thus dictate a maximum chain aberration which can be expressed in wavefront distortion per slab per pass and lead to thermal recovery requirements. Aberrations are analyzed in terms of residual temperature differences in slabs and gas columns of amplifier cavities and beamtubes, and of wavefront distortions.
Thermal recovery of the NIF amplifiers
Steven B. Sutton, A. E. Erlandson, Richard A. London, et al.
With approximately 99 percent of the electrical energy supplied to the NIF appearing as heat in the amplifiers, thermal recovery of the NIF system is a major consideration in the design process. The NIF shot rate is one shot every 8 hours, with a goal of 4 hours between shots. This necessitates that thermal recovery take place in no more than 7 hours, with a goal of 3 hours for the accelerated shot rate. Residual optical distortions, which restrict the shot rate, are grouped into two discrete categories: (1) distortions associated with residual temperature gradients in the laser slabs, and (2) distortions associated with buoyantly driven convective currents in the amplifier cavity and beam-tube regions. Thermal recovery of the amplifiers is achieved by cooling the flashlamps and blastshields with a heat deposited in the slabs and edge claddings. Advanced concepts, such as the use of slightly chilled gas to accelerate some aspects of recovery, are addressed. To quantify recovery rates of the amplifiers, experiments and numerical models are used to measure and calculate the temperatures and optical distortions in NIF-like amplifier elements. The calculation results are benchmarked against AMPLAB temperature measurements, thus allowing a quantitative prediction of NIF thermal recovery. These results indicate that the NIF requirement of 7 hour thermal recovery can be achieved with chilled temperature cooling gas. It is further shown that residual temperature gradient driven distortions in the slabs reach an acceptable level, after 4 hours of thermal recovery.
Adaptive Optics and Wavefront Controls
icon_mobile_dropdown
National Ignition Facility (NIF) wavefront control system
A wavefront control system will be employed on NIF to correct beam aberrations that otherwise would limit the minimum target focal spot size. For most applications, NIF requires a focal spot that is a few times the diffraction limit. Sources of aberrations that must be corrected include prompt pump-induced distortions in the laser slabs, thermal distortions in the laser slabs from previous shots, manufacturing figure errors in the optics, beam off-axis effects, gas density variations, and gravity, mounting, and coating-induced optic distortions.
Preliminary design of the cavity-end deformable mirror of the megajoule laser
Xavier Bozec, Renaud Mercier-Ythier, Jean-Louis Carel, et al.
This paper describes a preliminary design to realize a 400 X 400 mm2 active deformable mirror in the framework of the LaserMegajoule French program. The proposed design is based on a force control strategy. Forces are generated by specific designed electromechanical actuators and transmitted to a Zerodur mirror through an annular soft pad. This pad is optimized to filter high frequency ripple generated by the spatial sampling of the efforts at the back of the mirror in order to decrease the needed number of actuators, and thus the cost of the deformable mirror, a specific optimization method has been developed and is applied to determine the best actuator pattern fitted on the wavefront aberrations to be corrected. Analysis, calculations, finite elements models, preliminary test and validations on breadboard models have shown that the proposed design in compliant with the functional and operation requirements. A design description and the main justifications, as the guidelines of mirror integration are given in this paper. Due to the simplicity of the concept and the use of validate and mastered technologies at SFIM Industries and REOSC, the design present a good reliability. Furthermore, a complete and very easy to work maintainability is favored by this deformable mirror definition. Each parts of the system is easily removable and replaceable on the laser line without carrying out a heavy procedure and complex tools.
Wavefront control of solid state lasers using an optically addressed light valve in an adaptive optics loop and applications to ultra-intense pulses
Jean-Christophe Chanteloup, Arnold Migus, Brigitte Loiseaux, et al.
High energy intense lasers are difficult to focus close the diffraction limit because of phase-front aberrations induced in the optics and amplifiers. This may be an issue in applications where the pulses need to be focused near the diffraction limit such as in ultra-intense femtosecond laser-matter interactions. We describe here a new technique that we have implemented and which significantly improves the focal spot quality by correction the wave front even from highly distorted beam. This correction method is based on an adaptive optical technique using an optically addressed light valve and an achromatic three wave lateral sheering interferometer. Results with strongly aberrated beams focused close to the diffraction limit are presented as well as with fluence as high as 100 mJ/cm2.
Target Chambers and Vacuum Barriers
icon_mobile_dropdown
LMJ target area design and engineering physics inside the LMJ target chamber
Daniel Schirmann, Luc Bianchi, Roger Courchinoux, et al.
The LMJ target chamber is designed to withstand high fusion energy produced by 20 MJ yield shots and 600 nm yield shots per year on a large variety of targets. We show how the target chamber and its equipment have to be protected from damaging by 14 MeV neutrons, high x-ray fluences, debris and shrapnel. A first wall made of louvers is required to prevent material ablated by x-rays form reaching the debris shields and reducing their live time. Constraints on the target design is also considered to limit the sources of vaporized or ionized mass and shrapnel coming directly from the target. The use of pre-shield for large mass targets is suggested to meet the requirement on the frequency cleaning of the debris shields.
Management of unconverted light for the National Ignition Facility target chamber
Pamela K. Whitman, Alan K. Burnham, Mary A. Norton, et al.
The NIF target chamber beam dumps must survive high x-ray, laser, ion, and shrapnel exposures without excessive generation of vapors or particulate that will contaminate the final optics debris shields, thereby making the debris shields susceptible to subsequent laser damage. The beam dumps also must be compatible with attaining and maintaining the required target chamber vacuum and must not activate significantly under high neutron fluxes. Finally, they must be developed, fabricated, and maintained for a reasonable cost. The primary challenge for the beam dump is to survive up to 20 J/cm2 of 1 micrometers light and 1-2 J/cm2 of nominally 200-350 eV blackbody temperature x-rays. Additional threats include target shrapnel, and other contamination issues. Designs which have been evaluated include louvered hot-pressed boron carbide or stainless steel panels, in some cases covered with transparent Teflon film, and various combinations of inexpensive low thermal expansion glasses backed by inexpensive absorbing glass.
Constraints on target chamber first wall and target designs that will enable NIF debris shields to survive
Alan K. Burnham, Michel Gerassimenko, J. M. Scott, et al.
The NIF target chamber interior materials and target designs themselves have to be compatible with survival of the final- optics debris shields. To meet the planned maintenance and refinishing rate, the contamination of the debris shields cannot exceed about 1 nm equivalent thickness per shot of total material. This implies that he target mass must be limited to no more than 1 gram and the ablated mass released to the chamber from all other components must not exceed 3 grams. In addition, the targets themselves must either completely vaporize or send any minor amounts of shrapnel towards the chamber waist to prevent excessive catering of the debris shields. The constraints on the first-wall debris will remobilize at a rate fast enough to require cleaning every 3 weeks, about three times more frequent than possible with planned robotics. Furthermore, a comparison of ablatants from B4C and stainless-steel louvers suggest that remobilization of target debris by x-rays will be greater than that of the base material in both cases, thereby reducing the performance advantage of clean B4C over much cheaper stainless steel. Neutronics calculations indicate that activation of thin Ni-free stainless steel is not a significant source of maintenance personnel radiation dose. Consequently, the most attractive first wall design consists of stainless-steel louvers. Evaluation of various unconverted-light beam dump designs indicates that stainless steel louvers generate no more debris than other matrices, so one single design can serve as both first wall and beam dumps, eliminating beam steering restrictions caused by size and location of the beam dumps. One reservation is that the allowable contamination rate of the debris shield is not yet completely understood. Consequently, it is likely that either a protruding beam tube, a rapid post-shot gas purge of the final optics assembly, or thin polymeric pre-shield will be required to prevent low-velocity contamination from reaching the debris shield.
Fail-safe design for square vacuum-barrier windows
Tayyab I. Suratwala, Jack H. Campbell, William A. Steele, et al.
Laser-induced damage on the tensile side of vacuum-barrier fused silica optics can result in catastrophic fracture. This fracture can lead to two possible modes of failure: a benign failure resulting in a slow air leak into the vacuum chamber or an implosion. In previous work, we measured fracture in round vacuum windows and lenses and proposed a 'fail-safe' design that would insure the benign failure mode by fracturing into only two parts, thus eliminating the possibility of implosion. In this paper we extend the previous work to include square vacuum-barrier windows and lenses.
Laser Glass Manufacturing and Performance
icon_mobile_dropdown
Energy storage efficiency and small signal gain measurements in Nd:phosphate amplifiers
Claude Gamache, Denise Descamps, Daniel Husson, et al.
Neodynium doped phosphate glass laser has been chosen as the gain medium of large laser facilities devoted to ignition such as NIF or LMJ [1] because it can be cast into a great variety of form and size with excellent homogeneity at relatively low cost. In this respect Megajoules class laser design relies on previous experimental and numerical studies on energy storage and gain capabilities. These parameters are particularly needed as inputs in the modeling of the amplifying stages in order to dimension and caracterize laser systems. Although there is a abondant litterature on Nd:phosphate glass, we found it necessary to make absolute value measurements. We have so developed different experimental scheme based on amplifiers pumped by a laser [2, 3] and by flashlamps [4]. In parallel, we have developed numerical models which take into account the exact configuration of the amplifiers. The main result of these different experiments is that the use of an adjustable parameter (so-called arbitrary quantum yield) is always needed to obtain a good agreement between experimental and numerical results on gain measurements. Whatever method of pumping was used, we had to introduce a quantum yield close to 0.8 [2, 3, 4] which encompasses all unknown loss mechanisms and all hypothesis assumed in the models (section 2). We have analysed the different energy pathways and we put into evidence loss mechanisms correlated to spectroscopic properties of neodymium ions in host glasses. These mechanisms are energy transfer up-conversion [5]and Excited State Absorption (ESA) of both pump and laser radiation (amplified pulse or florescence from the upper laser state) [6, 7]. The two latter processes are correlated to a high population of excitated states, but all of them can significantly affect the energy storage efficiency and the gain capability of these laser systems by depopulating the upper laser level or reducing the state lifetime. These mechanisms have already been reported for some materials such as neodymium doped fluorite host medium [8] or crystals [7, 9]. Surprinsingly enough few works have been published in case of neodymium doped glasses [10]. Our goal in this work, was to identify and quantify each spectroscopic process that could exist in Nd :doped glasses. We report here spectroscopic experiments and the use of absolute characteristic values as new inputs in the modeling of gain capabilities of a Nd :phosphate amplifying stage (section 3). However part of the losses is also due to non spectroscopic factors such as the pumping geometry. For instance, in case of slab transversally-pumped, not taking into account Fresnel reflectivity could be at the origin of strong losses. This is the reason why we have developed in collaboration with the Lawrence Livermore National Laboratory (LLNL), a full 3D ray trace code that we have used to simulate gain measurement (section 4) [11]. Note that this study is of larger interest for NIF-LMJ program since rare earth doped-glasses appear also promising for future broad-band solid-state systems which would be diode-pumped.
Yb3+ P2O5-RO phosphate laser glasses
Lili Hu, Shixun Dai, Zhonghong Jiang
P2O5-RO-Yb2O3 phosphate laser glasses were reported in this work. Relationship among the integrated absorption area, fluorescent behaviors of Yb3+ ion and glass components in P2O5-RO-Yb2O3 glasses was examined. Calculated by Fuchtbauer-Ladenberg equation, the stimulated emission cross section of Yb3+ P2O5-RO glass can be larger than 1.5 X 10-20 cm2. The fluorescent lifetime of P2O5-RO-Yb2O3 is usually larger than 1msec. A comprising relationship between stimulated emission cross section and fluorescent lifetime was found in Yb3+ P2O5-RO glass.
Yb3+ borate laser glasses containing high-valency cations
Tetsuro Izumitani, Lili Hu, Shixun Dai, et al.
A new kind of Yb3+ borate laser glass containing high valency cations was reported in this work. B2O3-ZnO and B2O3-BaO glasses were chosen as the base glasses of Yb$3+)-BaO glasses were chosen as the base glasses of Yb3+ ions. Yb3+ ion has a large integrated absorption area in the former and longer fluorescent lifetime in the latter. The effect of Al3+, La3+, Ti3+, Zr4+, Nb4+ Ta5+, W6+ high valency cations on the absorption and fluorescent behaviors of Ba2O3-ZnO- RmOn-Yb2O3 and B2O3-BaO-RmOn-Yb2O3 glasses was examined. Some Yb3+ borate laser glasses with high cross section for stimulated emissions, good stability, good stability against devitrification and lower non-linear refractive index were presented.
Development of continuous glass melting for production of Nd-doped phosphate glasses for the NIF and LMJ laser systems
Jack H. Campbell, M. J. McLean, Ruth A. Hawley-Fedder, et al.
The NIF and LMJ laser systems require about 3380 and 4752 Nd-doped laser glass slabs, respectively. Continuous laser glass melting and forming will be used for the first time to manufacture these slabs. Two vendors have been chosen to produce the glass: Hoya Corporation and Schott Glass Technologies. The laser glass melting systems that each of these two vendors have designed, built and tested are arguably the most advanced in the world. Production of the laser glass will begin on a pilot scale in the fall of 1998.
Large-Optics Fabrication and Characterization
icon_mobile_dropdown
Laser ablative figuring of optical elements for phase control
Takahisa Jitsuno, Keiu Tokumura, Masahiro Nakatsuka
A new scheme for the control of the phase of the optical component using laser ablation has been developed. The surface shape of optical plastic coated on a glass plate is ablated using 193 nm laser light to control the transmission wave front. The surface shape is monitored fluence is about 45 mJ/cm2, and the ablation depth per pulse is about 0.01 micrometers per pulse. Flat and spherical surfaces are generated using this method to reach the wave front distortion of 0.2 (lambda) . Micro-lenses for the control of the beam profile of the laser diode has been tried.
Construction of the prototype of an optical system for measurement of small wavefront distortion of laser radiation in optical elements: proposal for the ISTC project
A. Potemkin, Anatoly Malshakov, Alexandr Makarov, et al.
Technique testing of quality the transparent component of optical devices with application of self-focusing effect is offered. In measurement of small wavefront distortions a method of comparison of laser beam parameters before and after passage of a tested optical element is used. With the purpose of increase of sensitivity it is offered for overcoming negative diffraction action to use self-focusing effect of probe beam. Application of self-focusing effect allows to reach sensitivity no less than (lambda) /600 and in future up to (lambda) /3000. On simple samples experimental checks of a method are made.