This paper gives an overview of recent soft x-ray laser research at Colorado State University. Progress related to capillary discharge source development includes the observation of emission from the 13.2-nm laser line of Nickel-like Cd in a plasma column generated by a high power capillary discharge. This result suggests it might be possible to extend capillary discharge lasers to significantly shorter wavelengths. In another approach to the generation of coherent soft x-ray radiation we analyzed the possibility of amplifying high order harmonic pulses in a discharge-pumped amplifier. The study of the already well- characterized 46.9-nm Ne-like Ar laser was extended with new spatial coherence and laser wavefront measurements, in work conducted in collaboration with U. California Berkeley and U. of Paris-Sud groups. In the field of applications, we have extended our previous results of plasma interferometry with a tabletop laser to plasma densities up to 0.9 x 10²¹ cm^-3. Sequences of soft x-ray laser interferograms of plasmas generated by a Nd-YAG laser at intensities between 1 x 10¹¹ W cm^-2 and 7 x 10¹² W cm^-2 show the development and evolution of a concave electron density profile. The detailed mapping of this phenomenon with soft x-ray interferometry exemplifies the usefulness of compact soft x-ray lasers in increasing the understanding of high density plasmas.

Analysis of the capillary discharge plasma as a possible preformed active medium of the collisionally pumped soft X-ray lasers (XRLs) working within the transient inversion scheme is presented. Temporal dependence of the plasma column resistance is compared with a 1D-model used in the numerical simulations of the plasma dynamics. Numerical analysis of the laser absorption determines the pump parameter range for efficient excitation of the longitudinally pumped hybrid XRLs.

In this work we report our numerical modeling results of laser-generated transient inversion and capillary discharge X-ray lasers. In the search for more efficient X-ray lasers we look closely at other approaches in conjunction with experiments at LLNL. In the search for improved X-ray lasers we perform modeling and experimental investigations of low density targets including gas puff targets. We have found the importance of plasma kinetics in transient X-ray lasers by expanding the physical model beyond hydrodynamics approach with Particle In Cell (PIC) and Fokker-Planck codes. The evidence of the Langdon effect was inferred from the recent experimental data obtained with the Ni-like Pd X-ray laser. We continue modeling different kinds of capillary discharge plasma configurations directed toward shorter wavelength X-ray lasers, plasma diagnostics and other applications.

We report in this article the experimental and numerical tools, developed at the LSAI, for a complete characterization of an x-ray laser (XRL) beam. First, a Michelson interferometer has been used to realize a Fourrier transform spectroscopy experiment. A full comprehension of the measured linewidth requires a comparison of the XRL beam amplification in the plasma to raytrace simulation. Results of transient pumping XRL simulations are presented in this article. The last section is dedicated to a description of the XUV Shack-Hartmann wavefront sensor we have developed, and to the study of the capillary discharge XRL beam.

We report the observation of line emission at 13.2 nm from the 4d ¹S₀ -4p ¹P₁ laser transition of Ni- like Cd in a plasma column created by a high-power capillary discharge. Time-resolved soft x-ray pinhole images of the plasma column show that the plasma column maintains good symmetry up to the time of maximum compression. The observed dynamics of the plasma column is compared with model simulations. These results advance the possibility of observing laser amplification in Ni-like Cd at 13.2 nm and in other short-wavelength Nickel-like ion transitions in a discharge-created plasma.

Soft x-ray lasers have become practical light sources for applications such as interferometric diagnosis of plasmas, the measurement of optical constants, and the characterization of EUV optics. For many potential applications involving light-matter interaction and nonlinear physics, the achievable light intensity is a critical parameter. High intensity could be realized by focusing high pulse energy light with good beam quality. Recent progress at Colorado State University has realized milli-joule level pulse energy at 46.9 nm from a capillary discharge-pumped tabletop soft x-ray laser. Direct measurements of the spatial coherence of the laser beam using a two-pinhole interference method have shown very high spatial coherence of the beam. These results imply the possibility of achieving very high intensity in the soft x-ray region. In this paper we report the results of a focusing experiment conducted with the laser mentioned above and designed to measure the focusability of the beam. The spatial profile of the focused beam is measured with knife-edge scanning technique and characterized with an M-squared factor. The results suggest that 10¹³ W/cm² intensity is achievable with modestly tight focusing.

The effect of the transverse-direction electrical fields on the stability and dynamics of a capillary discharge Z-pinch, at conditions for which soft x-ray lasing in Ne-like Ar has been demonstrated, is studied. It is shown that the transverse electrical fields of the sliding surface discharge provide the instability-free compression and heating of the plasma. The stable and homogeneous heating and compression allows achievement of the appropriate conditions for the soft x-ray lasing in Ne-like Ar. Numerical calculations using the MHD model of the discharge yield new predictions for dynamics and stability of the plasma collapse in the presence of the transverse electrical fields and explain details of experimental observations without artificial adjustments.

We have observed lasing on Ne-like 3s-3p line from titanium (32.4 nm), Ni-like 4p-4d line from silver (13.9 nm) and tin (11.9 nm) with the transient collisional excitation (TCE) scheme that uses combinations of a long pre-pulse (approximately ns) and a short main pulse (approximately ps) or a short pre-pulse (approximately ps) and a short main pulse (approximately ps). A gain coefficient of 24cm^-1 have been measured for plasma length up to 4mm with silver slab targets and 14cm^-1 up to 6 mm with tin slab targets. We have installed a step mirror in the focusing system to generate traveling wave on the target. The traveling speed on the target is measured to be 3.08 cm/s and very close to the traveling speed of light. The traveling wave system improves the gain coefficient to 35cm^-1 from 24cm^-1 for Ni-like Ag and to 30cm^-1 from 14cm^-1 for Ni-like Sn. The strong gain saturation has been observed for the Ni-like Ag and Ni-like Sn. The output energy of the N-like Sn x-ray laser is 20 (mu) J. Spatial beam profiles of propagating x-ray lasers through gain plasma have been measured and are indicating localization of very high gain area and x-ray laser refraction.

The development of the transient collisional excitation x-ray laser scheme using tabletop laser systems with multiple pulse capability has progressed rapidly in the last three years. The high small-signal gain and strong x-ray output have been demonstrated for laser drive energies of typically less than 10 J. We report recent x-ray laser experiments on the Lawrence Livermore National Laboratory (LLNL) Compact Multipulse Terawatt (COMET) tabletop facility using this technique. In particular, the saturated output from the Ni-like Pd ion 4d - 4p x-ray laser at 146.8 angstrom has been well characterized and has potential towards a useable x-ray source in a number of applications. One important application of a short wavelength x-ray laser beam with picosecond pulse duration is the study of a high density laser-produced plasma. We report the implementation of a Mach-Zehnder type interferometer using diffraction grating optics as beam splitters designed for the Ni-like Pd laser and show results from probing a 600 ps heated plasma. In addition, gas puff targets are investigated as an x-ray laser gain medium and we report results of strong lasing on the n equals 3 - 3 transitions of Ne-like Ar.

This paper summarizes our recent progress achieved in the characterization and understanding of the Ni-like Ag transient x-ray laser pumped under traveling wave irradiation. At the Rutherford Laboratory CPA laser facility, we measured the temporal history of the 13.9 nm laser pulse with a high-resolution streak camera. A very short, approximately 2 ps x-ray laser pulse was directly demonstrated for the first time. More recently we carried out an experiment at the LULI CPA laser facility. Several diagnostics that recorded the plasma emission at the XRL wavelength or in the keV range indicate the presence of small-scale spatial structures in the emitting XRL source. Single-shot Fresnel interferograms at 13.9 nm were successfully obtained with a good fringe visibility. Strong lasing was also observed on the Ni-like 4f-4d line at 16 nm.

Experimental investigations on the conditions to achieve transient gain in neon-like Ti and nickel-like molybdenum XUV laser pumped by a 10-Hz sub-Joule femtosecond laser are presented. The 4d-4p (J equals 0-1) (lambda) equals 18.9 nm and 4f-4d (J equals 1-1) (lambda) equals 22.6 nm lines in Ni-like Mo as well as the 3p-3s (J equals 0-1) (lambda) equals 32.6-nm line in neon-like titanium have been observed. The Ni-like laser lines show a threshold behavior with respect to the pump irradiance as they appear only above 10¹⁵ W/cm². Simulation for the fs-laser pumped Ni-like Mo XUV laser are also presented.

We report on the results of a series of experiments aimed at the full characterization of gain-saturated nickel-like soft x-ray lasers at wavelengths between 14.7 nm and 12.0 nm. The experiments include measurements of the gain-length product, output energy, near-field and far-field intensity distributions, and coherence properties of Pd and Sn x-ray lasers. Most of the experiments were done at a drive irradiance of 10 TW/cm², using a 1054-nm drive energy of 30 J in 100-ps pulses.

The technique of using a nsec pulse to preform and ionize the plasma followed by a psec pulse to heat the plasma has enabled low-Z nickel-like ions to achieve saturated output when driven by small lasers with less than ten joules of energy. We model experiments done using the COMET laser at LLNL and the P102 laser at Limeil to produce Ni-like Pd and Ag lasers. The COMET experiments use a 2 J, 600 ps prepulse followed 700 psec later by a 6 J, 6 psec drive pulse in a 1.6-cm long line focus. The P102 experiments used a somewhat larger energy and were able to use different combinations of frequency doubled light for both the prepulse and short pulse drive. The LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the CRETIN code, which then does the kinetics calculations to determine the gain. The temporal and spatial evolution of the plasmas are studied both with and without radiation transport included to understand the role of the self photopumping process on the gain of the Ni-like 4f implies 4D laser lines as well as the gain of the usual collisionally driven Ni-like 4d implies 4p laser lines. In particular we study why the 4f implies 4d line lases well only when frequency doubled light is used with the prepulse in the P102 experiments. Experimental results are presented for Ni-like Pd including two- dimensional near-field and far-field images.

Recent high temporal resolution Ni-like x-ray laser experiments have yielded important insights into the output characteristics of picosecond pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics which are critical to the gain generation within the x-ray laser medium. A theoretical study of the Ni-like Silver x-ray laser has therefore been undertaken to compliment our experimental results, in an attempt to further our understanding of the processes at play in yielding the observed x-ray laser output. Preliminary findings are presented within this paper.

Large amplification at 25.5 nm in neon-like iron has been demonstrated in experiments where prepulses are used. We show that the interaction between the x-ray laser beam and the amplifying medium must be taken into account in a reliable modeling of the saturation regime. Two approaches for intensity calculations are presented in this contribution. The first one combines the radiative transfer equation and the population rate equations. This approach is fully consistent, in the sense that beam amplification and population kinetics are treated simultaneously. A formalism based on a paraxial Maxwell-Bloch approach is presented. The Maxwell-Bloch calculations give the variation with length of intensity, local gain...Moreover, in the small-signal regime, it is possible to define an effective gain which is comparable to the measured gain. The second approach is based on a raytrace calculation where the saturation effect has been introduced empirically. The two codes need the electron density and the electron and ion temperatures as inputs. These quantities are given by the hydrocode EHYBRID. The two approaches give similar results.

A Ne-like TCE (transient collisional excitation) x-ray laser at 19.6 nm (J equals 0 implied by 1, 3p implied by 3s) was investigated numerically using a sophistic hydrodynamic code for a 100-micrometers thick Ge planar target irradiated by a nano- second prepulse followed by a picosecond main optical laser pulse. The simulations indicate that for a given peak intensity, the main pulse has an optimal duration to generate the maximum effective gain. An effective gain as high as 200 cm^-1 was obtained for the optimized drive pulse configuration.

A high peak current, low emittance, short pulse electron beam can produce intense, laser-like radiation in a single pass through a long periodic magnetic structure. The construction of such free-electron lasers (FELs) based on self-amplified spontaneous emission (SASE) has become feasible by recent advances in accelerator technologies. Since SASE FELs do not require any optical components they are promising sources for the generation of intense, sub- picosecond laser pulses which are continuously tunable over a wide wavelength range in the vacuum ultraviolet (VUV) and X-ray region. In the first phase of the VUV-FEL (phase I) at the TESLA Test Facility at DESY, SASE was achieved for the first time in the VUV at wavelengths between 80 and 180 nm. The concept of the VUV FEL at DESY and first experimental results are presented. The second phase of the TESLA Test Facility (phase II), which includes an increase of the electron beam energy to 1 GeV, aims at the construction of a SASE FEL operating in the soft X-ray region. An overview of the current status and the activities toward a soft X-ray FEL user facility is given.

We propose a scheme for bright, sub-100-femtosecond x-ray radiation generation using small-angle Thomson scattering. Coupling high-brightness electron bunches with high-power ultrafast laser pulses, radiation with photon energies of 8- to 40-keV can be generated with pulse duration comparable to that of the incoming laser pulse with peak spectral brightness of approximately 10²⁰ photons s^-1 mm^-2 mrad^{-2 per 10(superscript -3} bandwidth, close to those of the third-generation synchrotron light sources. The scheme is scalable to shorter pulse duration and high peak brightness, depending on the performance of the laser system.

High-intensity quasi-monochromatic x-ray irradiation from the linear plasma target is described. The plasma x-ray generator employs a high-voltage power supply, a low- impedance coaxial transmission line, a high-voltage condenser with a capacity of about 200 nF, a turbo-molecular pump, a thyristor pulse generator as a trigger device, and a new flash x-ray tube. The high-voltage main condenser is charged up to 60 kV by the power supply, and the electric charges in the condenser are discharged to the tube after triggering the cathode electrode. The flash x-rays are then produced. The x-ray tube is of a demountable triode that is connected to the turbo molecular pump with a pressure of approximately 1 mPa. As the electron flows from the cathode electrode are roughly converged to the nickel target by the electric field in the tube, the plasma x-ray source, which consists of metal ions and electrons, forms by the target evaporating. Both the tube voltage and current displayed damped oscillations, and their peak values increased according to increases in the charging voltage. In the present work, the peak tube voltage was almost equal to the initial charging voltage of the main condenser, and the peak current was about 29 kA with a charging voltage of 60 kV. When the charging voltage was increased, the linear plasma x-ray source formed, and the characteristic x-ray intensities of K-series lines increased. The quasi- monochromatic radiography was performed by a new film-less computed radiography system.

Effects of laser polarization were studied on behaviors of fast electrons produced from an aluminum target irradiated by obliquely incident laser pulses at 8x10¹⁵ W/cm². Jet emission of outgoing fast electrons collimated in the polarization direction was observed for the s-polarized laser irradiation, whereas for the p-polarized irradiation, very directional emission of outgoing fast electrons was found close to the normal direction of the target. The behaviors of in-going fast electrons into the target for s- and p-polarized irradiation were also investigated by observing x-ray Bremsstrahlung radiation at the backside of the target.

Intense soft x-ray radiation is observed when a high temperature laser produced plasma collides with a solid surface (wall). The laser plasma is produced by Nd:YAG laser radiation (0.53 micrometers ; 200 mJ; 3 ns; 5x10¹² w/cm² focused onto a Mg target. The wall is placed in the path of the plasma expansion. Two experimental techniques are used. 1) One-dimensional x-ray images of the plasma-wall gap formed by a slit are observed using an absolutely calibrated CCD linear array. 2) X-ray spectra of [H]- and [He]-like Mg ions are recorded using a crystal focusing x-ray von Hamos spectrometer. At a measured electron temperature of T_e equals 195 eV, emission of [He]-like ions Mg XI gives the main contribution to the observed x-ray intensity. The spatial structure and the intensity of the radiation are studied at various laser plasma-wall distances (r₀ equals 0.3 - 3.0 mm). Intense radiation near the wall is caused by an electron density jump in a shockwave formed in front of the wall. At small wall distances r₀<<v(tau) (v is the expansion velocity, tau is the laser pulse duration) the radiation is caused by excitation processes, and at distances r₀<0.3 mm the near wall intensity exceeds the emission from the laser plasma itself. At distances r₀$GTR$GTRv(tau) the radiation causes recombination processes. These results are important for developing collision and recombination schemes for x-ray lasers in colliding plasmas.

Efficient harmonic generation was achieved using a long argon gas jet pumped by intense 28 fs laser pulses. By balancing diffraction and plasma defocusing with nonlinear self-focusing, we could achieve a new optimum condition for efficient high-order harmonic generation. Under condition of a self-guided laser pulse, generated high-order harmonic signal was strong enough to saturate an x-ray CCD in one laser shot of a few mJ.

We demonstrated the generation of highly-coherent high-order harmonics and their energy scaling in the soft x-ray region using 30-fs Ti:sapphire laser pulses. By adjusting the gas density and the pump laser focusing, an output energy as high as 0.5 (mu) J was achieved with good spatial beam quality having a beam divergence of 2 mrad at the 27th harmonic (30 nm).

We report recent investigations on collisional Optical-Field Ionization soft x-ray lasers. The amplifying medium is generated by focusing a high energy circularly polarized, 35 fs 10 Hz Ti: sapphire laser system in a few mm cell filled with gas (xenon or krypton). Using xenon, a gain of 67 cm^-1 on the 4d⁹5p-4d⁹5d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 10⁹ photons per pulse. Using krypton, a large amplification of the 3d⁹4d-3d⁹4p line at 32.8 nm has been observed for the first time. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

We proposed a method to generate highly spatial coherent x-ray laser, in which high order harmonics was used as a seed light of a laser-produced x-ray amplifier. In this case, the intensity and the spatial coherence of the output x-ray depended on the harmonic conversion efficiency and the spectral and spatial coupling efficiencies between the harmonics and the lasing line. Based on the present x-ray lasers using a transient collisional-excitation (TCE) scheme, we investigated the values of these efficiencies, which were needed to realize a high spatial coherence. For this purpose we constructed a Ti:Sapphire laser system in which the central wavelength and the spectral bandwidth were tunable, and we conducted a preliminary experiment. The neon-like Ti x-ray laser at a wavelength of 32.4 nm was taken as an example, and harmonics at the same wavelength was generated using Ar gas target under the conditions that the central wavelength of 810 nm and the pulse duration of 1 ps. The experimental result showed that the characteristics of the harmonics were good enough to use as a seed light of x-ray lasers.

We present a review of new progress performed in several laboratories (Laboratoire pour l'Utilisation des lasers Intenses, Rutherford Appleton Laboratory, Prague Asterix Laser System, Institute of Laser Engineering, Laboratoire d'Optique Appliquee). Concerning the realization of x-ray lasers sources, using different laser pumping techniques (600 ps, 100 ps, ns/ps, OFI) and the optimization of their optical properties, using curved and plane half-cavity mirrors. In parallel of these developments, we present the main results obtained with x-ray laser in interferometry applications. These studies concern on the one hand the Michelson interferometry with an x-ray laser emitting at 13.9 nm (recently realized at LULI), and on the other hand the Fresnel bi-mirror with an x-ray laser emitting at 21.2 nm (recently realized at PALS).

We report an extension of previous tabletop soft x-ray laser interferometry work to plasma densities approaching the critical density. The evolution of line-focus and spot-focus plasmas created with Nd-YAG laser intensities of 0.1 and 7.0 TW/cm² respectively were studied utilizing a 46.9-nm capillary discharge laser with a diffraction grating interferometer. In the latter case, the electron density was mapped to values up to 0.9x10²¹ cm^-3 (90% of the critical density for the lambda equals 1.06 micrometers pump laser). The interferograms show the development of concave electron density profiles with a minimum on axis and pronounced side lobes. Hydrodynamic model simulations show that the concave profile is the result of the hydrodynamic and radiation effects that enlarge the ablated target area. The measurements exemplify how soft x-ray lasers can be used to probe high density plasmas for the validation of hydrodynamic codes.

Processes going on at elevated temperatures between Sc and Si layers in Sc/Si coatings are studied by X-ray scattering and cross-sectional transmission electron microscopy. It is shown that the W layers of 0.5-0.8 nm placed at Sc-Si interfaces form effective barriers preventing the penetration of Si into Sc. The effects of Si-Sc diffusion and W-barriers on the reflectivity of coatings are calculated in good agreement with experimental results. Presented measurements show that the Sc/W/Si/W multilayers with the period of 20.5 nm fabricated by dc-magnetron sputtering possess thermal stability up to 250 C and the normal incidence reflectivity of 24% at wavelengths about 40 nm.

The Gesellschaft fuer Schwerionenforschung (GSI, Society for Heavy Ion Research) is currently the leading facility in the production of radioactive isotopes. Nuclear properties like charge radii, spin, and magnetic moments of exotic nuclei provide important data for testing of nuclear models. These properties are usually accessed by laser spectroscopy, which requires photon energies of around 100 eV in the case of lithium-like ions. We propose to use a transient gain X-ray laser (XRL) at the experimental storage ring (ESR) to perform this kind of spectroscopy. In this article we describe the planned experiments and give an overview of the current construction at GSI.

In this work we demonstrate a soft x-ray laser with neon- like argon ions using a gas puff target irradiated with a combination of long 600 ps and short 6 ps high-power laser pulses with a total of 10 J energy. The gas puff target was formed by pulsed injection of gas from a high-pressure solenoid valve through a nozzle in the form of a narrow slit. The target was irradiated in a traveling-wave excitation geometry. Lasing was observed on the 3p ¹S₀ implies 3s ¹P₁ transition at 46.9 nm and the 3d ¹P₁ implies 3p ¹P₁ transition at 45.1 nm. Gain of 11 cm^-1 was measured on these transitions for targets up to 0.9 cm long.