The absorption of intense laser light in plasmas with a long density scale length can be influenced by many different processes, ranging from collisional absorption to laser-driven instabilities. The mix of these coupling processes depends strongly on the collisionality of the plasma. Optimum coupling is obtained by the use of short wavelength laser light.

Intense, short duration laser pulses can be focused onto solids to produce high temperature, non-equilibrium, solid density plasmas. Electrons are heated within an optical skin depth in the target with little vaporization of the target during the laser pulse.

At high laser field intensities, mixing of many levels in molecules occurs as a result of the delocalization of the eigenstates of the molecule-field system, over the unperturbed states. These new states are called dressed states. A measure of this mixing or delocalization is shown here by calculating the dressed states for certain analytical models. Pulse propagation is examined for these model cases in order to examine the validity of the two-level approximation for which soliton propagation is known to occur.

Intense light fields can induce elastic and inelastic electron scattering, as well as above-threshold ionization in atoms. This paper reviews some recent atomic photoionization experiments that demonstrate the connection between these new effects and the quantum mechanics of wiggling electrons.

Studies of the spectral and spatial properties of propagation of intense (- 10¹⁶ Wi CM² ) subpicosecond 248 nm radiation in hydrogen plasmas for electron densities in the 101'- 10" cm^-3 range are reported . The influence of the ponderomotive potential on the character of the propagation is considered.

The yield of singly- and multiply- charged ions of krypton and xenon is presented as a function of laser intensity and frequency. The measurements were performed using the second harmonic output of a well-characterized, tunable picosecond dye laser in the range 285 to 310 nm at laser intensities from lx10¹² to 10¹⁴ W/cm². Enhancement of the Kr⁺ yield by two orders of magnitude by three-photon resonant, four-photon ionization is observed in the vicinity of the 4d'[5/2]₃ and the 4d[3/2)₁ intermediate states. A model incorporating line shifts and widths scaling linearly with intensity is in good agreement with the experimental results.

There are two principal approaches to inertial confinement fusion (ICF): direct and indirect drive. In the indirect or "hohlraum" approach to fusion, pursued primarily at the national weapons laboratories at Los Alamos and Livermore, the driver-beam energy is absorbed and converted to x rays by a radiation case made of a high-atomic-weight element, the x rays are then used to drive the target implosion. In the alternative direct-drive approach, a short-wavelength high-intensity laser pulse directly illuminates a bare, spherical target. The implosion of capsules by direct laser drive may be more efficient than indirect drive. The University of Rochester's Laboratory for Laser Energetics (LLE) is the primary focus in the United States used to investigate directly driven spherical target experiments. The LLE research program in ICF has two principal objectives: (1) to demonstrate the scientific feasibility of the direct-drive concept for inertial fusion, and (2) to investigate the fundamental physics of the interaction of intense laser radiation with matter. The first of these objectives is addressed by a program aimed at the demonstration of high compression (peak DT densities in the range of 10-20 g/cm³) with short-wavelength (0.35-μm) laser radiation and direct-drive targets. The primary elements of this program include (a) development and implementation of high-density diagnostic systems on the OMEGA facility; (b) theoretical simulations of laser-driven target implosions using one-dimensional and two-dimensional hydrocodes; (c) the achievement of a high degree of drive uniformity (±5%) on the 24-beam OMEGA laser; and (d) the development and implementation of solid and liquid fuel layer targets. The second objective (laser-matter interaction physics) is addressed by an experimental and theoretical effort including (a) laser-target coupling studies; (b) studies of parametric instabilities occurring in the coronal plasma; (c) investigations of electron-thermal energy transport; and (d) research on the hydrodynamic behavior of ablatively driven targets , including the study of Rayleigh-Taylor instabilities. Even though the specific research topics covered under this part of the program are motivated by the primary mission, the demonstration of the feasibility of direct-drive inertial fusion, the fundamental laser-matter interaction physics issues are also important to hohlraum driven targets.

Linear particle accelerators can be made 100-1000 times shorter than conventional ones if the electric field of an electron plasma wave is used instead of that of a microwave cavity. Intense plasma waves can be excited by particle beams or laser beams. The most advanced concept, the Beat-wave Accelerator, has been studied extensively in theory, simulation, and experiment. Small-scale experiments have already demonstrated fields of order 1GeV/m, the existence of accelerated electrons, the effects of com-peting processes, and the nature of wave saturation. A proof-of-principle experiment employing a subnanosecond CO₂ laser is in progress.

We report on a series of experiments aimed at the demonstration of controlled acceleration of externally injected test particles (electrons) by a relativistic plasma wave. The plasma wave is excited resonantly by beating two co-propagating laser beams at the plasma frequency. The first part of the paper describes the electron linear accelerator and the beam transport system, as well as the particle detection system. The electron macropulse energy spectrum and beam emittance are measured and are shown to be consistent with the requirements of the experiment. The electron beam is passed through a 0-pinch plasma. Although Raman scattering of the incident CO₂ laser has shown the plasma to be reproducible in density to within 15% from one shot to the next at peak compression, direct evidence of localized density inhomogeneities due to trapped magnetic fields is found via the deflection of the injected electron beam. The trapped fields persist even as the plasma disassembles following the compression up to the second B=0 point in the 0-pinch cycle.

The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, geophysics and astrophysics. High-pressure properties are studied in the laboratory using static and dynamic techniques. The two differ drastically in the methods of generating and measuring pressure and in the fundamentally different nature of the final compressed state. This article covers a very broad range of conditions, intended to present an overview of important recent developments and to emphasize the behavior of materials and the kinds of properties now being studied.

The spacing of layers in a sputtered, multilayer structure was rapidly and permanently increased by heating with a nanosecond laser pulse. During laser heating, the Bragg angle decreased and the peak reflectivity increased for diffraction of soft x-ray radiation at 4.4 nm. Measurements were made using a time and space resolving x-ray streak camera detector.

The behaviour of laser-driven shock waves in impedance-match, multilayered targets has been explored in hydrodynamic simulations as well as experimental measurements, taking aluminum-gold as the sample target. To achieve maximum pressure enhancement in the gold layer, one needs to consider not only the aluminum layer thickness but also the thickness of the gold layer. Such optimization was demonstrated in the numerical calculations which showed good agreement with observations.

X-ray holography offers the potential for obtaining high resolution three-dimensional images of in vitro biological microstructures. Significant progress toward this goal has been achieved with holography systems using synchrotron x-ray sources and recently spatial resolutions as small as 40 nm have been demonstrated. These experiments required x-ray exposures of an hour or longer, which makes high spatial resolution difficult to achieve in live biological specimens because of blurring of the image. This blurring is caused by specimen motion and will prohibit the imaging of dynamical processes within the specimen. A possible solution to this problem is to exploit the extremely high brightness and long coherence lengths produced by x-ray lasers and create the hologram with exposure times of less than 1 nsec. This report presents the results from an experiment in which an x-ray laser was used to produce x-ray holograms. The holography geometry used was a Gabor in-line type modified by the inclusion of a high reflectivity multi-layer x-ray mirror used as a narrow bandpass filter. The x-ray mirror had a flatness and roughness of less than X/100 (where X is the wavelength) at the x-ray laser wavelengths.

In this paper we present the first observation of ion correlation effects in a dense plasma. A range of experiments has been carried out at the SERC Rutherford Appleton Laboratory, UK, using their high power laser system (VULCAN) for shock compression and heating of thin aluminium foil targets. The plasma is produced in aluminium Ising a colliding shock technique. Short range order within the plasma is observed using the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of the aluminium K absorption edge. Densities ≈2X solid density have been measured from the EXAFS spectrum. These results are in reasonable agreement with the computer predictions.

High z materials can produce large amounts of x-rays in laser plasma interactions. The efficiency of this process depends on the average intensity and wavelength of the laser light.1 We have demonstrated that the detailed intensity distribution of the laser focal spot is also an important factor in determining the x-ray conversion efficiency. An increase in the low energy x-ray yield from gold targets was noted when induced spatial incoherence (ISI)² was used to provide a smooth spatial laser intensity distribution when compared to a nominal high powered laser focal intensity distribution.

A programme of research by UK Universities and overseas collaborators at the $ERC Central Laser Facility is directed to the development of XUV lasers based on recombining laser produced plasmas. Laser amplification has been demonstrated for recombination to hydrogenic ions C VI and F IX with H_α laser amplification at 182 and 81 Å respectively, and for lithium like ions, namely Aℓ, XI with 4f-3d and 5f-3d laser amplification at 150/154 and 106/104 Å respectively and Cℓ XV 4f-3d at 83 A. An initial study of Na-like Cu XIX has been made. Current work is looking for ways to improve the efficiency, increase the gain length and reduce the wavelength to reach the 44 Å edge of the water window. The review summarises past work and outlines current work.

Recent progress in soft x-ray laser research has been marked by significant advances in source development and x-ray laser applications. In the first area, efforts have focused on producing amplification at higher powers and shorter wavelengths. The push to shorter wavelength is closely related to efforts to demonstrate significant applications, as the utility of techniques such as x-ray laser holography l is enhanced when performed with radiation in the "water window" between the K absorption edges of carbon (43.76 Å) and oxygen (23.32 Å). In this paper, we report on recent results aimed at demonstrating gain at shorter wavelengths. The first selection will discuss measurements of amplification in Ni-like ions, where amplification has been shown at wavelengths as short as 50.26 Å in Ni-like Yb. This is 6.5 Å away from the water window threshold at 43.76 Å. We also discuss results from recent recombination experiments. These schemes have been investigated because of their possible higher efficiency due to rapid scaling to shorter wavelength with ion charge Z.

The analysis of soft x-ray emission from plasmas created by intense short-wavelength laser radiation can provide much useful information on the density, temperature and ionization distribution of the plasma. Until recently, limitations of sensitivity and the availability of suitable x-ray optical elements have restricted studies of soft x-ray emission from laser plasmas. In this paper we describe novel instrumentation which provides high sensitivity in the soft x-ray spectrum with spatial and temporal resolution in the micron and picosecond ranges respectively. These systems exploit advances made in soft x-ray optic and electro-optic technology. Their application in current studies of laser fusion, x-ray lasers, and high density atomic physics will be discussed.

The CHROMA laser facility at KMS Fusion has been used to irradiate a variety of microdot targets. These include aluminum dots and mixed bromine dots doped with K-shell (magnesium) emitters. Simultaneously time-and space-resolved K-shell and L-shell spectra have been measured and compared to dynamic model predictions. The electron density profiles are measured using holographic interferometry. Temperatures, densities, and ionization distributions are determined using K-shell and L-shell spectral techniques. Time and spatial gradients are resolved simultaneously using three diagnostics: a framing crystal x-ray spectrometer, an x-ray streaked crystal spectrometer with a spatial imaging slit, and a 4-frame holographic interferometer. Significant differences have been found between the interferometric and the model-dependent spectral measurements of plasma density. Predictions by new non-stationary L-shell models currently being developed are also presented.

The process of analyzing plasma spectroscopic data is a difficult one involving several separate, but coupled steps. We briefly describe this process and its application to several experiments being conducted at Livermore, closing with two theoretical projects being developed at LLNL to aid the spectroscopist and improve the analysis.

Experimental results from laser driven implosions of argon-and argon/krypton-filled polymer shell targets are presented. Analysis of the results is discussed with regard to the physics involved and the potential use of spectral features as density diagnostics.

Single picosecond pulses have been amplified to the terawatt level by a tabletop Nd:glass amplifier by using the technique of chirped pulse amplification (CPA). The addition of a grating stretching stage made possible the generation and amplification of 1 ns chirped pulses, while preserving a final pulse width of 1 ps. Apart from being compact and powerful, the CPA laser provides a simple way of selecting the pulse duration, a feature which should prove very useful for high power laser-matter interaction study.

Single and multi-frame gated x-ray images with time-resolution as fast as 150 psec are described. These systems are based on the gating of microchannel plates in a stripline configuration. The gating voltage comes from the avalanche breakdown of reverse biased p-n junction producing high power voltage pulses as short as 70 psec. Results from single and four frame x-ray cameras used on Nova are described. There has been much recent interest in gated x-ray cameras and spectrometers with gate times as short as 100 psec. In the ICF field, this time resolution is necessary to freeze implosions with velocities of 10⁷ cm/s and resolutions of 10 μm. There are two different techniques for gating, shuttering electro optic tubes and voltage gating of a proximity focussed device. Several schemes have been proposed for subnanosecond shuttering of electro-optical tubes. Such approaches are attractive in that several frames can in principle, be achieved, there is little problem from x rays that are transmitted through the cathode, and a uniform response can be obtained over a large photocathode area. However, these schemes are complex and to date have found little practical application. Gating a proximity focused device is much simpler. A voltage pulse is applied across either photocathode-phosphor gap, or across a microchannel plate. There are several disadvantages to this approach: a fast, high voltage drive is required because a large detector area implies a high capacitance, it is inherently a single-frame device, straight through hard x rays can cause problems, and the response across a large sensitive area will be non-uniform due to the finite propagation velocity of the gating voltage wave. Until recently, fast high voltage pulses for the electrical gating could only be generated by photoconductive switches with the concomitant complexity of a short pulse laser. However, the well known phenomenon of avalanche breakdown/ has now been developed so that high power electrical pulses as short as 70 psec can be produced by purely electronic drivers. These voltage pulses are applied to a strip transmission line with a microchannel plate as the dielectric. The temporal and spatial resolution are described in Sec. II. Using a voltage doubling scheme sufficient voltage from four paralleled reverse biased diodes can be produced to gate four microchannel plates arranged in the four corners of a rectangle. The field uniformity and gain width variation of this configuration are described in Sec. II. For use on Nova, these microchannel plates are gated at different times and detect four gated images from an array of four pinholes. The system and its timing fiducial are described in Sec. III. Representative results from direct-drive implosions are described in Sec. IV.

Multiphoton ionization of xenon with 620 nm pulses is reviewed. Near saturated ionization is observed at 2 x 10¹³ W/cm² using 0.9 psec pulses and 6 x 10¹³ W/cm² using 90 fsec pulses. These results indicate that atomic systems can be irradiated with very high intensity ultrashort pulses without significant ionization. Below the ionization threshold new nonlinear phenomena, such as continuum generation, can be investigated. We review measurements of continuum generation in gases made with ultrashort pulses. The nonlinearity responsible for self-focusing and self-phase modulation saturates with laser intensity. A diffraction limited input beam has its spatial profile reconstituted after self-focusing.

We have observed the self-reflection of intense, sub-picosecond 308 nm light pulse incident on a planar AI target and have inferred the electrical conductivity of solid density AI. The pulse lengths were sufficiently short that no significant expansion of the target occurred during the measurement.

Broad spectral lines have been observed in the high-resolution extreme ultraviolet (XUV) spectra from plasmas created by irradiating solid targets using picosecond laser pulses. Transitions of the type n=2-3 and n=2-4 in Li-like ions of the elements N, 0, F, and Al have linewidths up to 2 Å. Using quasistatic ion Stark broadening, the electron densities of the plasma emission regions have been derived from the linewidths. The derived electron densities are found to be inversely proportional to the lifetimes of the upper levels of the transitions. The electron densities range from 2 x 10²² cm^-3 for levels with lifetimes less than 4 picoseconds to 10²¹ cm^-3 for levels with lifetimes greater than 30 picoseconds. In effect, the XUV emission is time-resolved in the expanding plasma. The observation of emission from solid-density regions (greater than 4 x 10²³ cm^-3) will require transitions from levels in highly charged ions with lifetimes less than a picosecond.

Ultrashort (<10ps at lkeV) high-brightness (10²² photons/sec-mm²-eV into 2π steradians at lkeV) x-ray pulses are generated by exciting a preformed laser-produced plasma using a 1GW picosecond laser pulse. Time-integrated spectra and time-integrated spatially resolved emission yields are presented for various materials. Time-resolved measurements of the XUV and x-ray emission from a solid Au target are presented, as well as the dependence of radiation yield on the laser energy, angle of incidence and polarization. The production and short time duration of the x-ray emission is attributed to resonance absorption followed by rapid plasma cooling via thermal conduction.