The field of soft x-ray laser research is at a point between the first demonstrations of x-ray amplification and the onset and widespread application of modest—sized x-ray lasers. The past several years have seen great strides in x-ray amplifier performance as well as the demonstration of proof—of—principle x-ray laser applications. In the near term, the challenge for the x-ray laser community will be to develop compact x-ray lasers and to carry out an applications experiment of crucial importance to an identifiable community of interested users. Achieving these goals will require furthering our understanding of basic amplifier physics and increasing our technological capabilities with x-ray lasers. In this paper, we summarize the results obtained to date in these areas, with emphasis on the most recent developments.

Systematic study of the collisionally pumped Ne-like germanium 3p-3s laser system has led to the observation of saturated output on the J = 2-1 lines at 23.2 and 23.6 nm when a double slab target is double passed using a normal incidence concave mirror. The about-1-MW output power on the same lines from a simple double plasma has been used to record preliminary images with a Schwarzchild condenser and zone plate microscope arrangement. Initial analysis of measurements on the 4d-4p J = 1-0 line at 7.3 nm from nickel-like samarium indicate a gain coefficient of between 0.4/cm and 0.8/cm for an incident irradiance of about 2 x 10 exp 13 W/sq cm using a l.06-micron heating laser on slab targets of SmF3.

Basic characteristics of soft x-ray lasers generated as amplified spontaneous emission are described. Experimental results on soft x-ray amplification in neon-like germanium ions, obtained recently at the Institute of Laser Engineering, are reported. By comparing the experimental results with a simplified model on amplified spontaneous emission, basic parameters such as coherence and brightness of the Ge soft x-ray laser are evaluated.

Difficulties involved in maintaining large gain coefficients in plasma columns longer than 1-2 cm are examined with particular reference to the potential use of lithium-like recombination schemes for pumping X-ray lasers with a moderate pumping power. The discussion focuses on the role of plasma nonuniformity along the amplification path due to large-scale and small-scale inhomogeneities of the pumping optical laser. Two methods of smoothing nonuniformities are considered: the use of random phase plates and wall-confined plasmas.

Slab germanium targets have been irradiated in a line focus geometry with 1-3 nsec FWHM, 1.06-micron laser pulses at irradiances of I equal to or less than 10 exp 13 W/sq cm. The effect of varying the rise-time of the driving laser pulse on the amplification of 3-3 soft X-ray lasing lines is investigated. Results of short-pulse (100 psec FWHM) experiments have also shown gain on the same lasing transitions, but at substantially higher irradiances of I equal to or greater than 3 x 10 exp 13 W/sq cm.

Experimental study of soft X-ray laser for the sodiumlike recombination scheme, carried out at the LF12 Laser Facility of Shanghai Institute of Optics and Fine Mechanics, is reported. Soft X-ray amplification was observed in sodiumlike copper ions by line-focused laser irradiation of slab targets. Based on the spatially-resolved measurements, the spatial distribution of the laser gain for the 6g-4f (72.22 A) transition of sodiumlike copper ions was obtained. The maxima of gain coefficients and gain-length products at 550 microns from target surface are 2.0/cm and 3.6, respectively. The evolution of spectrally discriminated spatial uniformity of line-shaped plasmas as lasing gain media produced by uniform laser illumination in line-focus is also investigated. It is found that, under this experimental condition, the line-shaped plasma is very uniform after optical laser's heating.

The effect of ion-ion scattering on the generation of plasma-based Raman laser is studied. The ion diffusion in velocity space is shown to broaden the detuning range and to increase the output power. The results are compared with the experiment made in argon laser plasma on anti-Stokes line 437 nm under 648 nm pumping. Agreement between the theory and measured generation spectra confirms that the collisions contribute to lasing processes.

Thomson scattering, X-ray diagnostics and optical interferometry are employed to characterize both collisionally excited and recombination X-ray laser plasmas. Spatially and time-resolved Thomson scattering spectra from carbon plasmas provide the electron and ion temperatures, which are compared with hydrodynamic computer modelling. The electron density is inferred from the recorded interferograms. Time-integrated X-ray spectra in the keV range are used to characterize very precisely the optimum irradiance conditions for lasing in Ne-like germanium plasmas. Also, electron temperatures obtained with the 5C/4C line ratio, assuming a PLTE model, are compared to Thomson scattering results.

The spatial coherence of a neon-like selenium x-ray laser operating at 206 and 210 angstroms has been measured using a technique based on partially coherent x-ray diffraction. The time integrated spatial coherence of the selenium x-ray laser was determined to be equal to that of a quasi-monochromatic spatially incoherent disk source whose diameter is comparable to the line focus of the visible light laser pumping the x-ray laser. The spatial coherence was improved by narrowing the line focus width. Laboratory x-ray lasers have been available for six years as potential tools for research. Their basic characteristics such as output energy, pulselength, linewidth, and divergence have been measured. Knowledge of these characteristics has resulted in x-ray lasers being used in some preliminary applications experiments including photo-ionization physics, contact microscopy of cells, and holography. Future applications of x-ray lasers, such as nonlinear x-ray optics and holographic microscopy of biological microstructures, require a detailed knowledge of the spatial coherence. This paper presents the first measurement of the spatial coherence of an x-ray laser.

This paper discusses recent progress in LLNL's high resolution XUV spectroscopy efforts with x-ray laser plasmas. We describe the instrumentation used, and we present preliminary time-resolved data on the spectral profiles of several XUV (extreme ultraviolet) lines from Ne- like Se and Ne-like Y x-ray lasers which have been obtained with instrumental resolutions ((lambda) /(Delta) (lambda) ) of approximately 10,000. The Se data indicates that the 206.4 angstroms J equals 2 - 1 laser line narrows below the expected 400 eV Doppler width (35 m angstroms) when amplified through approximately 6 gain lengths, while the Y data shows no evidence of the J equals 0 - 1 laser predicted to be nearly resonant with the J equals 2 - 1 laser at 154.9 angstroms.

We describe the design of a novel single element holographic toroidal grating spectrometer that would be nearly stigmatic between 100 - 300 angstroms. The instrument would be 7 meters from source to image, and would have a resolution of about 1 in 6000 for a 25 (mu) pixel size. Spatial resolution on the order of 10 (mu) in 2-D would be possible for a spectrally narrow line, such as that produced in a slow and low-density coronal plasma.

Lasing has been demonstrated at 44.83 angstroms in Ni-like Ta with a gain of order 3 cm-1 over a length of 3 cm. This wavelength is suitable for holography of biological specimens provided that sufficient energy is available. The duration of the x-ray laser pulse (FWHM of order 0.35 ns) allows the possibility of a second pass through the lasing medium to increase the output energy. The dependences of the output energy on the reflectivity of the multilayer mirror, the separation of the mirror from the lasing medium, and the duration of lasing are explored in this paper. The x-ray laser plasma is a strong source of x rays (energies up to 2 keV) that can damage the multilayer mirror. We examine the heating of the multilayer mirror as a function of the separation from the lasing medium. The effect of the mirror shape (radius of curvature and whether the surface is flat, concave, or convex) on the output energy and degree of coherence of the x-ray laser is given.

The whispering-gallery mirrors desired for use in short-wavelength laser cavities are seen to be highly aspheric and very different from the shapes encountered in conventional optics. Fabrication tolerances are established by examining the effects of various surface imperfections. The mirrors are found to be relatively insensitive to figure errors. The requirements on surface finish are shown to be fairly strict, though less severe than with normal-incidence optics.

Recent advances in theoretical and experimental research concerned with the feasibility of the gamma-ray laser are reviewed. The discussion covers the principal concepts, pump calibration, giant pumping resonances, and the development of a laser model. A quantitative model based on new data and concepts demonstrates the feasibility of the gamma-ray laser.

The use of recently developed high brightness subpicosecond lasers for the study of the interaction of solid matter is leading to the production of powerful incoherent X-ray sources associated with dense plasma environments. The use of these intense pulsed X-ray sources will enable the production of extremely high densities and levels of electronic excitation in materials while leaving the system kinetically cold during the interaction. This general condition is extremely conductive to the amplification of short wavelength radiation. The analysis of a particular case for amplification at a quantum energy of about 1 keV indicates that a total energy of about 1 J of ultraviolet radiation is necessary for excitation. The control of this class of physical processes is expected to lead to a new generation of amplifiers in the X-ray range.

We are investigating the generation of short pulse short wavelength x-rays for pumping inner- shell x-ray lasers by photo-ionization. In contrast with previous proposals, we are looking at the use of a single line as an efficient means of pumping these lasers. As a first step we are optimizing the flashlamp x-ray conversion efficiency and characterizing the x-ray pulse length.

Experimental results have demonstrated very high order harmonic generation in rare gases exposed to intense laser fields. We use time-dependent methods to calculate the single atom emission spectrum in a number of systems. These spectra show many of the same features seen experimentally, e.g. a broad plateau that depends in intensity and extent on the ionization potential. To explain the observed spectra, we fold these single atom results into a full solution of the propagation equations for the nonlinear medium. This leads to impressive agreement between theory and the experiment for xenon, both in the absolute number of photons observed as well as the intensity dependence of all harmonics through the 21st. The use of time-dependent methods also facilitates a comparison between photon and electron emission processes.

Experiments are reported in which a high-density aluminum plasma was generated by a 10-picosecond KrF laser pulse. The amplified spontaneous emission (ASE) level of the prepulse was controlled by using a saturable dye. The electron density was estimated from the intensity ration of the Ly-alpha satellite components. It is found that ASE has to be suppressed to less than 10 exp 9 W/sq cm to create a high electron density plasma.

Numerical solutions are given for the two-dimensional axisymmetric problem of self-focusing of powerful short-duration circularly polarized laser pulses in both initially homogeneous plasmas and static preformed plasma columns. These solutions account for (1) diffraction, (2) refraction arising from variations in the refractive index due to the spatial profile of the electron density distribution, (3) the relativistic electronic mass shift, and (4) transverse ponderomotively driven charge displacement. The most important spatial modes of propagation corresponding to the combined action of both the relativistic and charge-displacement mechanisms are described. It is demonstrated that the dynamical solutions of the propagation tend asymptotically to the lowest eigenmodes of the governing nonlinear Schroedinger equation.

We have been developing a small-scale EUV laser facility at MIT with which we hope to observe gain in nickel-like molybdenum at 191 angstroms. The physics involved in the formation of laser gain is examined through the use of simple models, and optimum operating temperatures and electron densities are determined. The upper state population is determined primarily through a balance between direct collisional excitation to and from the upper laser state; the lower state population follows primarily from a balance between indirect excitation and radiative decay. Models describing these balances are used to estimate optimum conditions and the resulting population inversions.

The possibility of achieving soft X-ray amplification by collisional recombination in a capillary discharge plasma is being investigated. The results of the soft X-ray spectroscopy study of carbon capillary plasmas excited by a 40 kV, 108 ns FWHM discharge are discussed. The first spectra corresponding to a fast 25 ns FWHM, 600 kV discharge are also reported.

The development of a pump laser for x-ray lasers that operate at wavelengths longer than 1 (mu) would permit the attainment of higher electron temperatures for a given pump intensity. Such a system would be of interest for electron collisional schemes at low Z, and would have the potential to improve the overall system efficiency. An initial design for an optical parametric oscillator that would down-convert 1 (mu) radiation to 2 (mu) is presented.

A small-scale X-ray laser system for operations near 191 A at a repetition rate of 6 ppm is being developed on the basis of the Ni-like collision excitation scheme of Maxon et al. (1986). Based on the idea of Hagelstein (1988), an attempt is made to use low-Z material (instead of high-Z materials commonly used in collisional excitation schemes), which is to be pumped with a series of short pulses rather than with one long pulse. It is predicted that favorable density and temperature conditions will be present after a few initial pulses to generate gain on the order of 5-10/cm.

A program aimed at the development of a table-top soft X-ray laser on the CVI 18.22 nm transition is reported. A gain-length product of GL = 2.5-3.5 has been observed for a variety of target geometries using a 3-ns 6-15 J Nd:glass driving laser emitting at 1.05 micron. Plans for future experiments are outlined.

We present preliminary experiments and calculations performed to optimize the photoionization of an He-like aluminum plasma. The high emissivity of the 3d-4f M-band of tantalum or tungsten is used as the pump. The plasmas are produced by two 500 ps duration beams of a frequency doubled Nd-glass laser. The pump beam is delayed by 1 ns with respect to the main beam. The Al plasma ionization state has been measured with K-alpha absorption measurements. X-ray diodes and space-resolving crystal spectrographs have been used to measure the intensity of the pump source in the desired spectral range. Optimization of the pumping scheme is analyzed with a numerical description of the photopumping process by a collisional-radiative modeling of the Al plasma including the X-ray pump.

A neonlike X-ray laser photoresonant pumping scheme is explored. The design is configured as a coaxial z-pinch consisting of an inner krypton lasant plasma surrounded by a carbon shell that itself is surrounded by a stagnated krypton plasma. The photoresonant radiation emitted from the outer plasma passes inwardly through the carbon shell and photoexcites lasant electrons to the 3s and 3d levels. It is calculated that monochromatic pump powers of 100 - 200 GW/eV can be achieved for the 3s and 3d neonlike resonance lines. The gain in several neonlike 3s-3p transitions is calculated as a function of temperature and density of the lasant plasma. Reasonable gain in the 3p-3s (J=0-1) transition is obtained for high density, high temperature lasant conditions. This gain is nearly independent of the pump. For low density, low temperature lasants, optimal gain is obtained in the 3p3s (J=2-1) transitions. Photoionization and photoexcitation by the pump are necessary to produce gain under these conditions. The attenuation of the resonance line pumps limits gain to the outer edge of modest density lasants.

We propose a laser scheme in which the upper levels of a lasing iron are pumped by narrow, quasi-continuous band radiation. In particular, we consider pumping of neon-like chlorine ions by N band radiation of Pb from 41 angstroms to 44 angstroms. We calculate small signal gain coefficients for the n equals 4 - n equals 3 transitions in neon-like chlorine. Both steady-state and time-dependent calculations have shown that the largest gain is found for the 2s22p-11/24f5/2(J equals 2) - 2s22p-11/23d3/2(J equals 1) transition at a wavelength of 264 angstroms, with a value of 5 cm-1. Achieving this value for the gain would require a pump radiation that has a brightness of 0.01 photons/mode, corresponding to an intensity of 1.6 X 1012 Wcm-2.

We present recent experimental data related to the resonant photopumping of Mo VII by Mo XII as a method of generating a VUV source near 600 A. The experiment is based on a scheme proposed by Feldman and Reader (1989), in which the excitation of the 4p5 6s - 4p6 transition in Mo VII by the 5s 2Ssub 1/2 - 4p 2Psub 1/2 transition in Mo XII produces a population inversion on one of several 6s-5p transitions in Mo VII. The generation of plasmas suitable for photopumping experiments is established from the presentation of measurements of line emission from Mo plasmas as a function of laser and target parameters. Results of the laser produced plasma experiments show the successful enhancement of the population of the Mo VII 4p5 6s upper lasing level when pumped by an adjacent Mo XII plasma. We also present results where improvements of the intensity of the Mo XII pump source, achieved using an additional pump laser, lead to the generation of a population inversion for a VUV transition.

We investigate the influence of ion collisional excitation on gain estimates for lasing between the two 3d - 2p fine-structure transitions in a recombining, hydrogen-like magnesium plasma. A sizable reduction of the 3d3/2 - 2p1/2 gain coefficient is found, resulting from both collisional population transfer and from lower state broadening of the emission profile. The need for care in evaluating the latter effect is discussed

The angular distribution of high-order harmonics produced in a jet of xenon gas is studied using a 1.053 micrometers wavelength laser. All the harmonics in the 9th to 25th range display approximately equal angular widths with cone angles that are approximately half that of the laser. These widths are significantly large than would be predicted by the lowest order perturbation theory.

The problem of the anomalously rapid decay rate of the principal radiative transition in interpreting expansion cooled recombination lasers is examined. The radiative transfer of the line is analyzed, and it is shown that, in typical situations, the net radiative bracket may be sufficiently larger than the commonly used planar escape factor to account for this discrepancy. The role of doubly excited states in the recombination cascade is also examined.

The ray tracing code SURF for x-ray optics simulation is presented. The code can be applied to x-ray imaging problems, and steering and concentration of x-ray beams. The results of the simulation of x-ray laser beam focusing with figured multilayer optics and pattern imaging with Schwartschild objective are given.

The ray tracing code SURF for x-ray optics simulation is presented. The code can be applied to x-ray imaging problems, and steering and concentration of x-ray beams. The results of the simulation of x-ray laser beam focusing with figured multilayer optics and pattern imaging with Schwartschild objective are given.

Possibilities for creating an active medium for hard X-ray lasers using superbright (intensity in excess of 10 exp 17 W/sq cm) laser radiation pumping are examined. Several possible X-ray laser schemes are evaluated, and their advantages and disadvantages discussed. It is shown that an inner ionization type scheme is particularly promising for the realization of hard X-ray lasers.

We present a calibration system for X-UV (40 - 400 angstroms) instrumentation used in inertial confinement research. A small pulsed laser, 4 Joules, 4 nanoseconds in the green (0.54 mm wavelength), is focused onto solid targets of different materials such as aluminum or gold with a repetition rate of one every 30 seconds. The radiation given by the plasmas produced on these targets is selected by a monochromator made with a grazing incidence grating associated with a toroidal mirror. The grating is planar and its period per mm is slightly variable in order to record the spectra in a plane when working as a spectrograph. The selected radiation from the monochromator enters a vacuum vessel, in the center of which is put a goniometer (theta) /2(theta) . This goniometer measures the reflectivity of multilayer mirrors, using XRD diodes absolutely calibrated. The system measures the response of photocathodes of streak cameras working in the X-UV range and other photon detectors in the same range.