We have succeeded in developing a laser-pumped x-ray laser with full spatial coherence at 13.9 nm. A highly directed x-ray laser beam with the divergence of 0.2 mrad was generated from the double target experiment, where a seeding light from the first laser medium was amplified in the second medium. The observed divergence is close to the diffraction limited value within a factor of two. The seeding light was amplified in the second medium without refraction influence and the gain coefficient was about 8 cm^-1. The gain region of the second medium was far away from the target surface compared with that of the first medium and located in the considerably low density region. From the measurement of visibility, it was found that the spatial coherent length is longer than the beam diameter.

Below saturation, X-ray laser output shows a reduction in pulse duration and frequency bandwidth as the gain-length product increases. Above saturation, both quantities can be expected to rebroaden. The duration of gain can be close to an order-of-magnitude longer than the output pulse duration. With gain-length products just below saturation, X-ray lasing at 13.9 nm in nickel-like silver has been measured with a pulse duration Δt of 3 - 4 ps and an estimated frequency bandwidth Δv of 5×10¹¹ Hz. Such values imply that the pulses are close to transform limited with Δt Δv ≈ 1.5. Measurements of x-ray laser pulse-lengths and gain duration will described in this paper.

We review our recent progress in the development of transient x-ray lasers and of their application to plasma diagnostic. The first observation of C-ray laser emission at the new PHELIX-GSI facility is reported. This TCE X-ray laser will be a promising tool for heavy-ion spectroscopy. We then present the main results obtained at the LULU-CPA facility with a compact high-resolution X-UV imaging device. This device was used to investigate the spatial source structure of the Ni-like silver transient X-ray laser under different pumping conditions. The key-role of the width of the background laser pulse on the shape of the emitting aperture is demonstrated. Finally the imaging device was used as an interference microscope for interferometry probing of a laser-produced plasma. We describe this experiment performed at APRC-JAERI.

Some prospects for development of collisional X-ray lasers with a high repetition rate based on the output characteristics of a transient Ni-like Ag soft X-ray laser pumped by a single picosecond laser pulse are analysed. Such problems as target technology, new driver development and the active medium parameters are discussed.

We report on 2D near-field imaging experiments of the 11.9-nm Sn x-ray laser that were performed with a set of novel Mo/Y multilayer mirrors having reflectivities of up to 40% at normal and at 45° incidence. Second-moment analysis of the x-ray laser emission was used to determine values of the c-ray beam propagation factor M2 for a range of irradiation parameters. The results reveal a reduction of M2 with increasing prepulse intensity. The spatial size of the output is a factor of ~2 smaller than previously measured for the Pd x-ray laser, while the distance of the x-ray emission with respect to the target surface remains roughly the same.

We present the longitudinal coherence measurement of the transient inversion collisional x-ray laser for the first time. The Ni-like Pd x-ray laser at 14.68 nm is generated by the LLNL COMET laser facility and is operating in the gain-saturated regime. Interference fringes are produced using a Michelson interferometer setup in which a thin multilayer-coated membrane is used as a beam splitter. The longitudinal coherence length for the picosecond duration 4d¹S₀ -> 4p¹P₁ lasing transition is determined to be ~400 µm (1/e HW) by adjusting the length of one interferometer arm and measuring the resultant variation in fringe visibility. This is four times improved coherence than previous measurements on quasi-steady state schemes largely as a result of the narrower line profile in the lower temperature plasma. The inferred gain-narrowed linewidth of ~0.29 pm is also substantially narrower than previous measurements on quasi-steady state x-ray laser schemes. This study shows that the coherence of the x-ray laser beam can be improved by changing the laser pumping conditions. The x-ray laser is operating at 4 - 5 times the transform-limited pulse.

Compact soft x-ray laser sources are now used routinely for various applications primarily because of their high repetition rate, high photon fluence and short pulse duration characteristics. For some of these applications, for example interferometry of high density laser-produced plasmas, longer optical drive pulses, 6 - 13 ps (FWHM), have been implemented to maximize the x-ray output and coherence. It is therefore important to know the x-ray laser pulse length, shape and repeatability for these specific experiments as a baseline measurement but also to better understand the temporal behavior as a function of the pumping conditions in general. We report a detailed temporal characterization of the picosecond-driven 14.7 nm Ni-like Pd ion x-ray laser on the Compact Multipulse Terawatt (COMET) laser at LLNL using an ultrafast x-ray streak camera measurement of a horizontal slice of the near-field x-ray laser pattern. This is measured as a function of the chirped pulse amplification pumping laser conditions, including varying the pump pulse from 0.5 - 27 ps (FWHM), varying the plasma column length as well as investigating traveling wave (TW) and non-TW irradiation conditions.

Simulations are performed to clarify the mechanisms that generate high spatial coherence x-ray lasers with the longitudinally pumped nickellike molybdenum scheme. Various factors that affect the x-ray laser output is also investigated, and we clarify the experimental conditions that maximize its performance.

We present a numerical investigation of the parameters characterizing the recombination gain in the LiIII 2→1 transition (13.5nm). The numerical model includes the initial Optical Field Ionization (OFI) of the plasma by an intense 100fs laser pulse, taking into account ATI heating, particle collisions and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools down. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions. In addition we present the latest experimental results on Raman amplification of ultrashort pulses in the pursuit for a better and more compact scheme to enable a ``table-top'' x-ray laser.

Output characteristics of a transient Ni-like Ag X-ray laser pumped by a single picosecond laser pulse are demonstrated. The small-signal gain coefficient, beam divergence, output pulse profile and beam dimensions in the near-field were measured. The influence of the pump pulse structure on the lasing process and the output signal is analysed theoretically.

We present measurements of electron densities of plasmas with fs resolution. The plasmas are generated by laser pulses with different intensities at different time delays. Such plasmas are of great interest as preplasmas for transient, collisionally excited X-ray lasers. The prepulse is generated by stretching part of a 130-fs laser pulse of the ATLAS titanium-sapphire laser of our institute. Focusing this radiation to a line on molybdenum and silver targets generates preplasmas highly interesting to research directed towards a 10 Hz sub-Joule soft X-ray laser. The electron density is measured as a function of distance from the target by interferometry using a Wollaston prism. The ultrashort probe pulse allows one to obtain data as close as 10 - 20 μm from the target surface. Experimental data are compared with simulations using the MULTI hydrocode. The results allow optimization of prepulse-main pulse delay times and compare ablation from a hard (Mo) and a soft (Ag) material.

We describe the first demonstration of a collisionally-excited optical field ionisation laser driven within a gas-filled capillary waveguide. Lasing on the 4d⁹5d - 4d⁹5p transition at 41.8 nm in Xe⁸⁺ was observed to be closely-correlated to conditions under which the pump laser pulses were guided well by the waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.

We give an overview of recent results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8 x 10¹⁷ Wcm^-2 into a low-density gas cell containing xenon or krypton, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d⁹5d(¹S₀) - 4d⁹5p(¹P₁) transition at 41.8 nm in Pd-like Xe, and strong lasing at 32.8 nm on the 3d⁹4d(¹S₀) - 3d⁹4p(¹P₁) transition in Ni-like Kr has been demonstrated. Under optimum pumping conditions the Xe IX laser provides about 5 x 109 photons per pulse while the Kr IX laser delivers up to (2-3) x 10⁹ photons per shot. The repetition rate of these X-ray lasers is 10 Hz. The experimental results are discussed and compared to computational simulations in which issues related to plasma formation, electron energy distribution, and atomic processes are addressed.

The recent progress in the development of the tabletop soft x-ray lasers pumped by capillary discharges has opened the possibility of the widespread use of these coherent sources for applications. In this paper, we present the latest results achieved in our laboratory concerning the optimization of a 46.9nm capillary discharge pumped soft x-ray laser and the imaging of the laser beam on Lithium Fluoride (LiF, films), which appears to be a new promising x-ray detector. The use of LiF allows a high spatial resolution (smaller than 1μm) on large areas, high-contrast imaging and simple manipulation. The laser, which is pumped by 18-20kA, 150-180ns long current pulses at a repetition rate of 0.1Hz, is produced in 3.2mm diameter up to 45cm length alumina capillary tubes filled with Ar at the initial gas pressure of 300-600mTorr. At the capillary lengths examined, the laser is operating in the saturation regime giving an output energy of about 300μJ per pulse and high coherence degree.

We report the generation and characterization of plasma waveguides in a highly ionized Ar plasma created by a fast capillary discharge. Their results are of interest for the efficient longitudinal laser excitation of collisional soft x-ray lasers. A discharge-driven hydrodynamic compression guides progressively lower order modes through a plasma with increasing density and degree of ionization. The mode structure and guiding properties were investigated using near and far field imaging, and transmission measurements. The lowest order mode (FWHM diameter ~ 50 um) is guided with an energy transmission of ~ 75% over a 5.5 cm long plasma a fraction of a ns before the discharge plasma column reaches the conditions for lasing in Ne-like Ar. The rapid expansion of the highly ionized plasma column after the pinch forms a significantly more leaky and absorbent waveguide.

We summarize results of several successful dense plasma diagnostics experiments realized combining two different kinds of table-top soft x-ray lasers with an amplitude division interferometer based on diffraction grating beam splitters. In the first set of experiments this robust high throughput diffraction grating interferometer (DGI) was used with a 46.9 nm portable capillary discharge laser to study the dynamics of line focus and point focus laser-created plasmas. The measured electron density profiles, which differ significantly from those expected from a classical expansion, unveil important twodimensional effects of the dynamics of these plasmas. A second DGI customized to operate in combination with a 14.7 nm Ni-like Pd transient gain laser was used to perform interferometry of line focus laser-created plasmas with picosecond time resolution. These measurements provide valuable new benchmarks for complex hydrodynamic codes and help bring new understanding of the dynamics of dense plasmas. The instrumentation and methodology we describe is scalable to significantly shorter wavelengths, and constitutes a promising scheme for extending interferometry to the study of very dense plasmas such as those investigated for inertial confinment fusion.

We present within this paper a series of experiments, which yield new observations to further our understanding of the transient collisional x-ray laser medium. We use the recently developed technique of picosecond x-ray laser interferometry to probe the plasma conditions in which the x-ray laser is generated and propagates. This yields two dimensional electron density maps of the plasma taken at different times relative to the peak of the 600ps plasma-forming beam. In another experimental campaign, the output of the x-ray laser plasma column is imaged with a spherical multilayer mirror onto a CCD camera to give a two-dimensional intensity map of the x-ray laser output. Near-field imaging gives insights into refraction, output intensity and spatial mode structure. Combining these images with the density maps gives an indication of the electron density at which the x-ray laser is being emitted at (yielding insights into the effect of density gradients on beam propagation). Experimental observations coupled with simulations predict that most effective coupling of laser pump energy occurs when the duration of the main heating pulse is comparable to the gain lifetime (~10ps for Ni-like schemes). This can increase the output intensity by more than an order of magnitude relative to the case were the same pumping energy is delivered within a shorter heating pulse duration (< 3ps). We have also conducted an experiment in which the output of the x-ray laser was imaged onto the entrance slit of a high temporal resolution streak camera. This effectively takes a one-dimensional slice of the x-ray laser spatial profile and sweeps it in time. Under some conditions we observe rapid movement of the x-ray laser (~ 3um/ps) towards the target surface.

X-ray laser induced time-of-flight photoelectron spectroscopy has been used to probe the core-level and valence band electronic structure of room-temperature bulk materials with picosecond time resolution. The LLNL COMET compact tabletop x-ray laser source provides the necessary high photon flux, high energy, monochromaticity, picosecond pulse duration, and coherence for probing ultrafast changes in the chemical and electronic structure of these materials. Valence band and core-level spectra were recorded for transition metal surfaces. In situ sputter etching with Ar ions at 30° incidence will be implemented to improve the surface purity and consequently increase core-level and valence-band photoemission intensity. This work demonstrates a powerful new technique for probing reaction dynamics and for probing changes of local order on surfaces on their fundamental timescales. Future work will include the study of fundamental phenomena such as non-thermal melting, chemical bond formation, intermediate reaction steps, and the existence of transient reaction products.

We report results of the development of capillary discharge driven metal-vapor plasma waveguides for the development of efficient laser-pumped soft x-ray lasers; and of the use of a previously developed capillary discharge Ne-like Ar 46.9 nm laser in study of the interaction of intense soft x-ray laser with materials. The guiding of a laser beam in a dense capillary discharge plasma channel containing a large density of Ag ions is reported. In term of applications we have conducted studies of materials modification and ablation with focalized 46.9 nm laser radiation at fluences between 0.1 and 100 J cm^-2. The experiments demonstrated that the combined high repetition rate and high energy per pulse of the capillary discharge laser allows for the first time the processing of large surface areas with intense soft x-ray laser radiation. The damage threshold and damage mechanism of extreme ultraviolet Sc/Si multilayer mirror coatings was studied . Damage threshold fluences of ~ 0.08 J/cm² were determined for coatings deposited on both borosilicate glass and Si substrates. Scanning and transmission electron microscopy, and small-angle X-ray diffraction techniques revealed the thermal nature of the damage mechanism. These results provide a benchmark for the use of Sc/Si multilayer mirrors in high fluence applications, and for the development of higher damage threshold mirrors. Soft x-ray laser ablation studies were also conducted for silicon and several plastic materials, including PMMA, Polyamide and PTFE.

We present results from soft x-ray laser interferometry/shadowgraphy of current-driven thin wire plasmas using a capillary discharge pumped 46.9 nm laser and a diffraction grating interferometer. We have obtained series of soft x-ray images of exploding Al wires 15 μm or 25 μm in diameter. The high photo-ionization cross sections of atoms and low charge ions at this wavelength allow the soft x-ray laser probe to detect the early stages of formation of a coronal plasma surrounding the wire core. Wires of 25 μm diameter excited by current pulses with a 78 A/ns increase rate are observed to expand uniformly. However, an increase in the rate of energy deposited per unit of mass is observed to give rise to significant instabilities. The simultaneous analysis of the fringe shift and soft x-ray absorption data suggests the coronal plasma contains a significant concentration of low Z impurities. The results illustrate that table-top soft x-ray lasers are a powerful new tool for the diagnostics of dense plasmas.

Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (δλ/λ~10^-6). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*10¹⁴ Wcm^-2 by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as λ/120 at 13 nm. Commercial developments are under way.

We model recent experiments done using the COMET laser at Lawrence Livermore National Laboratory to illuminate slab targets of Pd up to 1.25 cm long with a two joule, 600 ps prepulse followed 700 psec later by a six joule, six psec drive pulse. The experiments measure the two-dimensional near-field and far-field laser patterns for the 14.7 nm Ni-like Pd x-ray laser line. The experiments are modeled using the LASNEX code 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. Using a ray tracing code to simulate the near and far-field output the simulations are then compared with experiments. In addition we model recent experiments that used a streak camera to measure the time duration of the Pd X-ray laser when pumped with a constant energy short pulse with different time durations that ranged from 0.5 to 27 ps.

Advances in transient collisional x-ray lasers have been demonstrated over the last 5 years as a technique for achieving tabletop soft x-ray lasers using 2 - 10 J of laser pump energy. The high peak brightness of these sources operating in the high output saturation regime, in the range of 10²⁴ - 10²⁵ ph. mm^-2 mrad^-2 s^-1 (0.1% BW) ^-1, is ideal for many applications requiring high photon fluence in a single short burst. However, the pump energy required for these x-ray lasers is still relatively high and limits the x-ray laser repetition rate to 1 shot every few minutes. Higher repetition rate collisional schemes have been reported and show some promise for high output in the future. We report a novel technique for enhancing the coupling efficiency of the laser pump into the gain medium that could lead to enhanced x-ray inversion with a factor of ten reduction in the drive energy. This has been applied to the collisional excitation scheme for Ni-like Mo at 18.9 nm and x-ray laser output has been demonstrated. Prelimanry results show lasing on a single shot of the optical laser operating at 10 Hz and with 70 mJ in the short pulse. Such a proposed source would have higher average brightness, ~10¹⁴ ph. mm^-2 mrad^-2 s^-1 (0.1% BW) ^-1, than present bending magnet 3rd generation synchrotron sources operating at the same spectral range.

During recent months we have continued investigations of many different aspects of x-ray lasers to characterize and improve the source and applications. This work has included temporal characterization of existing laser-heated x-ray lasers under a wide range of pumping conditions. We have also looked into more details at different applications of x-ray lasers among which was the interferometry of laser-produced and capillary discharge plasmas in several irradiation conditions for different target Z materials. The reduction of pump energy remains the most important for the generation of new compact x-ray lasers. Numerical studies show that there are some ways to improve several of the key parameters of x-ray lasers specifically repetition rates and efficiency.

High-repetition-rate, femtosecond x-ray lasers would be useful for dynamical study of ultra-fast phenomena in nature. One of routes to get fs x-ray lasers is to use inner-shell processes in atomic and ionic systems. In this paper, the two inner-shell schemes recently proposed will be reviewed and compared in detail. One of important issues using inner-shell schemes is fast and intense x-ray pumping source. One of good candidate sources for that purpose is Larmor radiation produced by electrons under an intense fs laser field. The relativistic, nonlinear Thomson scattering by an electron of an intense laser field is investigated in computer simulation. Under a laser field with a pulse duration of 20 fs Full Width Half Maximum and an intensity of 10²⁰ W/cm², the motion of an electron is highly relativistic and generates an ultra-short radiation of 2 attoseconds with photon energies of 100 to 600 eV. An interesting modulated structure of the spectrum is observed and analyzed. A radiation produced by the zigzag motion of an electron under a linearly polarized laser has better characteristics than by a helical motion under a circularly polarized laser pulse in terms of an angular divergence and an energy spectrum.

The Picosecond Laser-Electron Inter-Action for the Dynamic Evaluation of Structures (PLEIADES) facility, is a unique, novel, tunable (10-200 keV), ultrafast (ps-fs), hard x-ray source that greatly extends the parameter range reached by existing 3rd generation sources, both in terms of x-ray energy range, pulse duration, and peak brightness at high energies. First light was observed at 70 keV early in 2003, and the experimental data agrees with 3D codes developed at LLNL. The x-rays are generated by the interaction of a 50 fs Fourier-transform-limited laser pulse produced by the TW-class FALCON CPA laser and a highly focused, relativistic (20-100 MeV), high brightness (1 nC, 0.3-5 ps, 5 mm.mrad 0.2% energy spread) photo-electron bunch. The resulting x-ray brightness is expected to exceed 10²⁰ ph/mm²/s/mrad²/0.1% BW. The beam is well-collimated (10 mrad divergence over the full spectrum, 1 mrad for a single color), and the source is a unique tool for time-resolved dynamic measurements in matter, including high-Z materials.

The high brilliance expected from the X-ray Free-Electron Lasers (XFEL’s) now under construction suggest re-investigating the feasibility of a photopumped soft X-ray laser. We present simulations of a Lyman-α X-ray laser in hydrogenic He (λ = 30.4 nm) pumped by XFEL radiation with parameters of the TESLA Test Facility, phase II, at DESY/Hamburg. The simulations show that high gain can be achieved at a pump intensity of 10¹⁵ W/cm². The realization of such a laser could provide a better understanding of the physics of photopumped lasers and thus help to develop table-top X-ray lasers.

Intense directional X-ray emission was observed from metal targets (Pd and Ti), which served as the cathodes in a pulsed high current (100-400 mA) and low voltage (1.0 - 2.0 keV) deuterium/hydrogen glow charge. X-ray measurements showed an intense (I_x = 10¹³-10¹⁴ s^-1-cm^-2) soft X-ray emission (with a mean energy of quantum E_x = 1.2-1.5 keV) directly from the Pd or Ti cathode. The X-ray yield is strongly dependent on a deuterium diffusivity in the surface layer of the cathode. The X-ray emission can be associated with enhanced electron screening effects at metal surfaces and interfaces and a coherent oscillation of this screening layer.

The results of modeling and experimental investigations on the laboratory x-ray laser in RFNC-VNIIEF are presented. The computational model and TRANS computer code of transient amplified spontaneous emission (ASE) have been developed. The actual 3D medium, random polarization source, diffraction, refraction, gain with saturation, scattering by fluctuations of density can taken into account in the model. The 3D calculations of ASE in a Ne-like titanium x-ray laser with transient collisional excitation are presented. Results of experiments are described on Ne-like Ge x-ray laser realized at irradiation of the target by radiation of the ISKRA-5 laser facility. The quasi-steady-state ASE generation on 3p,J=0-3s,J=1 transition of Ne-like Ge (19.6 nm) is observed that is the first demonstration of c-ray laser in Russia in laboratory conditions. The divergence of ASE in a direction perpendicular to the target is about 10 mrad, and ASE beam deflection with respect to an optical axis is about 10 mrad. The experimental data are compared with simulation results.