We have demonstrated gain-saturated operation of high repetition rate table-top soft x-ray lasers producing microwatt average powers at wavelengths ranging from 13.2 to 32.6 nm in transitions of Ni-like and Ne-like ions. Lasing was also observed for shorter wavelength transitions of the 4d¹S₀→4p¹P₁ Ni-like isoelectronic sequence, with amplification approaching gain saturation in the 11.9 nm line of Ni-like Sn and progressively reduced gain for wavelengths as low as 10.9 nm in Ni-like Te. The results were obtained heating a pre-created plasma with a picosecond optical laser pulse with an energy of only 1 J impinging at optimized grazing angles of incidence for maximum pump energy deposition efficiency. This pumping geometry takes advantage of the refraction of the pump beam to increase the energy deposition efficiency of the pump beam into the gain region, making it possible to operate soft x-ray lasers in this wavelength range at significantly increased repetition rates. The results demonstrate the feasibility of producing high average powers of coherent radiation in the 100 eV spectral region for applications using a table-top source.

We studied seeding of two x-ray laser transitions, 4d-4p at 32.8 nm in Kr⁸⁺ and 5d-5p 41.8 nm in Xe⁸⁺. The amplifying medium is generated by focussing a high energy circularly polarized, 35 fs 10 Hz Ti: sapphire laser system in a few mm cell filled with gas (xenon or krypton).We succeeded to increase from a factor 10 to 200 the input HHG energy, without deteriorating their optical qualities. The resulting beam was polarized, coherent and we estimate the output energy to be about 1 μJ.

A new high-density gas puff target for experiments on laser-driven X-ray lasers is presented. The target is based on a double-stream gas puff target approach, which makes possible to form an elongated gas sheet with steep density gradient and high density of gas in the interaction region. In the paper the valve system to produce the gas puff targets and the results of the target characterization measurements performed using X-ray backlighting technique are presented.

We examine the development of soft X-ray lasers using collisionally pumped Ni-like ions to identify the possibilites for further improvements in the pumping efficiency as we move to shorter wavelengths. The underlying requirements of the pumping system are reviewed. Silver and samarium are examined by detailed simulation in the light of this analysis. It is found that there is significant potential for further reductions in the pump demands for silver using long pre-pulse/mainpulse delays and grazing incidence pumping. Samarium however offers less possibilty of improvement as recombination in the expanding plasma plume inhibits the use of long delays. We find that grazing incidence pumping at an appropriate angle can lead to significant reduction in pump energy.

Over the last decade, most laser-driven collisional excitation x-ray lasers have relied on the absorption of the pump energy incident at normal incidence to a pre-formed plasma. The main advantage is that the inversion can be created at various plasma regions in space and time where the amplification and ray propagation processes are best served. The main disadvantage is that different plasma regions regardless of the contribution to the inversion have to be pumped simultaneously in order to make the laser work. This leads to a loss of efficiency. The new scheme of grazing incidence pumping (GRIP) addresses this issue. In essence, a chosen electron density region of a pre-formed plasma column, produced by a longer pulse at normal incidence onto a slab target, is selectively pumped by focusing a short pulse of 100 fs-10 ps duration laser at a determined grazing incidence angle to the target surface. The exact angle is dependent on the pump wavelength and relates to refraction of the drive beam in the plasma. The controlled use of refraction of the pumping laser in the plasma results in several benefits: The pump laser path length is longer and there is an increase in the laser absorption in the gain region for creating a collisional Ni-like ion x-ray laser. There is also an inherent traveling wave, close to c, that increases the overall pumping efficiency. This can lead to a 3-30 times reduction in the pump energy for mid-Z, sub-20 nm lasers. We report several examples of this new x-ray laser on two different laser systems. The first demonstrates a 10 Hz x-ray laser operating at 18.9 nm pumped with a total of 150 mJ of 800 nm wavelength from a Ti:Sapphire laser. The second case is shown where the COMET laser is used both at 527 nm and 1054 nm wavelength to pump higher Z materials with the goal of extending the wavelength regime of tabletop x-ray lasers below 10 nm.

Recent development in the field of X-ray lasers is shown and discussed starting from transient inversion scheme in a double-pulse arrangement. Different variants of this scheme are discussed in detail from the point of view of reduction in the pump energy. The discussion is concentrated on the kinetic aspect of the plasma created and heated by a profiled pulse. Recently, a scheme referred to as GRIP (GRazing Incidence Pumping) has been proposed and demonstrated. This pump geometry opens a new real possibility to construct a repetitive X-ray laser. Some aspects of the pump scheme implementation are analysed. Finally, a specific injector-amplifier system giving a new perspective on the future of X-ray lasers is dicussed briefly as well.

We report the demonstration of a high repetition rate desktop-size capillary discharge laser emitting at (= 46.9 nm (26.5 eV) used for experiments in photochemistry. Laser pulses with energy ~ 13 uJ were generated at 12 Hz repetition rate by single pass amplification in a 21 cm long Ne-like Ar capillary discharge plasma column. The capillary lifetime is 2-3 10⁴ shots. This new type of portable laser is of interest for numerous applications requiring a compact intense source of short wavelength laser light. One such application that we are currently pursuing is the study of small molecules using time of flight mass spectroscopy. Molecules include ammonia, NO, oxygen, hydrogen bonded nanoclusters, and metal oxide nanoclusters. Through single photon ionization the reactivity and catalytic behavior of these molecules is studied.

We present a detailed analysis of an experiment carried out recently in which the temporal coherence of the Ni-like silver transient X-laser at 13.9 nm was measured. Two main consequences of this measurement will be discussed and interpreted with numerical calculations. First we show that the high temporal coherence length measured corresponds to an extremely narrow spectral width of the X-ray laser line. Second we show that the high temporal coherence helps to explain the presence of small-scale structures observed in the cross-section of all transient X-ray laser beams.

The absolute time of emission of the x-ray laser output with respect to the arrival of a 100-ps pump pulse has been measured with the aid of a calibrated timing fiducial. The results show the x-ray laser to appear up to 60 ps (80 ps) before the peak of the pump pulse in the case of the Sn (Pd) x-ray laser, which is in good agreement with results obtained from hydrodynamic, atomic physics, and raytracing simulations. The pulse duration was found to be ~40 ps for both the Sn and the Pd x-ray lasers.

Evidence from experimental measurements of the temporal duration Δt of x-ray lasing and measurements or estimates of the frequency bandwidth Δν show that the Fourier transform limit ΔνΔt ~ 1 has been approached in several experiments. It is important to understand and quantify this fundamental limit. The temporal behaviour of x-ray laser pulses of short duration at the Fourier transform limit is examined in this paper. Using numerical methods to model ASE output, the behaviour of the electric field with time E(t) is determined from the Fourier transform of the electric field variation with frequency E(ν). The expected time-bandwidth product is then presented for different gain length products.

Longitudinal coherence length in x-ray lasers depends strongly on the shape of the amplified line. We have modeled an experiment performed at the Lawrence Livermore National Laboratory. The experiment was devoted to the study of the temporal (longitudinal) coherence of the transient x-ray laser at 14.7 nm in Ni-like palladium (4d-4p transition). Only electron (collisional) and Doppler broadening play a role in the line profile of the 0-1 4d-4p transition. This allows us to use the Voigt shape in conditions where the amplifier, i.e., the plasma produced by the interaction of a high intensity laser with a slab target, is neither stationary nor homogeneous. Our calculations use a ray trace code which is constructed as a post-processor of the hydro-atomic code EHYBRID. In the saturation regime, there is need to account properly for the interaction between the x-ray laser field and the lasing ions. This is done in the framework of the Maxwell-Bloch formalism. The FWHM of the spontaneous emission profile is ~28 mÅ, while the width of the amplified x-ray line ~4 mÅ. Comparison with experiment is discussed.

The paper deals with the recent results of the experiments on soft X-ray imaging of various carbon-containing objects: biological samples, artificial carbon fibres, graphite slices, etc. The working wavelength was chosen to be 4.5 nm due to high penetration ability of these soft X-rays in the carbon materials. The experimental set-up included: laser plasma X-ray source (generated with the 2^nd harmonics of Nd:YAG laser), scandium-based thin-film filter and highly reflective spherical multilayer mirror. The Co/C multilayer's reflectivity was measured to be about 15 % at normal incidence that was high enough to produce soft X-ray images using one nanosecond-long exposure. The work demonstrates a possibility to produce high contrast images outside "water window" region for study of relatively thick (tens of microns) samples that may lead to new fields of applications of the soft x-ray microscopy.

Technological reasons stimulated enormous interest in the spectral range between 10 nm and 15 nm. One of the most important, apart from the potential to be applied in the microlithography, was the existence of the high-efficiency, spectrally highly selective (narrow-band) reflective multi-layer (ML) optics in this spectral range. Applying these optics to plasma based XUV (extreme ultra violett) sources the debris from the plasma is a serious problem. For transmissive multi-layer optics we have additionally the low figures of merit. For example, the best beam splitters have an efficiency of about 30% (energy in both parts of the splitted beam). This type of element is crucial for efficient single-shot interferometry being the main application using table-top soft x-ray lasers. We applied capillary optical elements, to our knowledge for the first time, to XUV radiation at 13.9 nm. These optical elements help overcome the limits discussed above or at least remarkably reduce the existing difficulties. A capillary beam splitter and a focussing capillary were applied to an incoherent XUV radiation source. For the beam splitter we measured a throughput of about 80%. With the focussing capillary we obtained a spot size of 27 μm (FWHM) with a gain (intensity in the focal spot compared to the intensity behind a pinhole of the focal spot size) of 600. Advantages and disadvantages of these optics in the discussed spectral range are analyzed.

Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.

We give an overview of recent advances in development and applications of deeply saturated Ne like zinc soft X-ray laser at PALS, providing strongly saturated emission at 21.2 nm. Population inversion is produced in the regime of long scale-length density plasma, which is achieved by a very large time separation between the prepulse (<10 J) and the main pump pulse (~500 J), of up to 50 ns. This pumping regime is unique in the context of current x-ray laser research. An extremely bright and narrowly collimated double-pass x-ray laser beam is obtained, providing ~10 mJ pulses and ~100 MW of peak power, which is the most powerful soft X-ray laser yet demonstrated. The programme of applications recently undertaken includes precision measurements of the soft X-ray opacity of laser irradiated metals relevant to stellar astrophysics, soft X-ray interferometric probing of optical materials for laser damage studies, soft X-ray material ablation relevant to microfabrication technologies, and pilot radiobiology studies of DNA damage in the soft X-ray region. A concomitant topic is focusing the x-ray laser beam down to a narrow spot, with the final goal of achieving ~10¹³ Wcm^-2.

We have used soft x-ray laser interferometry to study dense colliding plasmas produced by laser irradiation of semi-cylindrical targets. Results are reported on the evolution of 1 mm long plasmas created by heating 500 μm diameter half holhraum copper targets with an intensity of ~1.6 10¹² W.cm^-2 from 120 ps duration laser pulses of 800 nm wavelength. The setup combines a robust high throughput amplitude division interferometer based on diffraction gratings with a 46.9 nm table-top capillary discharge laser. Series of high contrast interferograms were obtained depicting the evolution of the copper plasmas into a localized plasma that reaches densities above 1×10²⁰ cm^-3 when the plasmas collide near the center of the cavity. The technique allows the generation of high resolution density maps of colliding plasma with various degree of collisionality for comparison with code simulations.

Time-resolved soft x-ray photoelectron spectroscopy is used to probe the non-steady-state evolution of the valence band electronic structure of laser heated ultra-thin (50 nm) metal foils and bulk semiconductors. Single-shot soft x-ray laser induced time-of-flight photoelectron spectroscopy with picosecond time resolution was used in combination with optical measurements of the disassembly dynamics that have shown the existence of a metastable liquid phase in fs-laser heated metal foils persisting 4-5 ps. This metastable phase is studied using a 527 nm wavelength 400 fs laser pulse containing 0.3 - 2.5 mJ laser energy focused in a large 500 × 700 μm² spot to create heated conditions of 0.2 - 1.8 × 10¹² W cm^-2 intensity. The unique LLNL COMET compact tabletop soft x-ray laser source provided the necessary high photon flux, highly monoenergetic, picosecond pulse duration, and coherence for observing the evolution of changes in the valence band electronic structure of laser heated metals and semiconductors with picosecond time resolution. This work demonstrates the continuing development of a powerful new technique for probing reaction dynamics and changes of local order on surfaces on their fundamental timescales including phenomena such as non-thermal melting, chemical bond formation, intermediate reaction steps, and the existence of transient reaction products.

We present early results of an application of X-ray laser, aimed at understanding the effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses. For the purpose of the experiment, a novel interferometric microscopy technique was designed and tested. The interferometric beamline employed a double Lloyd's mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8 along a plane inclined with respect to the propagation direction of the X-ray beam. The objects investigated were thin plane beamsplitters made of fused silica (SiO₂), irradiated by damaging laser light at 438 nm and in situ probed by the developed technique of interferometric microscopy. The soft X-ray beam was emitted by neon-like zinc laser, delivering up to 10 mJ at 21.2 nm. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage of the rear surface of the beamsplitter occurs approximately during of much after the laser pulse. Another issue examined by the X-ray interferometric microscopy technique was whether the surface perturbation seen shortly after the impact of the damaging pulse is associated or not with the pattern of permanent surface modifications.

This paper gives an overview of recent progress of x-ray laser research in Japan Atomic Energy Research Institute (JAERI). In the development of high quality x-ray laser beam, the progress includes the improvement of output energy of fully spatial coherent x-ray laser beam at a wavelength of 13.9 nm and generation of temporally coherent x-ray laser at 26.9 nm by use of seed x-ray injection technique. Beam stability is greatly improved to be better than 0.5 mrad by introducing new designed target chamber and target alignment system. In the application of the 13.9 nm laser, an experiment by use of x-ray speckle technique reveals firstly the existence of polarization clusters in ferroelectric substance. For the purpose of further application experiments, 0.1 Hz-repetition rate x-ray laser driver is being developed, which is based on an OPCPA pre-amplifier and a Nd:glass zigzag slab amplifier with two beam lines, and each line provides 10 Joules 1 ps pulse on target.

For decades the electron density of plasmas has been measured using optical interferometers. With the availability of good X-ray laser sources in the last decade interferometers have been extended into the wavelength range 14 - 47 nm, which has enabled researchers to probe even higher density plasmas. The data analysis assumes the index of refraction is due only to the free electrons, which makes the index less than one. Recent interferometer experiments in Al plasmas observed plasmas with index of refraction greater than one at 14 nm and brought into question the validity of the usual formula for calculating the index. In this paper we show how the anomalous dispersion from bound electrons can dominate the free electron contribution to the index of refraction in many plasmas and make the index greater than one or enhance the contribution to the index such that one would greatly overestimate the density of the plasma using interferometers. Using a new average-atom code we calculate the index of refraction in many plasmas at different temperatures for photon energies from 0 to 100 eV and compare against calculations done with OPAL. We also present examples of other plasmas that may have index of refraction greater than one at X-ray laser energies. During the next decade X-ray free electron lasers and other X-ray sources will be available to probe a wider variety of plasmas at higher densities and shorter wavelengths so understanding the index of refraction in plasmas will be even more essential.

We have demonstrated imaging at soft x-ray wavelengths in transmission and reflection modes using high repetition rate table-top soft x-ray lasers. Transmission mode imaging with a resolution better than 50 nm was demonstrated using the output from a 13.9 nm Ni-like Ag laser in combination with condenser and objective Fresnel zone plate optics. Reflection mode imaging of a microelectronic chip with a resolution of 120-150 nm was demonstrated using the illumination provided by the 46.9 nm output from a compact capillary-discharge Ne-like Ar laser. This microscope combines a Schwarzschild condenser and a zone plate objective. The results demonstrate the feasibility of practical nanometer-scale microscopy with compact soft-x-ray laser sources.

We report clear evidence of the existence of multiply ionized plasmas with index of refraction greater than one at soft x-ray wavelengths. Moreover, it is shown to be a general phenomenon affecting broad spectral regions in numerous highly ionized plasmas. The experimental evidence consists of the observation of anomalous fringe shifts in soft x-ray laser interferograms of laser-created Al plasmas probed at 14.7 nm and of Ag and Sn laser-created plasmas probed at 46.9 nm. The comparison of measured and simulated interferograms shows that these anomalous fringe shifts result from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements and at different wavelengths.

We have developed a double Lloyd's mirror wavefront-splitting interferometer, constituting a compact device for surface probing in the XUV and soft X-ray spectral domain. The device consists of two independently adjustable superpolished flat surfaces, operated under grazing incidence angle to reflect a diverging or parallel beam. When the mirrors are appropriately inclined to each other, the structure produces interference fringes at the required distance and with tuneable fringe period. The double Lloyd's mirror may be used alone for surface topography with nanometric altitude resolution, or in conjunction with an imaging element for interferometric XUV surface microscopy. In the latter case, resolution in the plane of the probed surface is about micron, which is given by the quality of the imaging element and/or by the detector pixel size. Here, we present results obtained using the double Lloyd's mirror in two separate X-ray laser and high harmonics generation (HHG) application projects. The first experiment was aimed at understanding microscopic nature of the effects involved in laserinduced optical damage of thin pellicles, exposed to sub-ns laser pulses (438 nm) producing fluence of up to 10 Jcm^-2. The probing source in this case was a QSS neon-like zinc soft X-ray laser, proving a few mJ at 21.2 nm in ~100-ps pulses. The second experiment was carried out using a narrowly collimated HHG beam near 30 nm, employed to topographically probe the surface of a semiconductor chip.

The ablation of plain aluminium foil and aluminium foil with a thin (50 nm) iron coating was observed using a neon-like zinc x-ray laser. The 21.2 nm x-ray laser was produced by a double pass of a 3 cm long zinc target at the PALS centre in Prague. The x-ray laser was used to probe the sample targets as they were heated by a separate laser beam of 10 J, focussed to a 100 micron diameter spot. The data from the experiment are presented and compared with Ehybrid simulations and simple ablation rate calculations.

The determination of EUV optical constants in rare-earth metals is much hampered by the high reactivity and easy air contamination of these materials. The most difficult regions are a long wavelengths part of the EUV interval and the vicinity of absorption edges. In this study the optical constants of La and Tb are determined in a wide energy interval 14-400 eV. The study is performed by our recent method, which is suitable for reactive materials and for intervals around the absorption edges of elements. The samples are identically protected films with thickness ~ 10 nm or ~ 100 nm, which are deposited on silicon photodiodes. Mathematical treatment separates the contributions of a capping layer and a rare-earth metal and provides data which are free from the effects of air contamination. The precision and behavior of optical constants, as well as the parameters of the O_2,3 and N_4,5 edges in La and Tb are analyzed.

An optical-field-ionization soft x-ray laser with prepulse-controlled nanoplasma expansion in a cluster gas jet was demonstrated. Pd-like xenon lasing at 41.8-nm with 95 nJ pulse energy and 5-mrad divergence was achieved, indicating near-saturation amplification. In addition, by using deflectometry of a longitudinal probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials.

Main practical applications of X-rays lie in the important for the society fields of medical imaging, custom, transport inspection and security. Scientific applications besides of fundamental research include material sciences, biomicroscopy, and protein crystallography. Two types of X-ray sources dominate now: conventional tubes and electron accelerators equipped with insertion devices. The first are relatively cheap, robust, and compact but have low brightness and poorly controlled photon spectrum. The second generate low divergent beams with orders of magnitude higher brightness and well-controlled and tunable spectrum, but are very expensive and large in scale. So accelerator based X-ray sources are mainly still used for scientific applications and X-ray tubes--in commercial equipment. The latter motivated by the importance for the society made an impressive progress during last decades mostly due to the fast developments of radiation detectors, computers and software used for image acquisition and processing. At the same time many important problems cannot be solved without radical improvement of the parameters of the X-ray beam that in commercial devices is still provided by conventional X-ray tubes. Therefore there is a quest now for a compact and relatively cheap source to generate X-ray beam with parameters and controllability approaching synchrotron radiation. Rapid developments of lasers and particle accelerators resulted in implementation of laser plasma X-ray sources and free electron lasers for various experiments requiring high intensity, shrt duration and monochromatic X-ray radiation. Further progress towards practical application is expected from the combination of laser and particle accelerator in a single unit for efficient X-ray generation.