This PDF file contains the front matter associated with SPIE Proceedings Volume 6702, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We have recently demonstrated high repetition rate tabletop lasers operating at wavelengths as short as 10.9 nm based on collisional transient excitation of ions in plasmas created by laser heating of solid targets. As a further step in the development of these lasers into very high brightness and fully coherent soft x-ray sources, we have demonstrated injection seeding of the amplifiers with high harmonic seed pulses. We report results of an experiment in which a 32.6 nm Ne-like Ti amplifier was used to amplify a seed pulse from the 25^th harmonic of Ti:Sapphire into the gain saturation regime. Simultaneous amplification of the 27^th harmonic at 30.1 nm was also observed. The seeded soft x-ray laser beam was measured to approach full spatial coherence. We have demonstrated that this scheme is scalable to shorter wavelengths and that is capable of producing extremely bright soft x-ray laser pulse with essentially full coherence.

We report on gain-saturated operation of the 11.9-nm x-ray lasing line in Ni-like Sn using the grazing-incidence pumping scheme (GRIP). The experiments were done with 2-ps duration pump pulses and energies up to 5 J. Strong gain saturation with ⪆10-microJ output was measured for the Sn laser at a grazing angle of 45° and a pump pulse energy of 5 J. This was achieved with a 4.5%, 2-ps duration prepulse 1.6 ns ahead of the main pulse and also incident at grazing incidence. Increasing laser output was observed at GRIP angles from 22.5° to 45°. At this angle, the minimum energy required for saturated lasing was determined as ~2 J.

Soft x-ray interferometry was used to measure the evolution of dense converging plasmas created by laser irradiation of 500 μm diameter semi-cylindrical carbon targets. Optical laser pulses with an intensity of ~1×10¹²W cm^-2 and 120 ps duration were used to heat the surface of the cavities. The dense plasma formed expands from the walls converging slightly off the semi-cylinder's axis, giving rise to a bright localized high density plasma region. A sequence of electron density maps were measure using a 46.9 nm wavelength tabletop capillary discharge soft x-ray laser probe and an amplitude division interferometer based on diffraction gratings. The measured density profiles are compared with simulations conducted using the multi-diminensional hydrodynamic code HYDRA. The benchmarked model was then used to simulate particle trajectories which reveal that the increase in electron density near the axis is mainly the result of the convergence of plasma that originated at the bottom of the groove during laser irradiation.

Using the grazing incidence pumping technique with a 600 mJ, 500 ps background pulse and a 250 mJ, 200 fs main pulse the lasing emission from a molybdenum target has been studied. A flat field spectrometer designed to observe the X-ray laser emission in both the first and second orders was used to record the time integrated data. Time resolved data was obtained by installing an Axis-Photonique PX1 X-ray streak camera to observe the first order output from the spectrometer whilst retaining the time integrated second order observation. In this paper both time integrated and time resolved data are presented for a range of grazing angles, target lengths, delays between pumping pulses and pumping energy. Comparisons are also drawn with simulations from the Ehybrid and Medusa codes. An additional experiment is also described in which a two colour pumping method is used to investigate lasing at short wavelength from high Z targets (Z ≥ 62).

Efficient optical-field-ionization x-ray lasers driven by femtosecond laser pulses have been demonstrated in clustered gas jets. With a tight focusing configuration, near saturation outputs of Pd-like xenon laser at 41.8 nm and Ni-like krypton laser at 32.8 nm are generated at low pump energy of 200 mJ. By using the axicon-ignitor-heater scheme to produce a 9-mm-long plasma waveguide in a pure krypton gas jet, the lasing photon number of Ni-like Kr laser at 32.8 nm is dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in a photon number of 8×10¹⁰ and an energy conversion efficiency of 2×10^-6 with a pump pulse of just 235 mJ. Besides for producing various OFI collisional-excitation x-ray lasers of sufficient photon number for practical applications, an optically-preformed plasma waveguide may also be a favorable choice for achieving OFI recombination x-ray lasers and inner-shell x-ray lasers.

The possibility of generating short wavelength laser action in plasma is considered by two different methods. In the first we examine the conditions under which laser action can be generated using laser heating of thin foils of materials with a high atomic number. It is shown that there is a way forward using reasonable pump laser energies tailored to the specific x-ray wavelength and material. Recombination lasers, in principle, have a high quantum efficiency, particularly if transitions to the ground state can be used. ATI offers a possible means to achieve this objective. We examine the conditions under which such inversion can be generated. Some simple scalings for the density and temperature are derived.

Since the first seeding of an OFI soft x-ray laser in 2004, we progressed towards the full characterization of the output beam. The final is to be able to deliver to users well-known beam. Temporal as well as spatial parameters have been measured for different conditions of amplification. We observed a strong enhancement of the spatial coherence due to the amplification process with a far-field pattern exhibiting an airy-like shape. The gain zone having strong discontinuity behaves like a hard pinhole. Spatial filtering has been also observed on the wave front (δ/5 root-mean-square, rms, before seeding and δ/20 rms after amplification). Temporal coherence has been studied thanks to the use of a Fourier- Transform spectrometer. Spectral widths, δδ/δ, around 10^-5 have been measured for different plasma lengths or gas pressures. Departure from Gaussian shape has been clearly observed on the spectral line for some cases.

Output characteristics of an X-ray laser based on the GRIP geometry are analysed by both the theoretical and experimental methods. Detailed analysis of the last experiments on GRIP X-ray lasers with a single profiled pulse is given as well as the consequences of this pump variant for the injector-amplifier scheme being developed. Especially dynamics of the gain coefficient and the spontanous emission flux are important for the injector-amplifier scheme. Discussion on medium dynamics and kinetics is supported by numerical simulations. Additionally, some preliminary results on seeding a Ni-like soft X-ray with high harmonic from neon at 13.9 nm are presented.

We present in this paper theoretical and experimental investigations of temporal and spectral properties of seeded soft x-ray lasers. Bloch-Maxwell simulations of the harmonic pulse propagation in a soft x-ray laser plasma have been performed. Results show a growing wake of coherent radiation formed after the harmonic pulse.We describe the first measurement of the spectral bandwidth of a seeded soft x-ray laser. Using a varying path difference interferometer the spectral profile of a seeded OFI x-ray laser has been experimentally determined, leading to a Fourier-transform pulse duration of 5ps. The measured bandwidth is in good agreement with simulations. Finally we present the progress toward the implementation of a seeded soft x-ray laser at 18.9 nm at the new LASERIX facility.

Free electron lasers are expected to become brightest hard X-ray sources in the next decade. Meanwhile a quest still exists for moderately bright but lab scale X-ray sources to fill in the gap between conventional X-ray tubes and synchrotron radiation beamlines. Thomson scattering of picosecond laser pulses on electron bunches is considered as possible solution to this problem.

Inverse Compton is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Co Compton ba mpton backscattering X-ray source driven by the multi 10 TW-L backscattering Laser installed at Daresbury (COBALD). Hard X-rays, with spectral peak ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of a laser pulse with an electron bunch delivered by the energy recovery linear accelerator prototype (ERLP) at Daresbury. X-ray pulses containing 9×10⁷ photons per pulse will be created from head on collisions, with a pulse duration comparable to that of the incoming electron bunch. For transverse collisions 8×10⁶ photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be ~ 10²¹ photons / s / mm² / mrad² / 0.1% ΔE/E, which is comparable to fourth generation synchrotron light sources.

We present a review of recent development and applications of soft x-ray lasers, undertaken at the PALS Centre. The applications benefit from up to 10-mJ pulses at the wavelength of 21.2 nm. We describe the pumping regimes used to produce this soft x-ray laser, and outline its emission characteristics. A significant fraction of applications carried out using this device includes probing of dense plasmas produced by IR laser pulses and high-energy-density-in-matter experiments. Results obtained in these experiments are reviewed, including x-ray laser probing of dense plasmas, measurements of transmission of focused soft x-ray radiation at intensities of up to 10¹² Wcm^-2, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering

The rate of laser ablation at irradiances of ~2x10¹⁴ Wcm^-2 of solid iron and aluminum has been measured using the transmission of a neon-like zinc X-ray laser at 21.2 nm through thin iron and aluminum targets. It is shown that the opacity of ablated material falls rapidly with increasing temperatures and decreasing density from the solid value. As ablated plasma becomes transparent to the X-ray laser flux, the thickness of solid, unablated material and hence the rate of ablation can be measured from time resolved X-ray laser transmission. A self-regulating model of laser ablation and fluid code simulations with absorption to thermal plasma of 5-10% show agreement with our measured ablation rates.

Combining a compact table top soft X ray laser with an interferometric lithography set up arrays of nanodots and nanoholes were directly patterned on the surface of different photoresists. Multiple exposures with a Lloyd's mirror interferometer printed arrays of holes and dots over an area of 0.5× 0.5 mm² with typical diameters down to 60 nm full width at half maximum. This laser-based soft X-ray interferometric tool demonstrated the possibility to print different nanoscale patterns using a compact table-top set up.

Sorf X-ray (SXR) Gabor's holograms recorded with a highly coherent table top λ = 46.9 nm laser were obtained with different numerical apertures yielding a spatial resolution of 164 nm. The holograms were recorded in a high resolution photoresist and digitized with an atomic force microscope. A detailed wavelet decomposition and correlation method was developed to assess the spatial resolution achieved in the numerical reconstruction of the holograms.

Metal oxide clusters are employed in studies to help understand an important, specific, type of surface chemical problem: the contamination of soft x-ray mirrors by carbon deposits. Herein we report nanocluster chemistry studies that are relevant to the use of silicon oxide and titanium oxide capping layers. Systems involving Si_mO_n, and Ti_mO_n metal oxide nanoclusters are generated in a pulsed supersonic expansion/ablation source and passed through a reactor containing any reactant desired. The reaction products of these gas phase clusters are ionized using single photon ionization from a desk-top sized 46.9 nm Ne-like Ar laser providing the advantage of little or no fragmentation of desired nanoclusters. The ionized products are analyzed by a time of flight mass spectrometer and experimental results supply useful information related to condensed phase soft x-ray optical surfaces. The results illustrate the great potential of the use of very compact soft x-ray lasers in photochemistry and photophysics studies.

We have demonstrated ablation of holes with diameter as small as 82 nm in polymethyl methacrylate (PMMA) by focusing the output of a capillary discharge soft x-ay laser with a Fresnel zone plate. We also report the first demonstration of laser induced breakdown spectroscopy with soft x-ray laser light.

Images with nanoscale resolution were obtained in both transmission and reflection modes using a full-field microscope that is illuminated by an extremely compact λ = 46.9 nm (hν; = 26.4 eV) soft x-ray laser. The microscope was used to image the surface of partially processed silicon semiconductor chips containing periodic patterns of polysilicon and metal lines. To characterize the microscope, modulation transfer functions were experimentally built for three different objective zone plates, and images with near-wavelength resolution were obtained.

Over the last decade the electron density of plasmas has been measured using X-ray laser interferometers in the 14 to 47 nm wavelength regime. With the same formula used in decades of experiments with optical interferometers, the data analysis assumes the index of refraction is due only to the free electrons, which makes the index less than one. Over the last several years, interferometer experiments in C, Al, Ag, and Sn plasmas have observed plasmas with index of refraction greater than one at 14 or 47 nm and demonstrated unequivocally that the usual formula for calculating the index of refraction is not always valid as the contribution from bound electrons can dominate the free electrons in certain cases. In this paper we search for other materials with strong anomalous dispersion that could be used in X-ray laser interferometer experiments to help understand this phenomena. An average atom code is used to calculate the plasma properties. This paper discusses the calculations of anomalous dispersion in Ne and Na plasmas near 47 nm and Xe plasmas near 14 nm. With the advent of the FLASH X-ray free electron laser in Germany and the LCLS X-FEL coming online at Stanford in 2 years the average atom code will be an invaluable tool to explore plasmas at higher X-ray energy to identify potential experiments for the future. During the next decade X-ray free electron lasers and other X-ray sources will be used 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.

Recent experiments were carried out on the Prague Asterix Laser System (PALS) towards the demonstration of a soft x-ray laser Thomson scattering diagnostic for a laser-produced exploding foil. The Thomson probe utilized the Ne-like zinc x-ray laser which was double-passed to deliver ~1 mJ of focused energy at 21.2 nm wavelength and lasting ~100 ps. The plasma under study was heated single-sided using a Gaussian 300-ps pulse of 438-nm light (3ω of the PALS iodine laser) at laser irradiances of 10¹³-10¹⁴ W cm^-2. Electron densities of 10²⁰-10²² cm^-3 and electron temperatures from 200 to 500 eV were probed at 0.5 or 1 ns after the peak of the heating pulse during the foil plasma expansion. A flat-field 1200 line mm^-1 variable-spaced grating spectrometer with a cooled charge-coupled device readout viewed the plasma in the forward direction at 30° with respect to the x-ray laser probe. We show results from plasmas generated from ~1 μm thick targets of Al and polypropylene (C₃H₆). Numerical simulations of the Thomson scattering cross-sections will be presented. These simulations show electron peaks in addition to a narrow ion feature due to collective (incoherent) Thomson scattering. The electron features are shifted from the frequency of the scattered radiation approximately by the electron plasma frequency ±ω_pe and scale as n_e^1/2.

To widen the field of application of the x-ray laser, higher repetition rate of the x-ray laser is needed. Moreover, the study on effective generation of x-ray laser by optimizing pumping conditions is very important in order to achieve the high repetition rate. At Japan Atomic Energy Agency (JAEA), we have been developing a new type of high repetition rate x-ray laser driver using zigzag slab amplifier for high repetitive x-ray lasing and an OPCPA (Optical Parametric Chirped Pulse Amplification) preamplifier for optimizing x-ray lasing conditions instead of a regenerative amplifier. We are now studying the optimization of pumping conditions for efficient x-ray lasing by the use of pump laser with very high pulse contrast from the OPCPA preamplifier. And we are planning to perform applications using the high repetition rate x-ray laser such as x-ray holography.

LASERIX is a high power laser facility intended to realise and use for applications transient collisional excitation (TCE) X-ray lasers (XRLs) at various wavelengths, using grazing incidence pumping (GRIP) configuration with 10 Hz repetition rate. In addition new types of XRL schemes giving rise to emission at short wavelengths will be developed using the high energy LASERIX driver with 0.1 HZ rep-rate. Thus, this laser facility will both offer Soft X-ray lasers in the 40-10 nm range and synchronised auxiliary IR beam that could be also used to produce XUV sources. This experimental configuration highly enhances the scientific opportunities of the facility. Indeed it will be possible to realise both X-ray laser experiments and more generally pump/probe experiments, mixing IR and XUV sources. Then, this facility will be useful for the community, opening a large scale of investigations, including imagery and irradiation.

Results of experimental and numerical researches of problems, connected with creation of compact EUV lasers with a close geometry of electrodischarge systems, with long transporting lines for its energy supply and preliminary gas preionization by running wave of avalanche sliding discharge are reported. Both pumping schemes with collision excitation on example of neon-like ions of argon and collision recombination on example of hydrogen-like ions of nitrogen are considered. The input energy balance and questions of matching between a transport line and a discharge load are studying. Processes, limiting radiative properties of active media in both pumping schemes are discussed.

Multilayer mirrors for the extreme ultraviolet (EUV) are key elements for numerous applications of coherent EUV sources such as new tabletop lasers and free-electron lasers. However the field of applications is limited by the radiation and thermal stability of the multilayers. Taking into account the growing power of EUV sources the stability of the optics becomes crucial. To overcome this problem it is necessary to study the degradation of multilayers and try to increase their temporal and thermal stability. In this paper we report the results of detailed study of structural changes in Sc/Si multilayers when exposed to intense EUV laser pulses. Various types of surface damage such as melting, boiling, shock wave creation and ablation were observed as irradiation fluencies increase. Cross-sectional TEM study revealed that the layer structure was completely destroyed in the upper part of multilayer, but still survived below. The layers adjacent to the substrate remained intact even through the multilayer surface melted down, though the structure of the layers beneath the molten zone was noticeably changed. The layer structure in this thermally affected zone is similar to that of isothermally annealed samples. All stages of scandium silicide formation such as interdiffusion, solid-state amorphization, silicide crystallization etc., are present in the thermally affected zone. It indicates a thermal nature of the damage mechanism. The tungsten diffusion barriers were applied to the scandium/silicon interfaces. It was shown that the barriers inhibited interdiffusion and increased the thermal stability of Sc/Si mirrors.

This paper presents novel approaches and techniques in development and application of multi-mirror X-ray optical systems operating in the spectral region of "carbon window" (λ ~ 4.5 nm). Two approaches for fabrication of the graded Co/C multilayer mirrors for Schwarzschild objective are presented. A pair of the spherical mirrors with Co/C multilayer coatings was tested in combination with scandium/carbon filters and laser produced plasma X-ray source for stereoimaging of low density materials at λ ~ 4.5 nm.

We report new results of evaluated gain for 13.38 nm radiation created due to the inversion population on the Balmer alpha transition of hydrogen-like nitrogen. Recombination pumping in non-equilibrium plasma during the capillary pinch decay is judged. Quantitative analysis is performed in four-dimensional region of optional parameters chosen in accordance with available experimental devices: current slope dI/dt_|t=0 (0.9-3.0 10¹² A/s), current peak value Imax (50-100 kA), capillary radius R₀ (0.16-0.25 cm) and filling atom density N₀ (1.0-6.0 10¹⁷ cm^-3). Requirements on the current pulse shape are stated. Diameter of the created active medium is estimated.

We report on the development of ultrafast coherent soft X-ray beamline at the Prague Asterix Laser System (PALS) Research Center intended for interdisciplinary applications such as ablation and controlled surface modification of solid materials for a micro/nano-pattering, soft X-ray interferometry and holography for surface probing with nanometric resolution, and improvement of focusing optics for soft X-ray beams. The beamline is based on 1 kHz, tabletop, high-order harmonic generation (HHG) source capable to deliver fully coherent, tunable beam in the 13 - 40 nm spectral range. Ti:sapphire (810 nm, 1 kHz) laser pulses with a duration of 35 fs and energy 1.5 mJ have been focused into a gas jet or gas cell containing conversion medium (Ar). To achieve highly efficient HHG we will apply the technique of guided laser pulses and the two-color laser field. Results on the optimization of HHG near 21 nm are presented. The beamline consists of a tandem of two vacuum chambers: one for the HHG source and its diagnostics, and second for the application experiments. After completion, access to this new installation will be opened to external users. This table-top system will be complementary to the existing, high energy (~10 mJ) Ne-like Zn soft X-ray laser at 21.2 nm developed at PALS. We will also present the first experimental results on the structural surface modifications of various solid materials (i.e., PMMA - poly(methyl methacrylate); amorphous carbon) caused by a few shot exposure to the focused HHG beam at 21.6 nm.

We have designed, developed and tested the avalanche diode structures operational as single photon counters with picosecond resolution on the basis of the GaP material. The background of this research is our development of avalanche structures based on various semiconductor materials designed for single photon counting devices in the last twenty years. All the semiconductor detectors operate at a room temperature or at thermoelectrically achievable temperatures. Electronic circuits for these detectors biasing, quenching and control have been developed and optimized for different applications. Circuits permitting operation of solid state photon counters in both single and multiple photon signal regimes have been developed and applied. Timing resolution of solid state photon counters as high as 50 picoseconds full width at a half maximum has been achieved when detecting single photon signals. The compact and rugged design, radiation resistance, and low operating voltage are attractive features of solid state photon counters in various applications including the space projects. The avalanche structures based on the GaP material exhibit several special features for x-ray operation namely the timing resolution as high 100 picoseconds may be achieved when detecting individual quanta. The application of GaP avalanche photodiode is proposed and reported for interaction experiments on currently developed lab with capillary discharge EUV laser source and triggering co-located femtosecond laser system.

We present early prototype of a high repetition target delivery system, designed for nominal operation rate at 1 Hz but with possibility of running at up to 10 Hz. The system is primarily intended to deliver complex solid-state targets, such as composite foils or structured shapes, in vacuum environment into the focus of a high-repetition laser. The system in its full version consists of a carousel exchanger accommodating 450 target frames, and a feeding block positioning the individual frames into the laser focus, with nominal precision of about 10 microns. A kinematical prototype of this device was built and its performance is currently evaluated.

The point diffraction interferometer (PDI) is a simple self-referencing interferometer, designed here to measure the wavefront profile of a soft X-ray laser emitting at the wavelength of 21.2 nm. It is a monolithic device consisting of a thin filter and a very small pinhole (~1 μm), located near the axis of the X-ray laser focal spot. The foil material around the hole is semitransparent for the X-ray radiation of interest, acting like a neutral density filter. The small pinhole is a diffraction aperture and plays a spatial filtering role, generating spherical wave that acts a reference. The interferometric pattern is produced on a detector placed a few centimeters behind the foil. The beam wavefront profile is reconstructed from the information encoded in the pattern. In this paper we discuss the overall design of the PDI wavefront sensor setup, namely its geometry, fringe contrast, level of the detected signal, size of the pinhole, and candidate materials for the PDI foil.