Recent transient collisional excitation x-ray laser experiments are reported using the COMET tabletop laser driver at the Lawrence Livermore National Laboratory. Ne- like and Ni-like ion x-ray laser schemes have been investigated with a combination of long 600 ps and short approximately 1 ps high power laser pulses with 5 - 10 J total energy. We show small signal gain saturation for x-ray lasers when a reflection echelon traveling wave geometry is utilized. A gain length product of 18 has been achieved for the Ni-like Pd 4dyields4p J equals 0 - 1 line at 147 angstroms, with an estimated output of approximately 10 (mu) J. Strong lasing on the 119 angstroms Ni-like Sn line has also been observed. To our knowledge this is the first time gain saturation has been achieved on a tabletop laser driven scheme and is the shortest wavelength table-top x-ray laser demonstrated to date. In addition, we present preliminary results of the characterization of the line focus uniformity for a Ne-like ion scheme using L-shell spectroscopy.

We present experimental results of a high efficiency Ne-like Fe transient collisional excitation x-ray laser using the COMET 15 TW table-top laser system at LLNL. The plasma formation, ionization and collisional excitation of the x- ray laser have been optimized using two sequential laser pulses: a plasma formation beam with 5 J energy of 600 ps duration and a pump beam with 5 J energy of 1.2 ps duration. Since the observation of strong lasing on the 255 angstroms 3p - 3s J equals 0 - 1 transition of Ne-like Fe, we have achieved high gains of 35 cm^-1 and saturation of the x-ray laser. A five-stage traveling wave excitation enhances the strongest Fe 3p - 3s 255 angstroms lasing line as well as additional x-ray lines. A careful characterization of the plasma column conditions using L-shell spectroscopy indicates the degree of ionization along the line focus.

Recently, the technique of using a nsec pulse to preform and ionize the plasma followed by a psec pulse to heat the plasma has enabled low-Z neon-like and nickel-like ions to lase driven by small lasers with only ten joules of energy. In this work we model recent experiments done using the COMET laser at LLNL to illuminate 1 cm long slab targets of Ti with a 4.8 J, 800 ps prepulse followed 1.6 nsec later by a 6 J, 1 psec drive pulse. The LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities of the XRASER code, which then does the kinetics calculations to determine the gain.

Recent experiments, performed at the C.E.A./Limeil-Valenton P102 laser facility on the Ni-like transient collisional scheme, are reported in this paper. They mainly aimed at enhancing the efficiency and improving the optical properties of the already demonstrated 4d J equals 0/4p J equals 1 Ag¹⁹⁺ x-ray laser at 13.9 nm. The now classical 2- stage traveling-wave irradiation of slab targets was used, the illumination sequence being constituted of a long (600 ps) low-flux (0.5 - 11 J) laser pulse followed (200 ps later) by a short (< 1 ps) high intensity (1 - 20 J) one. The work novelty was the use of frequency-doubled pulses, either for the pre-forming or the pumping one. Various combinations ((omega) -(omega) , 2(omega) -(omega) , (omega) - 2(omega) ) have been investigated in terms of lasing performances. High gains, around 34/cm, have been measured and saturation achieved for target lengths above 4 mm. A strong enhancement, up to a few (mu) J, of the x-ray laser output has been observed, due to traveling-wave irradiation method, while the emission duration was decreased to less than 10 ps, resulting in a 300 kW source. Moreover, under specific laser conditions, a second lasing line at 16 nm was detected. Finally, the possibility of cavity operating transient collisional x-ray lasers has been demonstrated.

Results of measurements on a plasma created by a discharge in a sulfur capillary and irradiated by a picosecond laser pulse are presented. Time-resolved transmission measurements and the corresponding XUV spectra are discussed. It is found that the transmission of the laser through the capillary decreases abruptly after the peak of the discharge current, and that the profile of the output laser beam is symmetrized before the onset of the opaque period. The spectra are evidently affected by the laser pulse and show significant intensification on the 3p-3s transition in Ne-like sulfur at 60.84 nm. This observation, combined with the fact that the emission of this line appears spatially displaced from all other lines in the detector and presents a lower divergence, suggest that lasing is occurring.

High resolution 2D imaging experiment on the saturated 18.9 nm Ni-like soft X-ray laser is presented. The imaging experiment allows measurement of the absolute output energy and intensity of the X-ray laser, while gives detailed information on the spatial characteristics of the X-ray laser for understanding the physics and further improving the performance of the X-ray laser.

We report experimental results on Ni-like x-ray laser at the wavelength as short as 4.4 nm. The performance of x-ray lasing pumped by various types of pulse trains which were composed of 100 ps pulses was investigated with a double slab targets which were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi traveling wave pumping. The well collimated double target amplification was successfully demonstrated with two beam irradiation for Yb and Hf lasing at 5.0 nm and 4.7 nm whose gain length products were 11 and 6, respectively. The Ni- like lasing line of Ta have been observed at 4.5 nm. Based on these results, we will report the suitable pumping condition for the saturated water window x-ray lasers.

We report on the experimental demonstration of saturated x- ray laser output from collisionally pumped Ne-like Fe at 25.5 nm as well as Ni-like Ag and Pd at 14.0 nm and 14.7 nm, respectively, using a 100-ps drive pulse irradiation. A double-prepulse scheme and a 3-m radius-of-curvature target resulted in a gain-length product of gL equals 16.5 in the case of Fe. With a single prepulse and flat slab targets, gain- length products of 15.3 and 15.8 were obtained for Ag and Pd, respectively. Saturation was also confirmed by the observed reduction in beam divergence with increasing target length. The required drive energy used was only 30 J in a 100-ps pulse, corresponding to an irradiance of 12 TW/cm². A key role in the achievement of these results was played by the reduction in the roughness of the target surface in the case of the Pd x-ray laser.

Saturated operation of an X-ray laser is desirable as a high output irradiance is obtained with reduced shot-to-short variation. The potential of saturated X-ray laser output in probing plasma samples is first investigated. The laser pumping requirements to scale Ni-like saturated X-ray laser output to shorter wavelengths is then analyzed using published atomic physics data and a simple 4-level laser model for gain. A model of amplified spontaneous emission has been modified to accurately predict experimentally observed saturation behavior obtained in different experiments at the Rutherford Appleton Laboratory. In particular, the effects of traveling wave pumping with short duration (approximately 1 ps) laser pulses are investigated. Simulations of Ne-like Ge resonance line emission are compared to experimentally measured spectra.

Intense lasing at 18.9, 20.3 and 28.5 nm from nickel-like molybdenum, niobium and neon-like chromium ions has been observed by using two 200 ps laser pulses with a total energy of 50 J at 1.053 micrometers from XingGuang II laser facility. This shows the possibility of extending nickel- like and neon-like x-ray lasing in low-Z elements and paves the way towards small scale x-ray lasers for applications at university laboratories. A comparison has been made of performance of the neon-like chromium soft x-ray lasing at 28.5 nm driven by a double 900 ps pulse at 6 TW(DOT)cm^-2, with that driven by a double 200 ps pulse at similar irradiance. The double 200 ps pulse has been found to be more efficient to drive the neon-like x-ray lasing.

A numerical code COLAX solving the Maxwell-Bloch equations has been developed and applied to the modeling of soft X-ray lasers in hot laser-produced plasmas. First results in the case of Ne-like Zn from solid targets are presented, including refraction, saturation, coherence properties, and the effect of a half-cavity mirror.

When fs laser pulses interact with solid surfaces at intensities I(lambda) ² > 10¹⁸ W/cm² micrometers ², collimated relativistic electron beams are generated. These electrons can be used for producing intense X-radiation (bremsstrahlung or K_(alpha )) for pumping an innershell X-ray laser. The basic concept of such a laser involves the propagation of the electron beam in a material which converts electron energy into appropriate pump photons.

The development of keV X-ray lasers based on inner-shell atomic transitions requires extremely fast energy deposition on a target in order to effectively compete with the inherently fast (0.1 - 20 fs) atomic decay processes. The duration of ultrahigh peak power laser systems is now reaching this timescale. In principle, these systems can be used to produce sufficient duration and energy X-rays or electrons for pumping inner-shell transitions. In this paper X-ray laser schemes are described in which the Coster-Kronig Auger-process is the dominant lower level decay mechanism. Such systems have inherently short lower level lifetimes and it is shown that under certain conditions they can be inverted with excitation by energetic electrons as well as X-rays. They are therefore relatively immune to secondary electron ionization.

The experimental results on generation of high gain-length product for 13.5 nm radiation from 2 - 1 transition in hydrogen-like Li III ions are presented for 1 micrometers subpicosecond pumping laser. The comparison with earlier results, obtained with 0.25 micrometers subpicosecond pumping laser, is discussed in terms of gain generation efficiency. The results for discharge created pre-plasma in L equals 4 mm and L equals 14 mm microcapillaries are also presented.

We propose a Ni-like Kr soft x-ray laser operating on the 4d¹S₀ - 4p¹P₁ transition at 32.8 nm by a novel excitation scheme in which optical field ionization prepares Ni-like Kr ion with almost 100% abundance and inverse bremsstrahlung heats the plasma efficiently. The numerical calculation based on the hot-spot model predicts that ultra-high gains greater than 1000 cm^-1 in the low density plasma (N_e equals 5.6 X 10¹⁹ cm^-3) for a driving laser intensity of 1.75 X 10¹⁶ W/cm². With a longitudinally pumping, output energy greater than 1 (mu) J and an energy efficiency of approximately 10^-5 are expected with 200-torr Kr gas.

Millijoule-level laser pulses and multi-milliwatt average power have been obtained for the first time from a table-top soft x-ray laser. Laser pulses with energy up to 1 mJ and an average power of approximately equals 3.5 mW were generated at a wavelength of 46.9 nm by single pass amplification in a 34.5 cm long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft x-ray amplification in a highly saturated regime, resulting in very efficient energy extraction. An average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. The spatially coherent average power emitted by this 26.5 eV laser is comparable to that generated at this photon energy in a similar bandwidth ((Delta) (lambda) /(lambda) equals 10^-4) by a third-generation synchrotron beam-line, which its peak spatially coherent power exceeds that of the undulator by several orders of magnitude. With an estimated peak spectral brightness of approximately equals 1 X 10²³ photons/(s mm² mrad² 0.01% bandwidth) this table-top laser is one of the brightest soft x-ray sources to date.

Capillary discharge plasmas were generated with current pulses up to 200 kA and 10 - 90% rise time of about 10 ns using a water dielectric Blumlein generator. Time resolve spectra and pinhole images of the plasma columns generated in ceramic capillaries are presented. The data indicates that the plasma obtain in ceramic capillaries are hotter than those generated in polyacetal capillaries. The maximum degree of ionization, plasma dimensions and time evolution are in general agreement with model computations and show the generation of plasma columns 250 - 300 micrometers diameter with temperatures is congruent to 250 eV.

The dynamics of a capillary discharge experiment for collisional excitation x-ray laser in Ne-like Argon is investigated. Time resolved hard XUV radiation measurement (E > 80 eV) show fast rise time (approximately 5 ns) of the emitted radiation. The XUV radiation peak time, relative to the beginning of the current pulse, was measured for various initial Argon pressures and main current pulses. The measured data is in good agreement with our 1D-MHD code. XUV spectra from our fast capillary discharge indicate that high ionization stages of Ar ions are presented in the plasma (Ar⁶⁺ - Ar¹¹⁺) and that the Ar ionization abundance can be controlled by the main current pulse magnitude. The measured lines intensifies as a function of the main current pulse amplitude indicates that 40 - 50 kA (t_1/4 equals 51 ns) are needed for achieving maximum abundance of Ar(IX) in a 4 mm diameter Polyimid capillary with initial Ar pressure of 600 mT.

A fast capillary discharge for intense soft X-ray emission is under investigation for EUV lithography and X-ray laser studies. A high current pulse (60 kA) is driven into a 1 mm diameter polyethylene capillary and creates a highly ionized plasma of carbon produced by wall ablation. Input power density as high as 70 GW.cm^-3 was injected. The X- ray emission is analyzed with a spectrometer efficient from 20 to 450 angstroms. Time resolved spectra are recorded with a gated MCP. A visible spectrometer was used to measure the Stark broadening of the CIV line at 580 nm and to determine the electron density at low input energy. The FLY code was used to evaluate the electron temperature from the intensity ratio of two lines emitting in the same wavelength domain.

Since the pioneer work of J.J. Rocca et al. in the development of discharge pumped soft X-ray lasers, different experimental setup have been proposed at fast capillary discharge drivers. At GREMI, we developed a fast high voltage pulsed generator driving a Z-pinch capillary discharge capable of producing a plasma highly emissive in XUV. This work reports on the study of a Blumlein Pulse Forming Line generator. A model has been developed for a better understanding of produced currents. Comparison between experimental and calculated current waveforms show a good agreement. Short rate of rise time pulses as high as 1.2 X 10¹² A(DOT)s^-1 have been achieved. Modeling of the discharge led to the conclusion that a Z- pinch effect could occur. Argon time-integrated spectra showed that highly ionized argon lines, up to Ar XII, have been produced.

Kr-like ions are good candidates for FUV lasing since they can be produced in plasmas quite easily. We present results from a spectroscopic investigation of Y IV emission from a high current density, cold cathode reflex discharge. The Y II to Y V emission is recorded in the 200 - 3000 angstroms range using photometrically calibrated spectrometers, while the emission of trace aluminum ions serves for plasma diagnostics. The intensities of the Y IV 4d - 5p and 5s - 5p transitions strongly increase relative to lines from Y II and Y III with increasing plasma current. The spectra studied here are obtained at a current density of 1.75 A/cm². Experimental Y IV intensity ratios spanning several excited configurations are compared with collisional radiative predictions of the HULLAC atomic physics package. Good agreement is found for the measured and predicted ratios of 4p⁵5p to 4p⁵5s level populations per statistical weight. Finally, the response of the Kr-like system to a fast, transient excitation pulse is examined using the RADEX code. Large transient gains are predicted for several 5s - 5p transitions in Y IV, Zr V, Nb VI and Mo VII.

We report the first applications of a soft x-ray laser in materials processing and diagnostics. We have focused a Ne- like Ar capillary discharge laser ((lambda) equals 46.9 nm) operating at a repetition rate of 1 Hz using a spherical Si/Sc multilayer coated mirror. The energy density obtained significantly exceeded the threshold for the ablation of metals. Ablation of stainless steel, aluminum and brass samples is reported. The laser ablation patterns on brass were used in combination with ray tracing computations to characterize the focused beam. The radiation intensity within the 2 micrometers central region of the focal spot is estimated to be is congruent to 10¹¹ W/cm², with a corresponding energy density of is congruent to 100J/cm². In a separate experiment we have conducted laser reflectometry measurements for a number of different samples. These measurements resulted in the characterization of the reflectivity of Si/Sc multilayer mirrors as a function of angle and in the determination of optical constants for Si, GaP, InP, GaAs, GaAsP and Ir. The measurements of InP and GaAsP resulted in the first experimental values to our knowledge for these materials at this wavelength.

We report the first demonstration of plasma imaging and interferometry using a tabletop soft x-ray laser. A very compact capillary discharge Ne-like Ar laser operating at 46.9 nm was used as a backlighter to study the evolution of the plasma of a micro-capillary discharge. The resulting series of shadowgrams show the plasma created by ablation of the walls of the evacuated micro-capillary rapidly evolves from a non-uniform initial state into a symmetric column with minimum density on axis. In a second experiment we took advantage of the good spatial coherence of the capillary discharge laser to perform interferometry measurements in the plasma of a pinch discharge. In this experiment the laser was used in combination with a wavefront division interferometer based on Lloyd's mirror to map the electron density distribution in the cathode region of the discharge.

We report the measurement of the optical constants of Si, GaP, InP, GaAs, GaAsP and Ir at a wavelength of 46.9 nm (26.5 eV). The optical constants were obtained from the measurement of the variation of the reflectivity as a function of angle utilizing, as an illumination source, a discharge pumped 46.9 nm table-top laser operated at a repetition rate of 1 Hz. These measurements constitute the first application of an ultrashort wavelength laser to materials research.

We have performed soft x-ray interferometry of a laser- created plasma using a novel amplitude division interferometer in combination with a tabletop capillary discharge laser operating at 46.9 nm. The soft x-ray interferometer utilizes diffraction gratings as beam splitters in a Mach-Zehnder configuration to generate high contrast interferograms over a large field of view. An advantage of this interferometer scheme is that it can be used at any of the wavelengths covered by presently available soft x-ray lasers. This table-top system was used to probe a large scale (approximately 3 mm long) plasma crated by a Nd:YAG laser. The short wavelength of the probe laser has allowed mapping of the electron density in plasma regions with density gradients stepper than those that could be probed with the fourth harmonic of Nd:YAG for a plasma of this length.

An ultra-short pulse CPA laser system for x-ray laser driver has been developed with a combination of Ti:sapphire front end and Nd:glass rod amplifiers. This laser system has two beam outputs and each beam line produces 20J pre pulse and 20J main. This laser system is designed for x-ray laser pumping driver, especially for transient gain scheme. The new transient gain x-ray laser scheme with thin foil metal targets has been proposed. This scheme has higher laser energy efficiency and less x-ray laser refraction effect and makes possible to generate shorter x-ray wavelength with a compact table-top sized laser system. The electron temperatures of plasmas heated with a short pre pulse and short main pulse have been calculated with 1D hydrodynamic code and obtained electron temperature higher than 1 keV and 20 J laser energy. X-ray laser propagation is also calculated with gain guiding effect.

Theoretical analysis and preliminary experiment on ionization instability of intense laser pulses in ionizing plasmas are presented. The ionization instability is due to the dependence of the ionization rate on the laser intensity and scatters the laser energy off the original propagation direction.

Metal targets covered by micrometer layers of metal- phthalocyanines are studied here. An increase in EUV yield due to optimized absorption of the laser field is reported. Effects of high-temperature plasma rapid expansion (velocity about 10⁶ cm/s) were observed. Moderate power nanosecond and picosecond neodymium lasers are used to product an incident intensity of 10¹¹ to 10¹³ W/cm² on the targets. The plasma electron density was measured by fitting observed spectral profiles to theoretical profiles. Collisional, Doppler, and Stark broadening mechanisms were considered in the calculations. Our measurement technique makes it possible to determine the electron density and temperature dependence on distances from the target surface from 1 mm (where N_e equals 2.0 (+/- 0.5)10¹⁸ cm^-3 and T_e equals 14 eV are measured for aluminum plasma) up to approximately 5 mm (where N_e <EQ 10¹⁷ cm^-3). Electron temperature was measured by comparing intensities of spectral lines, belonging to the ions having a different degree of ionization. Preliminary experiments show that conversion efficiency for molecular coated targets is greater by a factor of approximately 1.5 than measured from bulk solid metal targets.

Our earlier papers showed that laser gain is possible on the innershell 2s-2p transitions of Ne-like ions. This result was confirmed by more comprehensive calculations. In more recent works it was shown that laser gain is also possible on transitions between highly excited states. We summarize these results and present atomic-kinetic calculations for gain values along the Ne-like sequence for argon, iron, krypton, silver and xenon. For these ions the optimum plasma conditions are found for all possible laser transitions. We have also found some unknown laser transition in Ni-like tantalum and the optimum plasma parameters for laser action on these transitions.

We present new progress in the optimization and understanding of the transient collisional pumping scheme using an ultra-short sub-ps heating pulse. The effect of traveling-wave irradiation in enhancing the lasing output of the 4d-4p Ni-like Ag line is studied in detail. A new irradiation scheme using a frequency-doubled 600 ps pulse to preform a plasma is tested. Strong lasing is also obtained on a new line at 16.05 nm that we identify to a 4f-4d transition in Ni-like Ag. Finally we review our recent work in the development of applications of the 21.2 nm zinc laser for imaging or exciting matter. New experiments include the probing of a plasma by imaging Fresnel interferometry and a first attempts to demonstrate two-photon ionization in a xenon gas.

Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.

Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.

The numerical study is carried out of amplified spontaneous emission (ASE) in an x-ray laser. Its gain zone appears in the laser-produced plasma at the prepulse irradiation of a thick optimally bent target and is characterized by an asymmetrical profile of the electron density. An effect of ASE transverse spatial coherence improvement is particularly investigated that takes place as a result of potential formation of a convex transverse density profile instead of a typical concave one. At the convex density profile one can obtain the partially coherent ASE whereas at concave density profile the ASE is practically non-coherent.

We describe the phase-matched high-order-harmonic generation of femtosecond Ti:sapphire laser pulses in a self-guiding channel. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. The harmonics around the 49th harmonic (16 nm) were enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High- harmonic conversion efficiency of 10^-6 was obtained, producing > nJ harmonics in the cutoff region around the 49th harmonic. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region.