We present a broad study of linear, clustered, noble gas puffs irradiated with the frequency doubled (527 nm) Titan laser at Lawrence Livermore National Laboratory. Pure Ar, Kr, and Xe clustered gas puffs, as well as two mixed-gas puffs consisting of KrAr and XeKrAr gases, make up the targets. Characterization experiments to determine gas-puff density show that varying the experimental parameter gas-delay timing (the delay between gas puff initialization and laser-gas-puff interaction) provides a simple control over the gas-puff density. X-ray emission (>1.4 keV) is studied as a function of gas composition, density, and delay timing. Xe gas puffs produce the strongest peak radiation in the several keV spectral region. The emitted radiation was found to be anisotropic, with smaller X-ray flux observed in the direction perpendicular to both laser beam propagation and polarization directions. The degree of anisotropy is independent of gas target type but increases with photon energy. X-ray spectroscopic measurements estimate plasma parameters and highlight their difference with previous studies. Electron beams with energy in excess of 72 keV are present in the noble gas-puff plasmas and results indicate that Ar plays a key role in their production. A drastic increase in harder X-ray emissions (X-ray flash effect) and multi-MeV electron-beam generation from Xe gas-puff plasma occurred when the laser beam was focused on the front edge of the linear gas puff.

Linearly polarized spectral emission is induced by non-Maxwellian electrons with anisotropic velocity distributions through collisional processes with ions in plasmas. X-ray line polarization is a viable diagnostic tool for the study of these electrons, which has been tested in controlled settings, e.g., the electron beam ion trap, but due to strong depolarization effects, has been relatively unexplored in laboratory-produced high-energy-density plasmas. This paper explores the X-ray line polarization for the study of Ne-like molybdenum X-pinch plasmas. Previous polarization studies have focused mostly on K-shell spectral lines, whereas the current paper expands on these studies by investigating L-shell line polarization. The X-pinch is favorable due to its high-density plasma formation, high radiation yield, fixed hotspot location, and reliable electron beam production. L-shell X-ray line polarization and spectral emission of Mo X-pinches were investigated through dual alpha-quartz (2d = 6.687 angstrom) crystal spectropolarimetry with axial and radial spatial resolution. In particular, line polarization is evaluated through a comparative analysis of relative line intensities observed over a range of Ne-like Mo transitions simultaneously recorded with two spectrometers with different sensitivity to polarization. An appreciable degree of X-ray line polarization (up to 15%) is observed in the Ne-like Mo plasmas and analyzed using depolarization effects. These results are the first polarization measurements in high-Z pulsed power plasmas.

Since the first wire-array tungsten (W) experiments on Z at SNL, where the record x-ray power of 200 TW and x-ray yield of nearly 2 MJ were achieved 1 , such arrays were actively studied and considered for various applications including inertial confinement fusion (ICF) 2 . More recently, W Double Planar Wire Arrays (DPWAs) were suggested and tested for indirect drive ICF 3 . DPWA consists of two parallel planes of wires of the same (uniform) or different (mixed) wire materials. W DPWAs have previously demonstrated the highest (among PWAs) radiation yield (up to 30 kJ), compact size (few mm), and strong electron beams at the Universityscale high-impedance generator 3 . During the last few years we have reported on the outcome of the experiments with uniform and mixed Al and stainless steel DPWAs on the University of Michigan’s low-impedance Linear Transformer Driver (LTD) MAIZE generator. Here we present the results of the most recent campaign with W and W/Al DPWAs recorded using filtered x-ray diodes, x-ray spectrometers and pinhole cameras, and a twelve frame shadowgraphy system. For the first time, implosion of W wire arrays on LTD generator in USA was demonstrated and analyzed. In particular, uniform W and mixed W/Al DPWAs with a mass up to 87 μg arranged in various configurations were successfully imploded at the current of 0.5 MA during $\sim 210$ns. The most interesting results were obtained with W/Al DPWAs where a long-term standing shock wave was consistently formed at the W side, which was also observed at the high-impedance Zebra generator at UNR. In addition, soft (4-7 Å) and hard (1-2.4 Å) line radiation was substantially suppressed by including the Al plane.

One of the first questions that should be considered to adequately describe radiation of high Z ions from High Energy Density (HED) plasmas and from even more extreme environments is: “How can we calculate and, most importantly, validate atomic properties and spectra of complex, high Z, highly charged ions in non-equilibrium as a function of plasma parameters and electron distribution function?” To contribute to answering this challenging question, we present a comprehensive experimental and theoretical study of the line emission from Mo HED plasmas benchmarked with LLNL EBIT data. Though there were some studies of Mo line radiation from pulsed power plasmas produced at UNR, Cornel University, and SNL before, there was no benchmarking with EBIT data. The analysis of X-ray spectra from high-Z HED plasma is always a very challenging topic because of contributions from numerous ionization stages and from multiple atomic processes in the plasma, such as, for example, dielectronic recombination, that is impossible to resolve in a rather narrow spectral range. We performed two types of Mo experiments at both the LLNL EBIT and at the Z-pinch generator at NTF/UNR to study line radiation in a spectral range between 3.5 and 5.5 Å. In particular, benchmarking experiments at the LLNL EBIT with Mo ions produced at electron beam energies from 2.75 keV up to 15 keV allowed us to break down these very complicated spectra into spectra with only few ionization stages and to select processes that influence them. The EBIT data were recorded using the EBIT Calorimeter Spectrometer and a crystal spectrometer with a Ge crystal. X-ray Mo spectra and pinhole images were collected from Z-pinch plasmas produced from various wire loads to provide different levels of opacity and electron beam effects. Non-LTE modeling and high-precision relativistic data were used to analyze L-shell Mo spectra from both experimental campaigns: an almost monoenergetic electron distribution function (EDF) for EBIT data and Maxwellian and non-Maxwellian EDFs for modeling of HED plasma spectra. The influence of different plasma processes including electron beams on Mo line radiation is summarized.

Many aspects of physical phenomena occurring when an intense laser pulse with subpicosecond duration and an intensity of 10^{18}-10^{19}W/cm^{2} heats an underdense plasma in a supersonic clustered gas jet are studied to determine the relative contribution of thermal and nonthermal processes to soft- and hard-x-ray emission from debris-free plasmas. Experiments were performed at the University of Nevada, Reno (UNR) Leopard laser operated with a 15-J, 350-fs pulse and different pulse contrasts (10^{7} or 10^{5}). The supersonic linear (elongated) nozzle generated Xe cluster-monomer gas jets as well as jets with Kr-Ar or Xe-Kr-Ar mixtures with densities of 10^{18}-10^{19}cm^{-3}. Prior to laser heating experiments, all jets were probed with optical interferometry and Rayleigh scattering to measure jet density and cluster distribution parameters. The supersonic linear jet provides the capability to study the anisotropy of x-ray yield from laser plasma and also laser beam self-focusing in plasma, which leads to efficient x-ray generation. Plasma diagnostics included x-ray diodes, pinhole cameras, and spectrometers. Jet signatures of x-ray emission from pure Xe gas, as well as from a mixture with Ar and Kr, was found to be very different. The most intense x-ray emission in the 1-9 KeV spectral region was observed from gas mixtures rather than pure Xe. Also, this x-ray emission was strongly anisotropic with respect to the direction of laser beam polarization. Non-local thermodynamic equilibrium (Non-LTE) models have been implemented to analyze the x-ray spectra to determine the plasma temperature and election density. Evidence of electron beam generation in the supersonic jet plasma was found. The influence of the subpicosecond laser pulse contrast (a ratio between the laser peak intensity and pedestal pulse intensity) on the jets' x-ray emission characteristics is discussed. Surprisingly, it was found that the x-ray yield was not sensitive to the prepulse contrast ratio.

Two new approaches of (i) simultaneous study of implosion and radiative characteristics of different materials in wire array Z-pinch plasmas in one shot and (ii) investigation of larger sized wire arrays (to enhance energy coupling to plasmas and provide better diagnostic access) were developed in experiments with 1.5–1.8 MA Zebra with a Load Current Multiplier. In particular, the larger sized multi-plane Planar Wire Arrays with two outer planes placed at 9 and 15 mm from each other and then as far as at 19 mm (compared with 6 mm studied before at standard 1 MA current) and with a modified central plane with 8 to 12 empty slots were investigated. Though K-shell Al and L-shell Ni, Cu plasmas have similar electron temperatures and densities, the ablation dynamics and radiation of Al and Ni, Cu planes are somewhat different, which was investigated in detail using the full set of diagnostics and modeling. Advantages of using such wire arrays at higher currents to study plasma flow and radiation from different materials and jets are highlighted.

A gas jet containing a mixture of monomers and clusters was characterized and studied as an x-ray radiation source produced by a TW-class laser pulse. Gas jet parameters such as average density and cluster size were measured at the UNR Radiation Physics Laboratory using both optical interferometry and Rayleigh scattering techniques, respectively. Several noble gases were used in the gas jet: Ar, Kr, and Xe. Additionally, mixtures of two or three of those gases were also tested. By changing the gas jet backing pressure as well as the gas delay time between jet initiation and laser interaction with the jet, both the density and cluster size of the gas jets can be varied. Having control over the composition, density, and cluster size of the gas jets is important when considering them as targets for intense laser pulses. Our gas jets were irradiated with the 1057 nm short pulse (350 fs) UNR Leopard laser with an intensity of 1019 W/cm2 in the focus spot. Time resolved diagnostics included filtered Si-diode detectors (1.4–9 keV), filtered absolutely calibrated PCDs (>2.4 keV), and Faraday cups. An x-ray spectrometer and two three-channel x-ray pinhole cameras provided time integrated diagnostics on the gas jet plasma. Anisotropy of x-ray radiation with respect to laser beam polarization was observed in all spectral regions. The coefficient of conversion of laser energy into x-rays was measured with a maximum of 10−3. Most importantly, the mixtures of two or three gases each produced higher x-ray yields than the pure gases. Non-LTE modelling and a molecular dynamics (MD) code have been employed to determine plasma and cluster parameters. Electron temperatures and densities of the laser plasma of the mixed gases were higher than the pure gases.

Experiments of x-ray emission from Ar, Kr, and Ar/Kr gas jet mixture were performed at the UNR Leopard Laser Facility operated with 350 fs pulses at laser intensity of 2 x 10(19) W/cm(2) and 0.8 ns pulses at an intensity of 1016 W/cm(2). Debris free x-ray source with supersonic linear nozzle generated clusters/monomer jet with an average density of >= 10(19) cm(-3) was compared to cylindrical tube subsonic nozzle, which produced only monomer jet with average density 1.5(-2) times higher. The linear (elongated) cluster/gas jet provides the capability to study x-ray yield anisotropy and laser beam self-focusing with plasma channel formation that are interconnecting with efficient x-ray generation. Diagnostics include x-ray diodes, pinhole cameras and spectrometers. It was observed that the emission in the 1-9 keV spectral region was strongly anisotropic depending on the directions of laser beam polarization for sub-ps laser pulse and supersonic linear jet. The energy yield in the 1-3 keV region produced by a linear nozzle was an order of magnitude higher than from a tube nozzle. Non-LTE models and 3D molecular dynamic simulations of Ar and Kr clusters irradiated by sub-ps laser pulses have been implemented to analyze obtained data. A potential evidence of electron beam generation in jets' plasma was discussed. Note that the described debris-free gas-puff x-ray source can generate x-ray pulses in a high repetition regime. This is a great advantage compared to solid laser targets. (C) 2016 Elsevier B.V. All rights reserved.

Gas jets accelerated through a linear supersonic and a conical nozzle, comprising a monomer/cluster mix, were characterized at UNR using a Mach-Zehnder type interferometer and Rayleigh scattering. A comparison of the two nozzle types is presented, showing that the linear nozzle produces gas jets of an order of magnitude denser than the conical nozzle. The linear gas jets of Ar, Kr, and Xe as well as triple mixtures with different percentages of each of the aforementioned gases were characterized. The densest gas jets used Ar as the target gas, while the least dense jets came from Kr. Cluster radii of the pure gases were measured, and Xe gas jets were found to produce the largest gas clusters. A study of X-ray generation by gas jet-laser plasma was performed at the UNR Leopard laser (1.057 μm, 350 fs, ∼1019 W/cm2) on the linear nozzle. The gas jets were irradiated with a high-intensity sub-ps laser pulse. An absolute X-ray output of the laser-gas jet interactions measured by the calibrated PCDs is presented and show that triple mixtures of Xe, Kr, and Ar each exhibited a higher X-ray yield compared to the pure gases. A strong anisotropy of X-ray radiation with respect to laser beam polarization direction is observed in all the gas jets. In fact, this anisotropy is different in three spectral regions (>1.4, 3.5 and 9 keV).

Summary form only given. Some aspects of physical phenomena occurring when an intense laser pulse with sub-ps duration and intensity > 1018-1019 W/cm2 heats under-dense plasma in supersonic clusters/gas jet from gas-puff system were studied to determine the relative contribution of these processes to soft and hard x-ray emission from debris-free plasmas. Experiments were performed at the UNR Leopard laser operated with 15J / 350 fs and different pulse contrast (107 or 10-5). The supersonic linear (elongated) nozzle generated Ar and Kr clusters/monomer jets as well as jets with Ar/Kr or Ar/Kr/Xe mixtures with density > 1019 cm3. In prior laser heating experiments, all jets were probed with optical interferometry and Rayleigh scattering to measure jet density and cluster distribution parameters. The elongated jet provides the capability to study laser beam self-focusing in plasma that is interconnecting with efficient x-ray generation. The laser radiation flux density in the focal spot was up to 2 × 1019 W/cm2. Plasma diagnostics included x-ray diodes, pinhole cameras and spectrometers. Jet signatures of x-ray emission from pure Ar carrier gas as well as from mixture with lower Kr concentration and Ar/Kr mixture with adding Xe gas, were found to be very different. The most intense x-ray emission was observed in a wide spectral region 1-9 keV from these gas mixtures compared to pure Ar or pure Kr carrier gases. Also, this x-ray emission was strongly anisotropic depending on the direction of laser beam polarization. Non-LTE models have been implemented to analyze the x-ray spectra. It was shown that Ar/Kr jet plasma was hotter than that from pure Kr. X-ray spectra of Ar carrier gas (only “cold” Ka line) were similar to the spectra obtained early from subsonic tube nozzle (no clusters in jet). It can be explained by the fact that Ar in Ar/Kr- mixture did not generate clusters as opposed to Kr. Evidence of electron beam generation in supersonic jets' plasma was found. Influence of fs laser pulse contrast level on x-ray emission characteristics is discussed. Future research will focus on the study of cluster formation in linear supersonic jets with Ar carrier gas and Kr and Xe impurities, varying its parameters for optimization of x-ray yield and power. Another goal is to determine optimal conditions for possible application of x-ray source with gas mixture jets at larger sub-ps laser facilities.