We have extensively studied plasma formation, implosion and radiative characteristics of two types of compact load configurations before: compact multi-planar and cylindrical wire arrays1, 2. The experiments with such loads were performed on the 1-1.7 MA Zebra generator at UNR1, 2, and also on the Saturn generator at SNL3, 4. It was demonstrated that such wire arrays of 3-6 mm diameter/width are excellent radiation sources and can be useful for novel ICF applications2, 3, however currently there is very limited research on compact wire arrays. During the last few years, there is also a renewed interest to study hard x-ray non-thermal inner-shell emission from Z-pinch plasmas of high-atomic-number materials on Sandia's Z and NRL Gamble II generators. We present here the analysis of three experiments with W wire arrays performed on the 1 MA Zebra in the same day. Two compact cylindrical wire arrays (CCWA) of 6 mm diameter with a similar array mass $(\mathrm{m}=91-92\ \mu \mathrm{g}/\text{cm})$ and the same number of wires (one with 24 W wires and another with alternating 12 W and 12 brass wires) and one single planar wire array (SPWA, 10 W wires, $\mathrm{m}=74\ \mu \mathrm{g}/\text{cm}$) are considered. A comprehensive set of diagnostics was implemented. We focus mostly on time-gated hard x-ray spectroscopy of “cold” W L-shell lines (1–1.6 Å) from W and W/brass CCWAs recorded from 10 ns before to 15 ns after stagnation and correlations between their relative intensities and x-ray diode and electron beam measurements. The results are compared with W SPWA, warm dense matter results, and modeling. In addition, Cu and Zn $\mathrm{K}\alpha$ lines from W/brass CCWA are investigated in the same spectral range and compared with brass CCWA5. Future work on using “cold” W L-shell lines for electron beam studies is discussed.

Charged particle acceleration has been investigated in a dense plasma focus (DPF) driven by the Hawk pulsed-power generator at the Naval Research Laboratory. Neutron yields of over $5.0\times 10^{10}$ have been measured at a peak current of $\mathrm{I}=670\text{kA}$ ) significantly above the yield expected at this current based on purely I 4.4 scaling from conventional DPFs. The high inductance (607 nH) and associated high voltage (640 kV) and fast rise time $(1.2 \mu \mathrm{s})$ of Hawk are unusual for a DPF driver, as is the initialization of the DPF using local injection of neutral gas and plasma into a vacuum chamber, rather than a conventional neutral gas fill. This method of initialization allows for the selection of an extensive array of initial conditions. A subset of those initial conditions has been explored in conjunction with multi-dimensional MHD simulations to affect the energy of the accelerated ions, as measured by an ion multi-pinhole camera, the neutron yield, as measured by rhodium-foil activation counters, and the end point energy of the emitted neutrons, as measured by neutron time-of-flight detectors.

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.

We present polarization measurements of the 2p(5)3d (1)P1 -> 2p(6) S-1(0) resonance line of Ne-like Mo32+, commonly known as 3C, as well as the 2p(5)3d D-3(1) -> 2p(6) S-1(0) intercombination line, commonly known as 3D, as a function of the electron impact energy ranging from near threshold to 6 times threshold (2.75-15 keV). In addition, the x-ray line polarization of the 2p(5)3p D-3(2) -> 2p(6) S-1(0) electric quadrupole forbidden line was measured at 3 and 4 keV. Excitation of these lines was achieved with the LLNL EBIT-I electron beam ion trap. The degree of polarization of the emission lines was derived by comparing spectra recorded with a flat Ge (111) crystal spectrometer, which is sensitive to polarization, to the spectra recorded by an x-ray calorimeter, which is insensitive to polarization. The experimental measurements were compared to theoretical predictions produced using the Flexible Atomic Code, and good agreement was found.

We begin with a brief review of the work on x-ray spectroscopy and imaging of tungsten (W) single wires, wire arrays, X-pinches and their applications [1]–[6], and move on to the most recent results. Two sets of the new experiments performed in 2017 on two university-scale Z-pinch generators with different architecture will be considered: W Double Planar Wire Arrays (DPW A) at the University of Michigan's (UM) low-impedance Linear Transformer Driver (LTD) MAIZE generator and W X-pinches at the University of Nevada, Reno's (UNR) high-impedance Marx bank Zebra generator. Though a comprehensive set of diagnostics was implemented in both types of experiments, here we will focus mostly on x-ray spectroscopy and imaging results and their interpretation, as well as applications to electron beam studies and spectropolarimetry of W pulsed-power plasmas. Two axially-resolved spectrometers were implemented: the first was the x-ray convex crystal spectrometer with a potassium acid phthalate (KAP) crystal (2d = 26.63 Å and R=51 mm) for the spectral region between 4 - 10Å and another spectrometer with a lithium fluoride (LiF) convex crystal (2d = 4.027 Å and R=25.4 mm) for hard x-ray radiation in a spectral band 1 - 2.4 Å. X-ray time-integrated pinhole cameras recorded the images with cutoff energies of 1.4 and 3.5 keV. The very complex soft x-ray M-shell W spectra recorded by the KAP spectrometers are modeled and studied in detail to provide valuable information about “hot” W plasmas and electron beams. The analysis of the hard L-shell W spectra recorded by the LiF spectrometers from much cooler W plasma as well as from the anode is applied to the studies of electron beams, and to explore the possible development of hard x-ray spectropolarimetry of W plasmas. In this regard, the results for W DPW As on UM-MAIZE and W X-pinches on Zebra at UNR are evaluated, compared, and discussed.

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.

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.