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In previous studies using the University of Nevada, Reno's (UNR's) high-impedance Zebra Marx generator (1.9 Ω, 1.7 MA, 100 ns), Double Planar Wire Arrays (DPWAs) proved to be excellent radiators, and Double Planar Foil Liners (DPFLs) proved useful for future inertial confinement fusion applications. This article presents the results of joint UNR/UM (University of Michigan) experiments with aluminum (Al) DPWAs, Al DPFLs, and tungsten (W) DPWAs using UM's Michigan Accelerator for Inductive Z-Pinch Experiments (MAIZE) generator, a low-impedance Linear Transformer Driver (LTD) (0.1 Ω, 0.5–1 MA, and 100–250 ns). The main goals of this study were twofold: the first was a pioneering effort to test whether a relatively heavy Al DPFL could successfully be imploded on a low-impedance university-scale LTD like the MAIZE generator, and, if so, to analyze the results and make comparisons to the optimized, lighter DPWA configurations that have been previously studied. The DPWAs consisted of two planes of micrometer-scale diameter Al or W wires, while the DPFLs consisted of two planes of micrometer-scale thickness Al foils. Diagnostics include filtered Si-diodes, an absolutely calibrated filtered PCD, x-ray pinhole cameras, spectrometers, and gated optical self-emission imaging. The implosion dynamics and radiative properties of Al DPWAs and DPFLs and W DPWAs on the MAIZE LTD are discussed and compared. Time-dependent load inductance calculations derived from measurements of the load current and a MAIZE circuit model provide a relative measurement of pinch strength. In experiments on MAIZE, W planar wire arrays exhibited a higher peak load inductance throughout the pinch than Al DPWAs and DPFLs, while x-ray pulses from Al DPFLs had the longest emission duration.

The presence of collimated 1.5–2.0 MeV electron beams in 0.9–1.0 MA high-Z X-pinch plasmas leads to the generation of an anisotropic hard x-ray radiation (more than 50–100 keV) in the crossing point of the wires, with a 1–2 mm size source and a time duration of 100–150 ns. The side-on measured electron temperature is higher than the end-on. Introducing of a synchrotron radiation in a side-on direction is used for the explanation of the phenomenon. The new results of time-resolved 0.16–10 keV x-ray imaging and spectroscopic study of the development of jets and hot spots in X-pinches are presented. The plasma parameters varied significantly for consecutive x-ray bursts (hot spots) during the same X-pinch discharge. A minimum duration of x-ray pulses was 1.1 ns. The x-ray energy output was up to 10–11 kJ, and a maximum power can reach up to 3×1011 W. The X-pinch coupled with the x-ray glass-capillary converter for collecting and focusing of x rays can generate soft x-ray energy density up to (1–10) J/cm2, an...

The University of Nevada, Reno has developed a collisional-radiative atomic kinetics model that has successfully described the spectra produced by widely different multicharged ion experiments. The application of the model to experiments that highlight the importance of particular kinetics effects, including time-dependence and non-Maxwellian electron distribution functions, will be presented. These experiments include ion beam collision and high-temperature femtosecond laser and X-pinch plasma experiments. Adequate descriptions of ion beam collision spectra and x-ray yield require time-dependent treatment of radiative cascades from high-n capture states. Spectra from plasmas with significant fractions of hot electrons, such as those produced by fs lasers and X-pinches, require calculation of hot electron effects on collisional rates. The kinetics model presented here extracts as much information as possible from advanced experimental diagnostics such as time- and space-resolved spectral measurements.

The complex measurements with spectral, temporal, and spatial resolution are very important in studies of properties of radiation from hot dense plasmas (Te up to 1.5 keV, Ne up to 1022 cm−3) generated by a pulse-powered Z-pinch machine with Imax∼0.9–1.0 MA and a current risetime of 100 ns. Plasma radiation from Ti, Fe, Mo or W X-pinches has been studied. The analysis and comparison of 1D TGS images in a spectral region 20–80 A with time-integrated and time-resolved images from pinhole cameras show differences in sizes and positions of images depending on the wavelength. A comparison of these data with data obtained from absolutely calibrated PCD, XRD, and the bolometer shows the input of radiation from this spectral range into the total yield. The temporal behavior and spatial structure were found to be different in keV and sub-keV regions along an anode–cathode gap.

X-ray spectra of high energy density (HED) Z-pinch plasmas have been analyzed by means of a comprehensive kinetic modeling. A new diagnostic, x-ray spectropolarimetry, was applied to study anisotropy of Z-pinch plasma. This diagnostic is based on theoretical modeling of polarization-dependent spectra measured simultaneously by spectrometers with different sensitivity to polarization. Specifically, K-shell emission from Ti X-pinches was recorded simultaneously with identical LiF crystal spectrometers with the dispersion plane perpendicular and parallel to the discharge axis. Spectroscopic results from seven Ti X-pinch shots have been analyzed. Similar K-shell Ti polarization-dependent spectra generated by a quasi-Maxwellian electron beam at the LLNL EBIT-II electron beam ion trap have been studied. Further, the EBIT-II M-shell W spectra have proved to be important in the development of M-shell diagnostics of HED Z-pinch plasma. The advantages provided by electron beam ion trap data in the interpretation of HED Z-pinch spectra will be presented.

X-ray spectropolarimetry is a powerful new tool for investigating the anisotropy of high-temperature plasmas. It is sensitive to the energetic electron distribution function and magnetic field, which, in general, have not been measured adequately. This diagnostic is based on the theoretical modeling of x-ray line polarization-dependent spectra measured simultaneously by spectrometers with different sensitivities to polarization. The increasing sophistication of spectroscopic monitoring of high-temperature plasmas has led to the necessity to account for both non-Maxwellian electron distribution functions and polarization properties of lines. This diagnostic can be applied to a broad range of plasmas, from low-density astrophysical plasmas to dense z-pinch plasmas. This work shows how x-ray spectropolarimetry complements the usual spectroscopic monitoring of hot plasmas and demonstrates the importance of accounting for x-ray line polarization in plasma diagnostics.

A wide variety of x-ray and extreme ultraviolet (EUV) diagnostics are being developed to study z-pinch plasmas at the Nevada Terawatt Facility at the University of Nevada, Reno. Time-resolved x-ray/EUV imaging and spectroscopy, x-ray polarization spectroscopy, and backlighting will be employed to measure profiles of plasma temperature, density, flow, and charge state, and to investigate electron distribution functions and magnetic fields. The instruments are state-of-the-art applications of glass capillary converters (GCC), multilayer mirrors (MLM), and crystals. New devices include: a novel GCC-based two-dimensional imaging spectrometer, a six-channel crystal/MLM spectrometer (“polychromator”) with a transmission grating spectrometer, and two sets of x-ray/EUV polarimeters/spectrometers. An x-pinch backlighter is under development. X-ray polarimeter/spectrometer, a survey spectrometer, a multichannel time-gated x-ray pinhole camera, and filtered fast x-ray diodes have observed the structure of Ti and Fe x pinches driven by a 0.9 MA current. X-ray yield and pulse duration depend sensitively on the wire load.

We have applied a new, more efficient diagnostic technique for studies of extreme ultraviolet (EUV) radiation from multicharged ions interacting with He gas. Following single and double electron capture and one electron capture plus target-ion excitation in He2++He and Ar8++He collisions, the subsequent emitted EUV photons are analyzed with a high-resolution 2.2 m grazing incidence monochromator in conjunction with a new type of glass capillary converter (GCC) specifically designed for the EUV wavelength region. This new optical device images a horizontal cylindrical ion beam segment onto the vertical entrance slit of the monochromator. With this new imaging technique a spectral intensity enhancement of about 10 has been achieved over a distance of about 60 cm. By further optimizing this method an enhancement of the flux density of EUV radiation of about 20–30 is expected. As prototype examples, new advanced EUV spectra arising from He2++He and Ar8++He collisions are presented and discussed.

Radiative properties of plasmas from X pinches with nichrome and conichrome wires have been studied using the presented diagnostic arrangement. The experimental results have been produced at the Cornell XP facility with a peak current of 450 kA and a full width at half maximum pulse duration of 100 ns. The spatially resolved, time-integrated x-ray line spectra from the region of the X-pinch cross point have been recorded using different crystal spectrometers. In particular, K-shell and L-shell x-ray spectra of Ni and Cr have been recorded through different filters in the same pulses for X pinches from two different wire alloys. A nonlocal thermodynamic equilibrium collisional–radiative atomic kinetic model of Ni has been developed to identify the useful diagnostic spectroscopic features and to model experimental spectra. The results of the modeling and radiative properties of different materials from nichrome and conichrome alloys are compared and discussed.

Electron beams in 400 kA peak current Mo and W X-pinches have been studied using 1 ns time-resolution Si diodes to monitor x rays >9 keV. Softer x rays were monitored by photoconducting detectors (PCDs). Three different types of higher energy x-ray bursts were observed. The first type appears to be produced by electrons generated starting at the moment of, or immediately after, the first thermal x-ray burst (typically 40–50 ns after the current start), and Si detector signals last 1–2 ns. The second type of harder x-ray burst occurs 50–80 ns after the current start, lasts 2.5–10 ns, and is typically not correlated with a thermal x-ray burst. These two types of bursts were generated near the cross-wire region. The third type of x-ray burst occurs 70–100 ns after the current start, and is also uncorrelated with PCD signals. The energetic electrons responsible for these x rays are generated for 10–30 ns, and the radiation is produced in the anode region.

Recently, the first X-pinch experiments were performed at enhanced current on the Zebra generator using the Load Current Multiplier (LCM). Previously, X-pinches were found to achieve the highest K-shell electron temperatures at 1 MA on Zebra and these new experiments were performed to determine how the increased current will affect the radiative properties of the X-pinches. A comparison of the linear radiation yields suggests an increase of around 50% for the LCM experiments (∼10 kJ/cm at 1 MA, ∼16 kJ/cm with LCM). These experiments used Cu or Ti alloy (6% Al, 4% V) wires for a first look at X-pinches at 1.5–1.7 MA at the University of Nevada, Reno. For Cu X-pinches, intense L-shell Cu radiation with electron temperatures >300 eV was recorded by both time gated and time integrated spectrometers. The time gated spectra show an evolution of line intensities from the high Rydberg states. For Ti alloy X-pinches, many interesting results from time gated spectra recorded during the Ti experiments were found such as: (i) the appearance of characteristic emission of Ti (wire material) and Fe (hardware material) in different orders of reflection beginning shortly before the first x-ray burst that was recorded for the next 15 ns, (ii) prominent K-shell Al radiation from the Ti alloy experiments despite the low percentage of Al in the alloy, and (iii) K-shell Al radiation that corresponds to 400–550 eV plasmas starting near the first x-ray burst. Time integrated spectra recorded intense K-shell Al radiation and K-shell Ti radiation from higher order reflections.

International audience; 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.

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).

X-ray spectroscopy is a useful tool for diagnosing plasma sources due to its non-invasive nature. One such source is the dense plasma focus (DPF). Recent interest has developed to demonstrate its potential application as a soft x-ray source. We present the first spectroscopic studies of krypton high energy density plasmas produced on a 3 kJ DPF device in Singapore. In order to diagnose spectral features, and to obtain a more comprehensive understanding of plasma parameters, a new non-local thermodynamic equilibrium L-shell kinetic model for krypton was developed. It has the capability of incorporating hot electrons, with different electron distribution functions, in order to examine the effects that they have on emission spectra. To further substantiate the validity of this model, it is also benchmarked with data gathered from experiments on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory, where data were collected using the high resolution EBIT calorimeter spectrometer.

Detection of low-energy ions via Thomson parabola mass analyzer in the absence of any additional electrical systems is examined. Numerous low-energy ions were recorded on UF-4 solid state emulsion films. Kinetic energies between 1 and 4 keV of ions generated by YAG laser focused on Al and Ti targets were obtained using Thomson parabola measurements. Characteristics of ion tracks on the UF-4 detector are discussed in terms of pressure ranges of vacuum chamber. Moreover, differences in charges of ions between this study and previous spectroscopic measurements are discussed.

Protons and multicharged ions generated from high-intensity laser interactions with thin foil targets have been studied with a 100 TW laser system. Protons/ions with energies up to 10 MeV are accelerated either from the front or the rear surface of the target material. We have observed for the first time that the protons/ions accelerated from the front surface of the target, in a direction opposite to the laser propagation direction, are turned around and pulled back to the rear surface, in the laser propagation direction. This proton/ion beam is able to create a self-radiograph of the target and glass stalk holding the target itself recorded through the radiochromic film stack. This unique result indicates strong long-living (ns time scale) magnetic fields present in the laser-produced plasma, which are extremely important in energy transport during the intense laser irradiation. The magnetic field from laser main pulse expands rapidly in the preformed plasma to rotate the laser produced protons. Radiation hydrodynamic simulations and ray tracing found that the magnetic field created by the amplified spontaneous emission prepulse is not sufficient to explain the particle trajectories, but the additional field created by the main pulse interaction estimated from particle-in-cell simulation is able to change the particle trajectories.

X-ray spectroscopy of mid-Z metal impurities is important in the study of tokamak plasmas and may reveal potential problems if their contribution to the radiated power becomes substantial. The analysis of the data from a high-resolution x-ray and extreme ultraviolet grating spectrometer, XEUS, installed on NSTX, was performed focused on a detailed study of x-ray spectra in the range 7-18 Å. These spectra include not only commonly seen iron spectra but also copper spectra not yet employed as an NSTX plasma impurity diagnostic. In particular, the L-shell Cu spectra were modeled and predictions were made for identifying contributions from various Cu ions in different spectral bands. Also, similar spectra, but from much denser Cu plasmas produced on the UNR Z-pinch facility and collected using the convex-crystal spectrometer, were analyzed and compared with NSTX results.

The results of experiments with combined aluminum (Al) and stainless steel (SS) alloy 304, nested wire arrays from the 1 MA COBRA generator at Cornell University are presented. The loads studied consisted of a 6 mm diameter inner array and a 13 mm diameter outer array with a different material in each array: SS or aluminum. Al implodes before SS in all loads studied, even when Al was on the inner array. The new wire ablation dynamic model and spectroscopic modeling are used to interpret these data. The observed implosion dynamics are likely a result of the higher ablation rate of Al. These initial results suggest that combining wire materials with different ablation rates in wire array loads could be developed into a useful technique for x-ray pulse shaping and radiation yield optimization.

Tracer aluminum alloyed wires (Al5056) are used to provide additional information for x-ray diagnostics of implosions of Cu planar wire arrays (PWAs). Specifically, the analysis of combined PWA experiments using the extensive set of x-ray diagnostics is presented. In these experiments, which were conducted at the 1MA pulsed power generator at University of Nevada, Reno, the Z-pinch load consisted of several (eight) Cu alloyed (main material) and one to two Al alloyed (tracer) wires mounted in a single plane row or double parallel plane rows, single planar wire array (SPWA) or double planar wire array (DPWA), respectively. The analysis of x-ray spatially resolved spectra from the main material indicates the increase in the electron temperature T(e) near the cathode. In general, the axial gradients in T(e) are more pronounced for SPWA than for DPWA due to the more "columnlike" plasma formation for SPWA compared to "hot-spot-like" plasma formation for DPWA. In addition, x-ray spectra from tracer wires are studied, and estimated plasma parameters are compared with those from the main material. It is observed that the x-ray K-shell Al spectra manifest more opacity features for the case of SPWA with about 18% of Al mass (to the total load mass) compared to the case of DPWA with about 11% of Al mass. The analysis of time-gated spectra shows that the relative intensity of the most intense K-shell Al line, small before the x-ray burst, increases with time and peaks close to the maximum of the sub-keV signal.

New extreme ultraviolet (EUV) spectroscopic diagnostics of relatively low-temperature plasmas based on the application of an EUV spectrometer and fast EUV diodes combined with glass capillary optics is described. An advanced high resolution dispersive element sliced multilayer grating was used in the compact EUV spectrometer. For monitoring of the time history of radiation, filtered fast EUV diodes were used in the same spectral region (>13 nm) as the EUV spectrometer. The radiation from the plasma was captured by using a single inexpensive glass capillary that was transported onto the spectrometer entrance slit and EUV diode. The use of glass capillary optics allowed placement of the spectrometer and diodes behind the thick radiation shield outside the direction of a possible hard x-ray radiation beam and debris from the plasma source. The results of the testing and application of this diagnostic for a compact laser plasma source are presented. Examples of modeling with parameters of plasmas are discussed.

For the past 2 years the planar wire array loads have proven their ability to create powerful x-ray radiation sources at the pulsed power facilities with the current level ranging from 1to3MA. Several key features of the implosion and ablation dynamics of the planar wire arrays distinguish them from the wire arrays of the conventional cylindrical design. The uneven current partition through the array wires in planar geometry results in a significant difference between the ablation rates of the outermost and the innermost array wires. This difference is even higher in a double row planar array geometry. According to the three-dimensional magnetohydrodynamic simulations the effect of the delayed ablation of the inner array wires can result in effective mitigation of the Rayleigh–Taylor instability modes. The high number (200–300) of wires in a cylindrical array is preferable to ensure fine azimuthal symmetry of an array implosion. However this requirement is not a great concern for the planar wire array loa...

Low wire number nested array Z-pinch experiments have been carried out with wires made of aluminum, stainless steel (uniform), and combinations of these two materials (mixed) on the 1MA COBRA generator at Cornell University [J. D. Douglass, J. B. Greenly, D. A. Hammer et al., in Proceedings of the 15th IEEE International Pulsed Power Conference (IEEE, Piscataway, NJ, 2005)]. The outer array consisted of eight wires, whereas the inner array had four or eight wires. The 10μm Al wires were alloy 5056 and the 6.25μm stainless steel wires were alloy SS304. The diagnostic suite included fast-x-ray and extreme ultraviolet (EUV) detectors, a time-gated x-ray pinhole camera, x-ray spectrometers, and laser shadow imaging. The main focus was made on the spectroscopic study of plasma evolution after the main x-ray burst though the data from photoconducting detector (PCD) and EUV signals over the whole period of current, and in addition laser shadowgraphy images before the main x-ray burst were analyzed. Modeling of t...

This paper presents the wire dynamics model (WDM), which can effectively replace the generic 0D (zero-dimensional) model in simulation of the implosions of arbitrary shaped wire arrays, including high-wire-number nested and planar array loads at multi-MA generators. Fast and inexpensive WDM modeling can predict the array implosion time and the rate of thermalization of the kinetic energy, and can estimate the timing of the x-ray pulse. Besides serving the purposes of the design and optimization of the wire array loads of complex configurations, the WDM reproduces the specific features of the wire array implosion dynamics due to the inductive current transfer, which makes the WDM a valuable amplification of the magnetohydrodynamic models.

Implosion of wire arrays was investigated at the 1MA Zebra accelerator by multiframe laser probing and gated x-ray self-emission diagnostics. Different regimes of implosion were observed in Al and Cu wire arrays. Implosion of Al loads with masses of 33–37μg∕cm produces a dense pinch 1–1.5mm in diameter. Strong instabilities are observed in the Z pinch at the time of stagnation. Implosion of “overmassed” loads produces a plasma column 3–4mm in diameter with a core. The plasma column does not collapse during the x-ray pulse. The core of the plasma column is not subjected to the kink instability and transforms to a chain of dense spots in the later stage. Different regimes of implosion were observed in Al 8×15μm loads presumably due to variations in the current pulse and load conditions. Observed regimes are compared to three-dimensional hybrid simulation of ideal and nonideal magnetohydrodynamics modes of implosion.