S.F. Keim

Radiation sources with planar wire arrays and planar foils for inertial confinement fusion and high energy density physics research

This article reports on the joint success of two independent lines of research, each of them being a multi-year international effort. One of these is the development of innovative sources, such as planar wire arrays (PWAs). PWAs turned out to be a prolific radiator, which act mainly as a resistor, even though the physical mechanism of efficient magnetic energy conversion into radiation still remains unclear. We review the results of our extensive studies of PWAs. We also report the new results of the experimental comparison PWAs with planar foil liners (another promising alternative to wire array loads at multi-mega-ampere generators). Pioneered at UNR, the PWA Z-pinch loads have later been tested at the Sandia National Laboratories (SNL) on the Saturn generator, on GIT-12 machine in Russia, and on the QiangGuang-1 generator in China, always successfully. Another of these is the drastic improvement in energy efficiency of pulsed-power systems, which started in early 1980s with Zuckers experiments at Nava...

Radiation from mixed multi-planar wire arrays

The study of radiation from different wire materials in wire array Z-pinch plasma is a very challenging topic because it is almost impossible to separate different plasmas at the stagnation. A new approach is suggested based on planar wire array (PWA) loads to assess this problem. Multi-planar wire arrays are implemented that consist of few planes, each with the same number of wires and masses but from different wire materials, arranged in parallel rows. In particular, the experimental results obtained with triple PWAs (TPWAs) on the UNR Zebra generator are analyzed with Wire Ablation Dynamics Model, non-local thermodynamic equilibrium kinetic model, and 2D radiation magneto-hydrodynamic to illustrate this new approach. In TPWAs, two wire planes were from mid-atomic-number wire material and another plane was from alloyed Al, placed either in the middle or at the edge of the TPWA. Spatial and temporal properties of K-shell Al and L-shell Cu radiations were analyzed and compared from these two configurations of TPWAs. Advantages of the new approach are demonstrated and future work is discussed.

Radiation from Ag high energy density Z-pinch plasmas and applications to lasing

Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8 keV) observed on the Zebra generator so far and upwards of 30 kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.5–5.0 A. With this, L-shell Ag as well as cold Lα and Lβ Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8 keV). Along with PCD signals, other signals, such as filtered XRD (>0.2 keV) and Si-diodes (SiD) (>9 keV), are analyzed covering a broad range of energies from a few eV to greater than 53 keV. The observation and analysis of cold Lα and Lβ lines show possible correlations w...

Producing Kiloelectronvolt L-Shell Plasmas on Zebra at UNR

Experiments with various wire loads from mid-atomic-number wires, which were performed on the university-scale 1-MA Zebra generator at the University of Nevada, Reno, during the last few years, are analyzed to assess the highest electron temperature reached. In particular, the results from experiments with planar wire arrays (PWAs) were considered. Load materials from mid-atomic-number such as stainless steel, Alumel, Cu, brass, Mo, and up to Ag were used to generate L-shell plasmas and to study plasma parameters. Though the full diagnostic set was utilized, the main focus was on X-ray spectroscopic data and on the non-local thermodynamic equilibrium kinetic modeling. As a result, the scaling of the maximum Te with the load material atomic number is presented for the first time in the range from Fe to Ag for L-shell plasmas from PWAs. The highest values of the electron temperature in L-shell plasmas, which are estimated from the modeling, were from both Ag PWAs and X-pinches. This work is important for the development of efficient X-ray radiators on university-scale Z-pinch generators.

Radiation from mid-atomic-number X-pinches at 1.5–1.7 MA

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 suc...

Inner-shell radiation from wire array implosions on the Zebra generator

Implosions of brass wire arrays on Zebra have produced L-shell radiation as well as inner-shell Kα and Kβ transitions. The L-shell radiation comes from ionization stages around the Ne-like charge state that is largely populated by a thermal electron energy distribution function, while the K-shell photons are a result of high-energy electrons ionizing or exciting an inner-shell (1s) electron from ionization stages around Ne-like. The K- and L-shell radiations were captured using two time-gated and two axially resolved time-integrated spectrometers. The electron beam was measured using a Faraday cup. A multi-zone non-local thermodynamic equilibrium pinch model with radiation transport is used to model the x-ray emission from experiments for the purpose of obtaining plasma conditions. These plasma conditions are used to discuss some properties of the electron beam generated by runaway electrons. A simple model for runaway electrons is examined to produce the Kα radiation, but it is found to be insufficient.

Analysis of K-shell HED plasmas in X-pinch and laser experiments at UNR

Summary form only given. High Energy Density (HED) plasmas were produced in two sets of experiments: using a Z-pinch generator and a high power laser at the Nevada Terawatt Facility (NTF) of the University of Nevada, Reno (UNR). Each experiment has its unique characteristics and can be used to gain better understanding of HED plasmas. X-pinch experiments were performed at enhanced current on the Zebra generator using the Load Current Multiplier (LCM) and generated radiation yield up to 19 kJ/cm. These experiments used Ti alloy wires (6% Al, 4% V) for a first look at X-pinches at 1.5-1.7 MA at UNR. A full set of diagnostics was fielded including time-integrated spatially-resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectrometers, time-integrated and time-gated pinhole x-ray cameras, and shadowgraphy. Many interesting results were found such as: i) the appearance of characteristic emission of Ti (wire material) and Fe (anode) in different orders of reflection at 30 ns prior to the first x-ray burst that was recorded for the next 15 ns, ii) prominent K-shell Al radiation despite the low percentage of Al in the alloy in both TGSI and TISR spectra, iii) K-shell Al radiation that corresponds to 400-550 eV plasmas starting near the first x-ray burst. Additionally experiments using the Leopard laser were performed on flat Al targets using a 0.8 ns pulse duration and two different target thicknesses (10 and 50 μm) that produced K-shell Al spectra of higher electron density. K-shell Al spectra of X-pinches, both TGSI and TISR, were modeled and compared with the laser-produced plasma results. The advantage of using alloyed Ti wires to study K-shell HED plasmas is highlighted. Future work is discussed.

Analysis of Al precursor wire array experiments on the 1 MA zebra generator at UNR

Summary form only given. Previous experiments on the 1 MA Zebra generator at UNR studied precursor plasmas with Ni-60 cylindrical wire arrays (CWA). Those precursor plasmas were shown to consistently have electron temperatures > 400 eV1. Continuing research on precursor plasmas at 1 MA on Zebra investigated first other mid-Z wire materials and then alternate arrays using mixed Al/mid-Z CWAs. Results found similar electron temperatures for the mid-Z elements present in the precursor with relatively colder temperatures for Al. A better understanding of the results from the mixed CWAs requires understanding pure Al CWAs. Recent experiments on Zebra using Al CWAs were performed to compare with the mixed Al/mid-Z CWAs. These CWAs consist of 6 wires evenly spaced in a 12 mm diameter, the same configuration as in previous experiments on precursor plasmas. Time-integrated spatially-resolved (TISR) and time-gated spatially-integrated (TGSI) X-ray spectral data, time-integrated and time-gated pinhole X-ray images, shadowgraphy, as well as optical streak camera images were obtained and analyzed. It was found that the Al precursor radiation starts, and stays pronounced until the main X-ray burst. This differs from the mid-Z precursors which show a defined precursor burst with an increase in radiation and decrease to zero before the main x-ray burst. Non-LTE kinetic models of Al have been applied to account for the K-shell radiation from precursor and main X-ray burst plasmas. The resulting plasma parameters from modeling of TGSI and TISR spectra together with analysis of corresponding images allow for the study of precursor plasma formation in time and in space, respectively.

Study of implosion dynamics and radiative mechanisms of planar foil liners in comparison with planar wire arrays at 1.7 MA UNR zebra generator

Summary form only given. Planar foil liners are alternative loads to wire arrays at multi-mega ampere generators as well as a promising object for the investigation of the magnetic energy dissipation mechanisms in Z-pinch plasmas. Experimental comparison of implosion dynamics and radiative mechanisms of Al planar foils and single planar wire arrays (SPWAs) of the same width and linear mass was performed for the 0.9-1.6-MA current region. Foils radiate approximately 80-90% of the total yield and power of SPWAs. The non-LTE code was applied to estimate the average electron temperature in Al planar foils that was found to be 20% higher, than that in SPWAs, and the average electron density in foils that was an order of magnitude lower than for SPWAs. Also, the foils are characterized by smaller axial gradient of electron temperature and density than SPWAs. In addition, anisotropic emission from Al planar foils was observed to be similar to Al SPWAs: the total yield registered orthogonally to the foil plane was 1.3 times higher than that along the plane (compared to 1.5 for SPWAs). The anomalous MHD resistivity consideration suggests that a significant part of foil radiation could be due to formation of strongly-inhomogeneous plasma through instabilities appearing on shadowgraphic images of a symmetric foil as a result of initial sharp edges inhomogeneity. This idea was tested in the recent experiments with modified foils where one edge was initially sharp and the other was folded with smaller initial inhomogeneity. The yield from a foil with a folded edge was 13-15% lower than that with both sharp edges as predicted by MHD modeling. Presented results on radiation from foils suggest them as potentially useful x-ray sources for various HEDP applications due to simpler load foil preparations compared to wire arrays. Preliminary results of the research we started on radiation from double foils in comparison with double planar wire arrays (DPWAs) are also discussed.

Radiation signatures of large sized multi-planar wire arrays

Summary form only given. Experiments on the Zebra generator with LCM (Load Current Multiplier, provides 1.5-1.7 MA) allow for implosions of larger sized wire array loads (including planar wire arrays) than at standard current of 1 MA. Advantages of larger sized planar wire array implosions include enhanced energy coupling to plasmas and better diagnostic access to observable plasma regions. A full set of diagnostics was implemented to study radiation in a broad spectral range from few Å to few hundred Å using PCD, XRD, and EUV detectors, X-ray/EUV spectrometers and X-ray pinhole cameras. In addition, laser shadowgraphy was utilized. In multi-planar wire arrays, two outer wire planes were each 4.9 mm width and made of eight mid-atomic-number (Alumel with 95% of Ni) wires with the inter-row gap increased from 3 or 6 mm (usually used at 1 MA current) up to 9 mm. A central plane located in the middle between the outer planes had empty slots and a few Al wires at the edges. Recently, we have shown that such configuration produces higher linear radiation yield. In the new experiments, the number of empty slots was further increased from 6 up to 10, increasing the gap inside the middle plane from 4.9 to 7.7 mm, respectively. This allows for more independent study of the flows of L-shell Ni plasma (between the outer planes) and K-shell Al plasma (which first fills the gap between the edge wires along the middle plane) and their radiation in space and time. When studying the combined wire arrays before, the time-gated X-ray spectra have always included radiation from both materials, even at early time. In the present work, for the first time we have observed that the K-shell Al radiation was delayed compared to L-shell Ni radiation when the number of empty slots was increased. In addition, the results of another new experiment are presented when a few Al wires on each edge were replaced by a thicker Cu wire to understand their influence on radiation from outer planes.