Compact hohlraum configuration with parallel planar-wire-array x-ray sources at the 1.7-MA Zebra generator.

A compact Z-pinch x-ray hohlraum design with parallel-driven x-ray sources is experimentally demonstrated in a configuration with a central target and tailored shine shields at a 1.7-MA Zebra generator. Driving in parallel two magnetically decoupled compact double-planar-wire Z pinches has demonstrated the generation of synchronized x-ray bursts that correlated well in time with x-ray emission from a central reemission target. Good agreement between simulated and measured hohlraum radiation temperature of the central target is shown. The advantages of compact hohlraum design applications for multi-MA facilities are discussed.

Wire ablation dynamics model and its application to imploding wire arrays of different geometries.

The paper presents an extended description of the amplified wire ablation dynamics model (WADM), which accounts in a single simulation for the processes of wire ablation and implosion of a wire array load of arbitrary geometry and wire material composition. To investigate the role of wire ablation effects, the implosions of cylindrical and planar wire array loads at the university based generators Cobra (Cornell University) and Zebra (University of Nevada, Reno) have been analyzed. The analysis of the experimental data shows that the wire mass ablation rate can be described as a function of the current through the wire and some coefficient defined by the wire material properties. The aluminum wires were found to ablate with the highest rate, while the copper ablation is the slowest one. The lower wire ablation rate results in a higher inward velocity of the ablated plasma, a higher rate of the energy coupling with the ablated plasma, and a more significant delay of implosion for a heavy load due to the ablation effects, which manifest the most in a cylindrical array configuration and almost vanish in a single-planar array configuration. The WADM is an efficient tool suited for wire array load design and optimization in wide parameter ranges, including the loads with specific properties needed for the inertial confinement fusion research and laboratory astrophysics experiments. The data output from the WADM simulation can be used to simplify the radiation magnetohydrodynamics modeling of the wire array plasma.

Extreme ultraviolet spectroscopy and modeling of Cu on the SSPX Spheromak and laser plasma "Sparky".

Impurities play a critical role in magnetic fusion research. In large quantities, impurities can cool and dilute plasma creating problems for achieving ignition and burn; however in smaller amounts the impurities could provide valuable information about several plasma parameters through the use of spectroscopy. Many impurity ions radiate within the extreme ultraviolet (EUV) range. Here, we report on spectra from the silver flat field spectrometer, which was implemented at the Sustained Spheromak Physics experiment (SSPX) to monitor ion impurity emissions. The chamber within the SSPX was made of Cu, which makes M-shell Cu a prominent impurity signature. The Spect3D spectral analysis code was utilized to identify spectral features in the range of 115-315 Å and to more fully understand the plasma conditions. A second set of experiments was carried out on the compact laser-plasma x-ray∕EUV facility "Sparky" at UNR, with Cu flat targets used. The EUV spectra were recorded between 40-300 Å and compared with results from SSPX.

X-ray absorption spectroscopy of aluminum z-pinch plasma with tungsten backlighter planar wire array source.

Absorption features from K-shell aluminum z-pinch plasmas have recently been studied on Zebra, the 1.7 MA pulse power generator at the Nevada Terawatt Facility. In particular, tungsten plasma has been used as a semi-backlighter source in the generation of aluminum K-shell absorption spectra by placing a single Al wire at or near the end of a single planar W array. All spectroscopic experimental results were recorded using a time-integrated, spatially resolved convex potassium hydrogen phthalate (KAP) crystal spectrometer. Other diagnostics used to study these plasmas included x-ray detectors, optical imaging, laser shadowgraphy, and time-gated and time-integrated x-ray pinhole imagers. Through comparisons with previous publications, Al K-shell absorption lines are shown to be from much lower electron temperature (∼10-40 eV) plasmas than emission spectra (∼350-500 eV).

Influence of induced axial magnetic field on plasma dynamics and radiative characteristics of Z pinches.

The influence of an induced axial magnetic field on plasma dynamics and radiative characteristics of Z pinches is investigated. An axial magnetic field was induced in a novel Z-pinch load: a double planar wire array with skewed wires (DPWAsk), which represents a planar wire array in an open magnetic configuration. The induced axial magnetic field suppressed magneto-Rayleigh-Taylor (MRT) instabilities (with m = 0 and m = 1 instability modes) in the Z-pinch plasma. The influence of the initial axial magnetic field on the structure of the plasma column at stagnation was manifested through the formation of a more uniform plasma column compared to a standard double planar wire array (DPWA) load [V. L. Kantsyrev et al., Phys. Plasmas 15, 030704 (2008)]. The DPWAsk load is characterized by suppression of MRT instabilities and by the formation of the sub-keV radiation pulse that occurs before the main x-ray peak. Gradients in plasma parameters along the cathode-anode gap were observed and analyzed for DPWAsk loads made from low atomic number Z (Al) and mid-Z (brass) wires.

Observation of >400-eV precursor plasmas from low-wire-number copper arrays at the 1-MA zebra facility.

Experiments with cylindrical copper wire arrays at the 1-MA Zebra facility show that high temperatures exist in the precursor plasmas formed when ablated wire array material accretes on the axis prior to the stagnation of a z pinch. In these experiments, the precursor radiated approximately 20% of the >1000 eV x-ray output, and time-resolved spectra show substantial emission from Cu L-shell lines. Modeling of the spectra shows an increase in temperature as the precursor forms, up to approximately 450 eV, after which the temperature decreases to approximately 220-320 eV until the main implosion.

Diagnostic of charge balance in high-temperature tungsten plasmas using LLNL EBIT.

Diagnostic of high-temperature M-shell W plasmas is challenging because of contribution of numerous ionization stages in a relatively narrow x-ray spectral region. A method using LLNL EBIT data generated at different electron beam energies has been established for the identification of prominent spectral features and for the determination of charge balance in x-ray M-shell W spectra between 3.5 and 8.5 A. It extends previous work [A. S. Safronova et al., Can. J. Phys. 86, 267 (2008)] which used only Ni-like lines to include the neighboring ionization stages. This diagnostic procedure was tested with results from Z-pinch plasmas produced on the 1 MA pulse power generator Zebra at UNR. These results are of particular importance for fusion research.

X-ray diagnostics of imploding plasmas from planar wire arrays composed of Cu and few tracer Al wires on the 1MA pulsed power generator at UNR.

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.