Planar wire array performance scaling at multi-MA levels on the Saturn generator.

A series of twelve shots were performed on the Saturn generator in order to conduct an initial evaluation of the planar wire array z-pinch concept at multi-MA current levels. Planar wire arrays, in which all wires lie in a single plane, could offer advantages over standard cylindrical wire arrays for driving hohlraums for inertial confinement fusion studies as the surface area of the electrodes in the load region (which serve as hohlraum walls) may be substantially reduced. In these experiments, mass and array width scans were performed using tungsten wires. A maximum total radiated x-ray power of 10 {+-} 2 TW was observed with 20 mm wide arrays imploding in {approx}100 ns at a load current of {approx}3 MA, limited by the high inductance. Decreased power in the 4-6 TW range was observed at the smallest width studied (8 mm). 10 kJ of Al K-shell x-rays were obtained in one Al planar array fielded. This report will discuss the zero-dimensional calculations used to design the loads, the results of the experiments, and potential future research to determine if planar wire arrays will continue to scale favorably at current levels typical of the Z machine. Implosion dynamics will be discussed, including x-ray self-emission imaging used to infer the velocity of the implosion front and the potential role of trailing mass. Resistive heating has been previously cited as the cause for enhanced yields observed in excess of jxB-coupled energy. The analysis presented in this report suggests that jxB-coupled energy may explain as much as the energy in the first x-ray pulse but not the total yield, which is similar to our present understanding of cylindrical wire array behavior

Compact Wire Array Sources: Power Scaling and Implosion Physics.

A series of ten shots were performed on the Saturn generator in short pulse mode in order to study planar and small-diameter cylindrical tungsten wire arrays at {approx}5 MA current levels and 50-60 ns implosion times as candidates for compact z-pinch radiation sources. A new vacuum hohlraum configuration has been proposed in which multiple z pinches are driven in parallel by a pulsed power generator. Each pinch resides in a separate return current cage, serving also as a primary hohlraum. A collection of such radiation sources surround a compact secondary hohlraum, which may potentially provide an attractive Planckian radiation source or house an inertial confinement fusion fuel capsule. Prior to studying this concept experimentally or numerically, advanced compact wire array loads must be developed and their scaling behavior understood. The 2008 Saturn planar array experiments extend the data set presented in Ref. [1], which studied planar arrays at {approx}3 MA, 100 ns in Saturn long pulse mode. Planar wire array power and yield scaling studies now include current levels directly applicable to multi-pinch experiments that could be performed on the 25 MA Z machine. A maximum total x-ray power of 15 TW (250 kJ in the main pulse, 330 kJ total yield) was observed with a 12-mm-wide planar array at 5.3 MA, 52 ns. The full data set indicates power scaling that is sub-quadratic with load current, while total and main pulse yields are closer to quadratic; these trends are similar to observations of compact cylindrical tungsten arrays on Z. We continue the investigation of energy coupling in these short pulse Saturn experiments using zero-dimensional-type implosion modeling and pinhole imaging, indicating 16 cm/?s implosion velocity in a 12-mm-wide array. The same phenomena of significant trailing mass and evidence for resistive heating are observed at 5 MA as at 3 MA. 17 kJ of Al K-shell radiation was obtained in one Al planar array fielded at 5.5 MA, 57 ns and we compare this to cylindrical array results in the context of a K-shell yield scaling model. We have also performed an initial study of compact 3 mm diameter cylindrical wire arrays, which are alternate candidates for a multi-pinch vacuum hohlraum concept. These massive 3.4 and 6 mg/cm loads may have been impacted by opacity, producing a maximum x-ray power of 7 TW at 4.5 MA, 45 ns. Future research directions in compact x-ray sources are discussed

Modeling experiments of new compact hohlraum configuration with multiple parallel-driven x-ray sources with application of VisRad code

International audienceA new compact Z-pinch x-ray hohlraum design with multiple parallel-driven x-ray sources was jointly proposed by the Sandia National Laboratories and the University of Nevada [1]. The first proof-of-principle experimental demonstration of the full configuration of this compact hohlraum with central reemission target and tailored shine shields (to provide a symmetric temperature distribution on the target) was achieved at the 1.7 MA UNR Zebra generator [2]. VisRad (PRISM Computational Sciences Co.), a 3-D view factor code, is used to simulate the multi-dimensional radiation environment within this new compact hohlraum configuration that incorporates multiple compact (mm-scale) planar wire array (PWA) x-ray sources that surround a reemission target in the center of the hohlraum cavity, allowing a reduction of hohlraum surface area and potentially providing a hotter x-ray environment. View factor modeling is a valuable design tool, allowing us to improve rapidly on experimental design and to demonstrate the feasibility of the concept for hohlraum and ICF studies on a 1-2 MA university-scale pulsed power platform. Double-PWA sources (DPWA) were modeled and used in experiments due to much better pulse shaping properties compared with single PWAs. Also, we are taking into account that the W DPWA is an anisotropic x-ray source and maximum radiation is emitted in the direction parallel to the wire planes. Different versions of compact hohlraum with two W DPWA sources and central cavity between them were analyzed using VisRad code. Simulations have predicted a reemission plastic target radiation temperature Trad ~ 39eV, showing good correlation to experimental data 37 3 eV The possibility of optimization of new compact configuration was demonstrated by changing relative volume of central cavity. Special emphasis is made on Trad uniformity at the reemission target surface by analysis of compact holraum configuration of 6 or more - PWA pinches proposed in Ref. [2] to reach better symmetry of hohlraum exposure. The scaling of this 6 DPWA sources hohlraum configuration using VisRad for higher current 20 MA generators (as Sandia National Laboratories Z facility) show that central target Trad ~ 85 eV is reachable. VisRad simulation has shown that x-ray power flux in new compact hohlraum might be ~1.3 times higher if W sources will be changed with Au sources

Radiative Signatures of Z-Pinch Plasmas at UNR: from X-Pinches to Wire Arrays

International audienceUniversity-scale Z-pinch generators are able to produce High Energy Density (HED) plasmas in a broad range of plasma parameters under well-controlled and monitored experimental conditions suitable for radiation studies. The implosion of X-pinch and wire array loads at a 1 MA generator yields short (1-20 nsec) x-ray bursts from one or several bright plasma spots near the wire cross point (for X-pinches) or along and near Z-pinch axis (for wire arrays). Such X- and Z-pinch HED plasma with scales from a few μm to several mm in size emits radiation in a broad range of energies from 10 eV to 0.5 MeV and is subject of our studies during the last ten years. In particular, the substantial number of experiments with very different wire loads was performed on the 1 MA Zebra generator and analyzed: X-pinch, cylindrical, nested, and various types of the novel load, Planar Wire Arrays (PWA). Also, the experiments at an enhanced current of 1.5-1.7 MA on Zebra using Load Current Multiplier (LCM) were performed. This paper highlights radiative signatures of X-pinches and Single and Double PWAs which are illustrated using the new results with combined wire loads from two different materials

Mid-Atomic-Number Cylindrical Wire Array Precursor Plasma Studies on Zebra

International audiencePrecursor plasmas from low wire number cylindrical wire arrays (CWAs) were previously shown to radiate at temperatures >300 eV for Ni-60 (94% Cu and 6% Ni) wires in experiments on the 1-MA Zebra generator. Continued research into precursor plasmas has studied additional midatomic-number materials including Cu and Alumel (95% Ni, 2% Al, 2% Mn, and 1% Si) to determine if the >300 eV temperatures are common for midatomic-number materials. In addition, current scaling effects were observed by performing CWA precursor experiments at an increased current of 1.5 MA using a load current multiplier. The results show an increase in a linear radiation yield of ~50% (16 versus 10 kJ/cm) for the experiments at increased current. However, plasma conditions inferred through the modeling of X-ray time-gated spectra are very similar for the precursor plasma in both current conditions

Optimization of Compact Hohlraum Design to ICF Studies

International audienceProposed optimization of a compact hohlraum design with parallel-driven Z-pinch x-ray sources is discussed. This design was jointly proposed earlier by SNL and UNR [1] and was experimentally demonstrated in full configuration at the 1.7 MA UNR Zebra generator [2]. A parallel driven multiple Z-pinch sources scheme was a significant advance in terms of driver requirements compared to a traditional double-ended cylindrical pinch scheme. Considering only geometric arguments from [1], it became evident that the new scheme could be more efficient by a factor of ~ 4.5 in terms of energy requirements. The prospective results of compact hohlraum scheme optimization are described: improvement of geometry of a compact hohlraum with parallel driven multiple Z-pinches sources; application of Au wires (Za = 79) instead of traditionally used W (Za = 74) wires; and employment of double planar foils liner (DPFL) instead of DPWA in hohlraums x-ray sources. This work was supported by NNSA under DOE grant DE-NA0003047. References [1] B. Jones et al., Physical Review Letters 104, 125001 (2010) [2] V.L. Kantsyrev et al., Physical Review E 90, 063101 (2014) ORAL CONTRIBUTIO

Load current pulse shaping on a nanosecond PFL-based accelerator using dynamic LCM technique

International audienceOral presentation Load Current Multiplier (LCM) continues to be successfully applied on Zebra at UNR. Here we consider further, expanded applications of LCM. Time-shaping of magnetic pressures on pulsed facilities is useful for such a number of High Energy Density Physics (HEDP) applications such as dynamic material properties studies1,2, high gain ICF research3 and instability growth studies4. The Dynamic LCM (DLCM) technique was recently developed for profiling the dynamic pressure ramp with good reproducibility on the microsecond SPHYNX generator (left plot)2. The present report analyses applications of this technique on Pulse Forming Line (PFL) nanosecond generators. The right plot presents simulated load currents Id and load liner accelerations ad for a 0.2 Ω PFL with the voltage Veq. without (dashed lines) and with (solid lines) DLCM for the imploding solid-state load of Ref. 2. DLCM here is an additional compact (5×10 cm) hardware installed between the PFL and the load with an imploding annular plasma shell inside. We report circuit analysis and MHD simulations that identify critical DLCM and load configuration parameters allowing to control the load current ramp. We demonstrate that not only the load magnetic pressure time-dependence can be controlled in the HEDP loads above but also the load current amplitude can be increased with respect to the direct PFL-driven case (right plot)

Application of VisRad modeling to design of hohlraum experiments on Zebra with enhanced current

International audienceVisRad (Prism Computational Sciences), a 3-D view factor code, is used to simulate the multi-dimensional radiation environment within a compact hohlraum. A new hohlraum design proposed in B. Jones, et al. [PRL, v.104, 125001, (2010)] incorporates multiple compact (mm-scale) planar wire array (PWA) x-ray sources that surround a target in the center of the hohlraum cavity, allowing a reduction of hohlraum surface area and potentially providing a hotter x-ray environment. The first experiments with this prototype of hohlraum with two magnetically-decoupled PWA sources were performed on the 1.7 MA Zebra at the University of Nevada, Reno without significant loss of radiation yields and power due to implementation of a new Load Current Multiplier (LCM). VisRad simulations have predicted a center hohlraum radiation temperature >; 30eV, showing good correlation to experimental EUV data (hν >; 17 eV). Special emphasis is made on Trad and uniformity at the test target surface. Also discussed is the scaling of the new hohlraum multisource configurations using VisRad for higher current, 20 MA-scale generators