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International audience; We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during asubstorm event on 23 July 2017. We analyzed Ohm’s law for each event and found that ions are mostly decoupled from the magnetic field byHall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electrondecoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from theelectromagnetic field to the plasma (J E > 0) ahead or at the DF, whereas it is the opposite (J E < 0) behind the front. This reversal ismainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy ismostly transferred from the plasma to the electromagnetic field (J E0 < 0) and should contribute to the deceleration of the fast flow.However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely relatedto lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.

A model has been considered to describe the development of a surface discharge over a conductor electrically exploding in a strong magnetic field. A simulation performed using this model has shown that in the initial stage of the conductor explosion, a plasma layer of several tens of micrometers thick with an electron temperature of several electronvolts is formed on the metal surface. Based on the theory of small perturbations, the development of thermal filamentation instabilities that form in the surface plasma layer has been analyzed. The characteristic growth rates and wavelengths of these instabilities have been determined. The theoretical results were compared with the results of experiments performed on the ZEBRA generator (providing load currents of amplitude about 1 MA and rise time about 100 ns) and on the MIG generator (providing load currents of amplitude about 2 MA and rise time about 100 ns). For the conditions implemented with these generators, the filamentation model gives rise times of thermal filamentation instabilities of tens of nanoseconds at characteristic wavelengths of the order of 100 μm. These values are in good agreement with experimental data, which indicates the adequacy of both the surface discharge development model and the filamentation model.

Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigan’s low-impedance linear transformer driver (LTD)-driven generator ( $0.1~\Omega $ , 0.5–1 MA, and 100–200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3–0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to $2\times 10^{20}$ cm $^{\mathrm {-3}}$ . Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot.

International audience; Proposed 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 hohlraum’s 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 CONTRIBUTION

Since the first wire-array tungsten (W) experiments on Z at SNL, where the record x-ray power of 200 TW and x-ray yield of nearly 2 MJ were achieved 1, such arrays were actively studied and considered for various applications including inertial confinement fusion (ICF) 2. More recently, W Double Planar Wire Arrays (DPWAs) were suggested and tested for indirect drive ICF 3. DPWA consists of two parallel planes of wires of the same (uniform) or different (mixed) wire materials. W DPWAs have previously demonstrated the highest (among PWAs) radiation yield (up to 30 kJ), compact size (few mm), and strong electron beams at the Universityscale high-impedance generator 3. During the last few years we have reported on the outcome of the experiments with uniform and mixed Al and stainless steel DPWAs on the University of Michigan’s low-impedance Linear Transformer Driver (LTD) MAIZE generator. Here we present the results of the most recent campaign with W and W/Al DPWAs recorded using filtered x-ray diodes, x-ray spectrometers and pinhole cameras, and a twelve frame shadowgraphy system. For the first time, implosion of W wire arrays on LTD generator in USA was demonstrated and analyzed. In particular, uniform W and mixed W/Al DPWAs with a mass up to 87 μg arranged in various configurations were successfully imploded at the current of 0.5 MA during $\sim 210$ns. The most interesting results were obtained with W/Al DPWAs where a long-term standing shock wave was consistently formed at the W side, which was also observed at the high-impedance Zebra generator at UNR. In addition, soft (4-7 A) and hard (1-2.4 A) line radiation was substantially suppressed by including the Al plane.

International audience; Double Planar Wire Arrays (DPWA), which consist of two parallel rows of wires, have previously demonstrated high radiation efficiency, compact size, and usefulness for various applications in experiments on a University-scale high-impedance Z-pinch generator1. Recently, we successfully performed two experimental campaigns with PWAs on the University of Michigan's low-impedance MAIZE (Linear Transformer Driver (LTD)-driven generator, 0.1ohm, 0.5-1 MA, 100-180 ns) in collaboration with the UM team. The details and the analysis of the results of the first experimental campaign can be found in Ref. [2]. The second experimental campaign was focused on studying the implosion and radiative characteristics of DPWAs using a diagnostic set similar to the first campaign, including: filtered X-ray diodes, X-ray spectrographs and pinhole cameras, and a new four-frame shadowgraphy system with 2-ns, 532 nm frequency doubled Nd:YAG laser. Here we present the results of four, mixed-DPWA shots with the load consisting of one plane with 6 Al wires of 10¿¿m diameter and another plane of 6 stainless steel wires of 5.1 ¿¿m diameter. The rise-time of the current varies between 175 and 225 ns and shadowgraphy images cover the broad span of time from as early as 116 ns to as late as 304 ns. The shadowgraphy images show ablating and imploding mixed DPWAs that are very different from the images of uniform DPWAs. There is a clearly observed asymmetry of implosions of two wire array planes dependent on the material of each plane, (early time images in particular), captured also by X-ray pinhole images. WADM is used for the analysis of shadowgraphy images. X-ray spectra display both K-shell Al and L-shell Fe features analyzed with non-LTE modeling. Advantages of using mixed wire arrays are discussed. [1] V. L. Kantsyrev et al, Phys. Plasmas 15, 030704 (2008). [2] A.S. Safronova, V.L. Kantsyrev, M.E. Weller, V.V. Shlyaptseva, I.K. Shrestha, M. Lorance, M. Schmidt-Petersen, A. Stafford. M. Cooper, A.M. Steiner, D.A. Yager-Elorriaga, S.G. Patel, N.M. Jordan, R.M. Gilgenbach, A.S. Chuvatin, IEEE TPS, Special Issue on Plenary and Invited papers from ICOPS 2015, to be published, April 2016. * This work was supported by NNSA under DOE Cooperative Agreement DE-NA0001984.

International audience; The SPHINX machine is a 6MA, 1 μs, LTD Z-pinch driver at CEA Gramat (France) and primarily used for studying radiation effects. Different power amplification concepts were examined in order to reduce the current rise time without modifying the generator discharge scheme, including the Dynamic Load Current Multiplier (DLCM) proposed by Chuvatin [1]. A DLCM device, capable of shaping the current pulse without reducing the rise time, was developed at CEA. This device proved valuable for isentropic compression experiments in cylindrical geometry [2]. Recently, we achieved a short pulse operation mode by inserting a vacuum closing switch between the DLCM and the load. The current rise time was reduced to ∼ 300 ns. We explored the use of a reduced-height wire array for the Dynamic Flux Extruder in order to improve the wire array compression rate and increase the efficiency of the current transfer to the load. These developments are presented. Potential benefits of these developments for future Z pinch experiments are discussed. [1] A.S. Chuvatin, Dynamic Current Multiplier; 14th Symposium on High Current Electronics, Tomsk, Russia, pp 232-235 (2006). [2] T. d'Almeida et al, Phys. Plasmas, 20, 092512-1 092512-16 (2013).

International audience; Two new approaches of i) simultaneous study of implosion and radiative characteristics of different materials in wire array Z-pinch plasma in one shot [Safronova et al, PoP 21, 031205 (2014)] and ii) investigation of larger sized wire arrays (to enhance energy coupling to plasmas and provide better diagnostic access) were further developed in experiments with 1.7 MA Zebra with a Load Current Multiplier. In particular, the largest multi-Planar Wire Arrays with two outer planes from alloyed Al wires placed as far from each other as at 19 mm (compare with 6 and 9 mm studied before) and with a modified central plane from Ni-60 (mostly Cu), were investigated. Though K-shell Al and L-shell Cu plasmas have similar temperatures and densities, the ablation dynamics and radiation of Al and Cu planes is 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. This work was supported by NNSA under DOE Cooperative Agreement DE-NA0001984 and in part by DE-NA0002075. SNL is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE under Contract DE-AC04-94AL85000.

International audience; Single Planar Wire Arrays (SPWA) and Double Planar Wire Arrays (DPWA), which consist of one or two parallel rows of wires, respectively, have demonstrated high radiation efficiency (up to 30 kJ), compact size (1.5-3 mm), and usefulness for various applications in experiments on the high-impedance Zebra (1.9Ω, 1 MA, 100 ns). For example, DPWAs are very suitable for the new compact multi-source hohlraum concept, astrophysical applications, and as an excellent radiation source. Their implosion dynamics strongly depends on the critical load parameter, the aspect ratio Φ (width to inter-planar gap Δ) as well as on load wire material and mass. We have studied implosion dynamics and radiative properties of DPWAs at the enhanced Zebra current of 1.5-1.7 MA and have demonstrated the new regimes of implosions with asymmetric jets, no precursor formation, and very early radiation for larger sized (Δ=9 mm, Φ=0.54) and precursor formation and strong “cold” Ka emission for standard sized (Δ=6 mm, Φ=1.28) DPWAs.

International audience; A 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.

International audience; Oral 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).

The radiative performance of Al, Ni, and W planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, is investigated. However, the planar wire arrays were shown to radiate much more energy in a short intense peak than possible due to dissipation of the kinetic energy. The planar array gives the unique possibility of seeing the evolution of the small-scale inhomogeneity of wire-array plasmas during wire ablation and implosion phases and highlights the importance of the Hall plasma phenomena and their impact on the dynamics, energy coupling, and radiation performance of wire-array Z-pinches

This work describes an inductive energy storage scheme intended for power multiplication at mega-Ampere currents. The key power multiplication element of the scheme is an opening switch generating the voltage of inductive origin. The switch represents an additional volume with magnetically accelerated solid-state or plasma conductor between the generator and the load. Motion of the conductor increases the inductance of the volume. A sufficiently fast increase of this inductance at the end of magnetic energy storage time ensures power multiplication. A critical requirement for the accelerated conductor is the possibility of temporal profiling of the inductance increase. A proof-of-principle experiment at GIT12 shows that such profiling is possible. We suggest a simple analysis of the scheme efficiency and illustrate this analysis for a multi-mega-Ampere class generator. The scheme is alternative to existing inductive energy storage technologies for pulsed-power conditioning at high currents.

In the following experiments, we studied implosions of different wire arrays and X-pinches produced on the 1-MA Zebra generator at the University of Nevada, Reno. Diagnostics included both spatially-resolved and time-gated X-ray imaging and spectroscopy, and laser probing. In particular, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in subkiloelectronvolt and kiloelectronvolt spectral ranges than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small-scale inhomogeneity of wire-array plasmas has a major effect on dynamics, energy coupling and radiation performance of wire-array Z-pinches. The study of Al, Alumel, and W cylindrical wire arrays shows a wide variety of characteristic behaviors in plasma implosions discussed hereinafter. Additional experimental results for symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X-pinches are also presented. New data for the total radiation yield are obtained. The planar structures of X-pinch plasma and the corresponding electron beam was observed for most of X-pinches. The generation of hot spots along original wires positions-cooler than those from the cross-wire region-and arc structures with hot spots between wires were found for X-pinches composed from Al, Cu, and W wires.

We discuss a new opportunity of using Z-pinch plasma radiation sources for generating Ar K-shell radiation and harder keV quanta. Our approach to keV X-ray generation is based upon an analogy with laser fusion, where the imploding shell compressionally heats the low-density inner mass. The suggested design of a Z-pinch load consists then of one or two heavy outer shell(s) with a lower mass on-axis fill (i.e., central gas jet) producing most of the radiation. The outer shell is not supposed to radiate and thus does not need to have high specific energy characterized by the large /spl eta/ parameter (Whitney et al., 1990). Thus, the heavy outer shell does not need to have a very large initial diameter for its implosion to be matched to the long-pulse current driver. Rather, we want to have a large amount of energy from the driver coupled to this shell by the moment when the shell collides with the low-density fill and eventually converts much of this energy to the thermal energy of the on-axis plasma. This configuration is investigated numerically in the framework of a one-dimensional radiation-magneto-hydrodynamics model for the case of Ar K-shell radiators. It is demonstrated that the Ar fill is heated in two stages. The first stage corresponds to the shock heating and thermal conduction in an initially low-density fill, and it allows preheating the fill while avoiding significant losses in soft radiation. The fill radiator is then compressed quasi-adiabatically and is heated-up to the temperature optimum for K-shell quanta generation. Diffusion of the driving magnetic field is shown to always suppress the conductive heat losses from the hot on-axis plasma to the cold outer shell. Absorption of the K-lines emitted near the axis in the surrounding plasma could be avoided by filling the outer shell with a different gas (like N-on-Ar), which allows a substantial increase in the observed keV X-ray radiation yields.

The PIAF generator was designed for low total energy and high energy density experiments with liners, X-pinch or fiber Z-pinch loads. These studies are of interest for such applications as surface and material science, microscopy of biological specimens, lithography of x-ray sensitive resists, and x-ray backlighting of pulsed-power plasmas. The generator is based on an RLC circuit that includes six NWL 180 nF–50 kV capacitors that store up to 1.3 kJ. The capacitors are connected in parallel to a single multispark switch designed to operate at atmospheric pressure. The switch allows reaching a time delay between the trigger pulse and the current pulse of less than 80 ns and has jitter of 6 ns. The total inductance without a load compartment was optimized to be as low as 16 nH, which leads to extremely low impedance of ∼0.12 Ω. A 40 kV initial voltage provides 250 kA maximum current in a 6 nH inductive load with a 180 ns current rise time. PIAF has dimensions of 660×660×490 mm and weight of less than 100 kg...

Confinement and compression of magnetic flux by plasma shells is of interest for a variety of applications associated with keV x-ray production in pulsed-power driven Z-pinch plasma radiation sources (PRS). Confinement of a Bz field with a Bθ field of the pinch current can help stabilize the implosion of a shell from a large initial radius. Compression of the azimuthal magnetic flux in the PRS (secondary) circuit with a plasma shell driven by Bθ field of the primary circuit may represent a new opportunity for using a low-cost, relatively slow pulsed power to generate large amounts of keV x rays. The magnetic field has to be compressed and/or confined by low-mass plasma shells emitting soft x-ray radiation that limits the temperature and conductivity of the shell plasma. The thickness of a plasma shell is established self-consistently during the implosion, and it is not obvious whether or not the shell becomes thick enough to confine or compress the magnetic flux. The results of analytical theory and numer...

International audience; Precursor 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.

Kr and Xe plasmas are very intensive emitters in the spectral range of 100-150 A, which is relevant for a number of applications (for example microlithography). We present investigations of the extreme utraviolet (EUV) emission from a slow capillary discharge with Kr and Xe fillings. The emission of Kr ions (Kr VIII to Kr XI) within the range of 70-150 A consists of three bands of lines of about 10 A width with maxima at 116, 103 and 86 A. Xe emission bands of about 15 A width have their maxima at 136 and 115 A (Xe IX to Xe XII). The radiation duration in this spectral range is ~150 ns for both elements. At the optimum conditions, the Kr emission at 103 A is 2-3 times more intense than the Xe emission at 136 A. The measured spectral energy of Kr radiation is about 0.1 J sr-1 A-1. Experimental results are compared with numerical modellings of the dynamics and emission of the capillary discharge plasma, which enables the determination of plasma parameters and the future use of the codes as additional instruments for plasma diagnostics.

International audience; 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.

International audience; Double Planar Wire Arrays (DPWA), which consist of two parallel rows of wires, have demonstrated high radiation efficiency (up to 30 kJ), compact size (1.5-3 mm), and pulse shaping capabilities in experiments at 1 MA Zebra. DPWAs are also very suitable for the new compact multi-source hohlraum concept. It was shown that their implosion dynamics strongly depends on the critical load parameter, the aspect ratio (width to inter-planar gap Δ). Recently, we studied larger sized DPWAs at the increased current of 1.5-1.7 MA that provided enhanced energy coupling in plasma and better diagnostic access to observable plasma regions. The new regimes of implosions with asymmetric jets, no precursor formation and very early radiation for larger sized DPWAs (Δ=9 mm) with low aspect ratio of 0.54 were demonstrated. As a development of this work, new experiments at the enhanced current with the DPWAs from Alumel (mostly Ni) with Δ=6 mm and higher aspect ratio were performed. The different implosion and radiative signatures were observed that are presented and analyzed such as formation of the precursor in the middle of the array, no “foot” pulse emission but a very broad XRD signal, and L-shell radiation before the XRD peak but not so early as for larger PWAs. Also, simultaneously with soft x-ray L-shell Ni radiation, hard x-ray K-shell Ni radiation was recorded in a broad range from 13 ns before up to 18 ns after the XRD peak showing the temporal evolution of characteristic “cold” Ni Kα emission as well as some adjacent spectral features from hotter plasmas. Non-LTE modeling of such K-shell features provided time history of ionization balance of Ni ions and is compared with results of L-shell modeling.

International audience

International audience; 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.

La combinaison des caracteres d'une decharge capillaire rapide avec ceux d'une decharge transitoire a cathode creuse a permis d'etudier une famille nouvelle de micro-plasmas. De telles decharges fonctionnent differemment des decharges capillaires usuelles decrites dans la litterature. Un faisceau d'electrons resultant de l'effet de cathode creuse transitoire cree une preionisation sur l'axe, favorable a la formation de la decharge. La paroi du capillaire agit alors seulement comme un limiteur et le materiau de la paroi ne contamine pas le plasma, sauf aux temps longs, dans la mesure ou le temps de montee initial du courant est suffisamment eleve. Une geometrie particuliere des electrode assure la tres faible inductance indispensable pour realiser cette condition. Un courant de decharge typique de plusieurs kA alimente la decharge avec un temps de montee nanoseconde, produisant une colonne de plasma chaud de taille sub-millimetrique qui rayonne de la region UVV aux X mous. Des mesures completes de spectroscopie UVV ont ete faites pour caracteriser le plasma. L'evolution temporelle du diametre de la source est mesuree par une technique de camera fente-fil, et montre que le processus trouve bien son origine sur l'axe.

International audience; Experiments on the UNR Zebra generator with Load Current Multiplier (LCM) allow for implosions of larger sized wire array loads than at standard current of 1 MA. Advantages of larger sized planar wire array implosions include enhanced energy coupling to plasmas, better diagnostic access to observable plasma regions, and more complex geometries of the wire loads. The experiments with larger sized wire arrays were performed on 1.5 MA Zebra with LCM (the anode-cathode gap was 1 cm, which is half the gap used in the standard mode). In particular, larger sized multi-planar wire arrays had two outer wire planes from mid-atomic-number wires to create a global magnetic field (gmf) and plasma flow between them. A modified central plane with a few Al wires at the edges was put in the middle between outer planes to influence gmf and to create Al plasma flow in the perpendicular direction (to the outer arrays plasma flow). Such modified plane has different number of empty slots: it was increased from 6 up to 10, hence increasing the gap inside the middle plane from 4.9 to 7.7 mm, respectively. Such load configuration allows for more independent study of the flows of L-shell mid-atomic-number 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. We demonstrate that such configuration produces higher linear radiation yield and electron temperatures as well as advantages of better diagnostics access to observable plasma regions and how the load geometry (size of the gap in the middle plane) influences K-shell Al radiation. In particular, K-shell Al radiation was delayed compared to L-shell mid-atomic-number radiation when the gap in the middle plane was large enough (when the number of empty slots was increased up to ten).

International audience; A new compact Z-pinch x-ray hohlraum design with parallel-driven x-ray sources was experimentally demonstrated in a full configuration with a central target and tailored shine shields (to provide a symmetric temperature distribution on the target) at the 1.7 MA Zebra generator. This presentation reports on the joint success of two independent lines of research. One of these was the development of new sources – planar wire arrays (PWAs). PWAs turned out to be a prolific radiator. Another success was the drastic improvement in energy efficiency of pulsed-power systems, such as the Load Current Multiplier (LCM). The Zebra/LCM generator almost doubled the plasma load current to 1.7 MA. The two above-mentioned innovative approaches were used in combination to produce a new compact hohlraum design for ICF, as jointly proposed by SNL and UNR. Good agreement between simulated and measured radiation temperature of the central target is shown. Experimental comparison of PWAs with planar foil liners (PFL) - another viable alternative to wire array loads at multi-MA generators show promising data. Results of research at the University of Nevada Reno allowed for the study of hohlraum coupling physics at University-scale generators. The advantages of new hohlraum design applications for multi-MA facilities with W or Au double PWAs or PFL x-ray sources are discussed.