Search

The LTE Saha–Boltzmann plasma ionization balance model and the Braginskii plasma electric resistance model are compared with the results by a suite of codes based on the average-atom model, which is a quantum-mechanical version of the Temperature Dependent Thomas–Fermi Theory. The analysis is focused on low-Z Al, mid-Z Cu and higher-Z Mo plasmas over broad ranges of electron temperature Te and electron number density ne. Calculations of mean ion charge by these two LTE models are compared to the results produced by non-LTE atomic kinetic codes. The applicability of the LTE and non-LTE models to the description of the radiative properties of highly-radiating z-pinch plasmas is also discussed. Two different approaches to the calculation of plasma resistance and their effects on line radiation mechanisms are analyzed.

The results of the recent experiments focused on study of x-ray radiation from multicharged plasmas irradiated by relativistic (I > 1019 W/cm2) sub-ps laser pulses on Leopard laser facility at NTF/UNR are presented. These shots were done under different experimental conditions related to laser pulse and contrast. In particular, the duration of the laser pulse was 350 fs or 0.8 ns and the contrast was varied from high (10−7) to moderate (10−5). The thin laser targets (from 4 to 750 μm) made of a broad range of materials (from Teflon to iron and molybden to tungsten and gold) were utilized. Using the x-ray diagnostics including the high-precision spectrometer with resolution R ∼ 3000 and a survey spectrometer, we have observed unique spectral features that are illustrated in this paper. Specifically, the observed L-shell spectra for Fe targets subject to high intensity lasers (∼1019 W/cm2) indicate electron beams, while at lower intensities (∼1016 W/cm2) or for Cu targets there is much less evidence for an electron beam. In addition, K-shell Mg features with dielectronic satellites from high-Rydberg states, and the new K-shell F features with dielectronic satellites including exotic transitions from hollow ions are highlighted.

Radiative emission from alloyed Al single, double and compact cylindrical wire arrays have been studied using the 1 MA Zebra UNR generator. Single planar wire arrays using ten wires and double planar wire arrays and compact cylindrical wire arrays (CCWA) that both had sixteen wires were utilized. The wire composition is Al-5056 (95% of Al and 5% of Mg). We have observed that implosion of these alloyed Al wire loads generated optically thick Al plasmas that can be diagnosed using K-shell Mg lines. In particular, among the considered loads, the K-shell lines of Al from implosions of the double planar wire arrays have the highest optical depth for He-like Al resonance transitions, which occurred near the stagnation phase. X-ray time-gated and time-integrated spectra and pinhole images as well as photoconductive detectors signals were analyzed to provide information on the plasma parameters; electron temperatures and densities, implosion dynamics features and power and yields of the X-ray radiation. Previously developed non-LTE models were applied to model axially-resolved time-integrated, as well as time-gated spatially-integrated, K-shell spectra from Al and Mg. The derived time-dependent electron temperature, density and axial opacity were studied and compared. In addition, the wire ablation dynamics model (WADM) was used to calculate the kinetic energy of the plasma, which with the aid of a Local Thermal Equilibrium (LTE) magneto-hydrodynamics (MHD) simulation, allowed to estimate the precursor and stagnated z-pinch plasma electron temperatures from implosions of wire array loads.

We continue to search for more efficient X-ray radiators from wire array Z-pinch plasmas. The results of recent experiments with single planar wire array (SPWA) loads made from mid-atomic-number material wires such as Alumel, Cu, Mo, and Ag are presented and compared. In particular, two new efficient X-ray radiators, Alumel (95% Ni, 2% Al, and 2% Si) and Ag, are introduced, and their radiative properties are discussed in detail. The experiments were performed on the 1 MA Zebra generator at UNR. The X-ray yields from such mid-atomic-number SPWAs exceed twice those from low-atomic-number SPWAs, such as Al, and increase with the atomic number to reach more than 27–29 kJ for Ag. To consider the main contributions to the total radiation, we divided the time interval of the Z-pinch dynamic where wire ablation and implosion, stagnation, and plasma expansion occur in corresponding phases and studied the radiative and implosion characteristics within them. Theoretical tools such as non-LTE kinetics and wire ablation dynamic models were applied in the data analysis. These results and the models developed have much broader applications, not only for SPWAs on Zebra, but for other HED plasmas with mid-atomic-number ions.

Experimental study of the characteristics of hard x-ray (HXR) emission from multi-planar wire arrays and compact-cylindrical wire arrays (CCWA) plasmas on the 1.6 MA Zebra generator at UNR has been carried out. The characteristics of HXR produced by multi-planar wire arrays such as single, double, and triple planar as well as compact-cylindrical wire arrays made from Al, Cu, brass, Mo, and W were analyzed. Data from spatially resolved time-integrated and spatially integrated time-gated x-ray spectra recorded by LiF spectrometers, x-ray pinhole images, and signals from fast x-ray detectors have been used to study spatial distribution and time history of HXR emission with different loads. The dependence of the HXR yield and power on the wire material, geometry of the load and load mass is observed. Both HXR yield and power are minimum for Al and maximum for W loads. The HXR yield increases with the rise of the atomic number of the material for all loads. The presence of aluminum wires in the load with the main material such as Cu, Mo, or W in combined wire arrays decreases HXR yield. For W plasma, the intensity of cold L-shell spectral lines (1–1.5 A) correlates with corresponding amplitude of HXR signals which may suggest the evidence of generation of electron beams in plasma. It is found that HXRs are generated from different plasma regions by the interaction of electron-beam with the plasma trailing mass, with the material of anode and due to thermal radiation from plasma bright spots. The theoretical assumption of thermal mechanism of HXR emission predicts the different trends for dependency of HXR power on atomic number and load mass.

The radiative performance of Z-pinches created by the imploding wire array loads is defined by the ablation and implosion dynamics of these loads. Both these processes can be effectively modeled by the Wire Ablation Dynamics Model (WADM), which extends the formalism exploited earlier for the cylindrical wire arrays to the loads of arbitrary geometries. The WADM calculates the ablation rates for each array wire and provides the important dynamic parameters, such as the specific mass and velocity of the imploding plasma, which can be used to estimate the shapes of the x-ray pre-pulse and, partially, the main x-ray burst. The applications of the WADM also extend to combined material wire array loads. The ablation and implosion dynamics of novel Prism Planar Wire Array (PPWA) and combined material (Mo/Al/Mo) Triple Planar Wire Array (TPWA) loads are discussed in detail. The combined WADM and radiation MHD simulation is applied to model the radiative performance of the precursor plasma column, created by the imploding stainless steel compact cylindrical wire array. As the radiation effects intensify with the mass accumulation at the array center, the simulation reveals the transformation of quasi-uniform precursor column into a heterogeneous plasma structure with strong density and temperature gradients. We find that radiative performance of the precursor plasma is greatly affected by the load geometry as well as by the wire material.

X-pinches are very good sources of x-rays and can be used for studying radiative properties of high density and temperature plasmas with scale from a few μm to several mm in size. An X-pinch is formed by the touch-crossing of two or more wires between the electrodes of a high-current pulsed-power generator. As a result of current quickly vaporizing and strongly ionizing the wire material, X-pinch yields short (few nsec) x-ray bursts from one or few bright plasma spots near the wire cross point. Other distinct features of X-pinches are strong electron beams, which make them attractive objects for x-ray spectropolarimetry, as well as plasmas jets for astrophysical applications. Recently, we spectroscopically studied x-ray L-shell and K-shell radiation from variety of X-pinches from different materials and load configurations. In the present work, the results of x-ray spectroscopy and imaging of X-pinches from stainless steel are presented. The application of these results to astrophysics is highlighted.

The spectroscopic features of implosions of Mo single- and double-planar wire arrays are studied. The experiments were performed on the 1 MA Zebra generator at UNR. Implosions of Mo planar wire arrays radiate high peak powers and produce high-temperature L-shell plasmas. In particular, plasma electron temperature of single-planar wire arrays of Mo reached ∼1375 eV. To estimate and analyze the evolution of plasma parameters in space and time, spatially resolved, time-integrated L-shell Mo X-ray spectra as well as time-gated and time-integrated pinhole images were collected and analyzed. A non-LTE kinetic model was applied to study the spatial structures, temperatures and densities of different wire loads of Mo planar wire arrays. Effects of non-Maxwellian hot electrons on L-shell radiation of Mo are discussed. Furthermore, Mo planar wire arrays generate bright spots or clusters of bright spots along the axial directions. Temperature and density gradients inside these bright spots or its clusters are also investigated. The results are compared with results of previous experiments with X-pinches on UNR Zebra and nested wire arrays at SNL-Z. Radiation magnetohydrodynamics modeling was performed to analyze the mechanisms of Z-pinch plasma heating in the presence of strong density gradients.

Laboratory experiments using a plasma wind generated by laser-target interaction are developed to investigate the creation of a shock in front of the magnetosphere and the dynamo mechanism. Magnetic obstacles are placed in the plasma wind and measurements of the electron density gradients surrounding the obstacles are recorded. The experiments are analyzed with the methods used in theoretical simulation of the solar-wind-driven magnetosphere interactions. The proposed experiments are thought to be relevant to understanding the electron acceleration mechanisms at work in shock-driven magnetic dipole confined plasma surrounding compact magnetized stars and planets. Electron shock acceleration mechanisms are discussed in some detail.

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

Implosion characteristics of cylindrical arrays of aluminum (Al) or tungsten (W) wires with low number of wires were studied on the 1-MA 100-150-ns current rise time COBRA generator at Cornell University using X-ray/extreme ultraviolet detectors, time-gated cameras, spectrometers, and electrical diagnostics. Total radiation yields ET of 2.8 and 4.1 kJ were measured for Al and W, respectively. The yield above 0.75 keV for W arrays was lower than for Al. Al spectra imply Te of 250-280 eV. X-ray spectra in the 2-3-keV range from W arrays showed only very weak spectral lines. A relatively uniform precursor plasma column was observed on time-gated images in the initial implosion phase for both Al and W. Rapid radiation cooling of W plasmas after precursor plasma formation leads to a decrease of plasma Te before the stagnation phase of the implosion. No evidence of the increased energy deposition that could be associated with the single-shell or closed toroidal magnetic structures models was found

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.

A wall confined plasma in an inverse-pinch configuration holds potential as a plasma target for Magnetized Target Fusion (MTF) as well as a simple geometry to study wall-confined plasma. An experiment is planned to study the inverse-pinch configuration using the Zebra Z pinch [B. S. Bauer et al., AIP Conference Proceedings Vol. 409 (American Institute of Physics, Melville, 1997), p. 153] of the Nevada Terawatt Facility at the University of Nevada, Reno (UNR). The dynamics of the discharge formation have been analyzed using analytic models and numerical methods. Strong heating occurs by thermalization of directed energy when an outward moving current sheet (the inverse pinch effect) collides with the outer wall of the experimental chamber. Two-dimensional magnetohydrodynamic simulations show Rayleigh–Taylor and Richtmyer–Meshkov like modes of instability, as expected because of the shock acceleration during plasma formation phase. The instabilities are not disruptive, but give rise to a mild level of turbu...

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.

Brass wire array implosions on Zebra have produced characteristic K-shell emission. These Kα and Kβ photons are a result of high-energy electrons (>∼10 keV) ionizing or exciting a 1s bound electron from ionization stages around the Ne-like charge states. The electron beam was measured using a Faraday cup. The K- and L-shell radiation was captured using time-gated and time-integrated spectrometers. The L-shell radiation comes from ionization stages around the Ne-like charge state that is mostly populated by a thermal electron energy distribution function. X-ray imaging was accomplished using time-integrated and time-gated pinhole cameras, with one camera using a Ross filter pair to image the K-shell emission. The diagnostic suite included various filtered x-ray diodes, bolometers, and laser shadowgraphy. A multi-zone non-LTE kinetics pinch model with radiation transport is used to obtain the plasma conditions by matching the observed x-ray emission. Monte-Carlo simulations are employed to infer information about the electron energy distribution function for the runaway electrons.

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.

International audience; 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.

International audience; 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 Å. 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 with electron beams and SiD signals. Recently, an interesting issue regarding these Ag plasmas is whether lasing occurs in the Ne-like soft x-ray range, and if so, at what gains? To help answer this question, a non-local thermodynamic equilibrium (LTE) kinetic model was utilized to calculate theoretical lasing gains. It is shown that the Ag L-shell plasma conditions produced on the Zebra generator at 1.7 maximum current may be adequate to produce gains as high as 6 cm−1 for various 3p → 3s transitions. Other potential lasing transitions, including higher Rydberg states, are also included in detail. The overall importance of Ag wire arrays and plasmas is discussed.

The dynamics of a capillary discharge is studied to achieve optimum conditions for the guiding of ultrashort intense laser pulses. A dynamic regime is revealed in which, after a short transient process, the discharge plasma is in dynamic and thermal equilibrium. Such plasma configuration is stable against MHD perturbations. It is shown that the radial inhomogeneity of the discharge plasma composition can provide the improvement of the focusing properties of a plasma waveguide. The radius of the region where electromagnetic radiation is localized is governed by a contact discontinuity between the plasma that initially fills the channel and the plasma that is produced due to ablation of the capillary wall material.

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.

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.

Extreme ultraviolet (EUV) radiation from z-pinch plasma sources has been shown to play a substantial role in the evolution of z-pinches, contributing significant amounts of radiation in the wire ablation, stagnation, and plasma expansion phases. Recent studies of Cu z-pinch plasmas from cylindrical wire arrays have also shown that high temperatures (up to 450 eV) exist in precursor plasmas, which have applications to inertial confinement fusion. The final expansion phase has shown that substantial EUV radiation continues even after the main x-ray bursts. In this work, EUV data were analyzed with the goal of understanding how the bulk cooler plasma might represent the main contribution to the total radiative output from z-pinch plasmas. In particular, a comparison and analysis of EUV data generated by two plasma sources is shown: the first set of experiments used Cu cylindrical wire arrays on the 1.0 MA Zebra generator at UNR. In addition to EUV data, x-ray data is also analyzed which shows dominant emission of Cu XX ions. The second set used Cu flat targets and was performed at the compact laser-plasma x-ray/EUV facility “Sparky” at UNR, which is used as a unique line calibration source. Moreover, spectral data generated by Sparky generally show more and better resolved lines. Cu L-shell lines in the range of 120-160 Å, specifically CuX to CuXIII ions, are identified. To help with the identification of lines, a non-local thermodynamic equilibrium (non-LTE) kinetic model was utilized and was also used to determine plasma parameters, such as electron temperature and density. Future studies will focus on attaining time-gated EUV spectra in order to better understand its role in the evolution of z-pinch plasmas.

International audience; The planar wire array (PWA) is a promising load for new multi-source inertial confinement fusion (ICF) hohlraums [B. Jones et al. Phys. Rev. Lett. 104 125001 (2010)]. The hohlraum radiation symmetry is an important issue for ICF. It was found that extreme ultraviolet and sub-keV photon emission from PWAs may have considerable anisotropy in the load azimuthal plane. This experimental result is obtained on the UNR 1–1.7 MA Zebra generator. The time-dependent anisotropy effect is detected. This feature is studied in 2D numerical simulations and can be explained by initial anisotropy of implosion of those non-cylindrical loads radiating essentially as surface sources in sub-keV quanta and also by radiation absorption in cold magnetized plasma tails forming in the direction of magnetic compression.

International audience; A new hohlraum configuration for radiation physics/ICF was proposed [B. Jones et al., PRL 104, 125001 (2010)]. Multiple compact (mm-scale) planar wire array (PWA) sources that surround a central hohlraum allow reduced hohlraum surface area and potentially provide a hotter x-ray environment than in a double-ended scheme with cylindrical arrays. The current multiplication technique became possible because of high impedance of Zebra and increase of the current up to 1.7 MA. We have demonstrated previously that the PWA plasma at stagnation dissipates the magnetic energy greater than ΔL*I2/ 2, operating mainly as a resistor R(t) [V. Kantsyrev et al., Dense Z-pinch 2011 CP]. This allowed us to redistribute the generator current between two magnetically decoupled PWA without significant loss of radiation yields. With half the current flowing through each PWA, the kinetic energy Ek coupled to it, which scales as I2, will be 1/4 of the energy coupled to a single PWA carrying the same total current and experiencing the same change of inductance ΔL at implosion. The total Ek delivered to two PWAs at stagnation would thereby be 1/2 of Ek delivered to a single PWA. In experiment, the current was split between two decoupled sources (0.75 MA to each), and the total yield EΣ was at least 85 % compared to total yield ET from single PWA at 1.6 MA. Then, new hohlraum configuration with two decoupled PWAs was tested. The VisRad code (Prism Comp. Sci.) predicted hohlraum radiation temperature >30 eV which correlates with strong hohlraum output radiation (hυ > 17 eV). Scaling using VisRad on different hohlraum multisource configurations for higher current 20 MA scale generators is discussed. The experimental comparison of planar foil liners (another promising x-ray radiator) with PWAs is also presented.

Summary form only given. Experiments on Zebra with a Load Current Multiplier (LCM, which provides an enhanced current of 1.5–1.7 MA) allow the implosion of larger wire array loads than possible with the standard current of 1 MA. Advantages of the larger wire arrays include enhanced energy coupling to plasmas and better diagnostic access to observable plasma regions. Diagnostics, fielded on 10 beam lines, included PCD, XRD, and EUV detectors, X-ray/EUV spectrometers and X-ray pinhole cameras, and laser shadowgraphy. For this work, we collected and analyzed the experimental results from standard and modified triple planar wire arrays (TPWA), as well as cylindrical wire arrays (CWA). The anode-cathode gap in these experiments with LCM was 1 cm, which is half the gap used in the standard mode. For TPWAs, two outer wire planes were made out of mid-atomic-number wire material (Ni or Cu) with the inter-row gap increased from 1.5 or 3 mm (usually used at 1 MA current) up to 4.5 mm. The different designs of a central wire plane from Al were implemented to investigate its role as a magnetic field extruder to prevent the formation of closed magnetic configurations around each wire plane. Previous work has shown that larger double planar wire arrays (at a standard current) can block the inward motion of ablated plasma jets. Therefore, multi-planar wire arrays provide a new test bed to study jet formations in larger wire arrays for laboratory astrophysics. In addition, we measured higher linear radiation yield and the high-temperature precursor Al plasmas, not previously observable in experiments on Zebra at 1 Ma current. However, the observation of high-temperature precursor plasmas from Cu CWAs at enhanced current was consistent with previous findings at a standard current of 1 MA.

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.

International audience; 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.

International audience; Previous experiments on the 1 MA Zebra generator at UNR were dedicated to studies of precursor plasmas with Ni-60 alloy (96% Cu) cylindrical wire arrays (CWA). Those precursor plasmas were shown to consistently have electron temperatures >; 400 eV 1. Continuing research at 1 MA on Zebra found precursors for Cu, Brass, and Alumel CWAs to be consistent with Ni-60 results2. Recent Ni-60 CWA experiments have been performed on Zebra using a Load Current Multiplier (LCM) that raises the current up to 1.7 MA. A full set of diagnostics included 10 beam lines. These CWAs consist of 6 wires evenly spaced in a 12 mm diameter as before, but with a linear mass density of 100 μg/cm, more than double the mass density in 1 MA experiments. The radiation yield per unit length in the LCM shots, 16 kJ/cm, (with decreased 1 cm anode-cathode gap) has approximately doubled the previous Ni-60 yield per unit length. The total integrated PCD energy (filtered with 8 μm Be) increases up to 750 J (compared to the previous ~500 J). The percentage of the PCD due to the precursor has remained relatively constant. Non-LTE kinetic models of Cu and Ni have been applied to account for the L-shell radiation from the 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.

The dynamics of a laser ablation plasma expanding in an external magnetic field have been investigated with imaging interferometry and shadowgraphy. The diagnostics reveal a new interaction mechanism, namely, the redirection of the explosive plasma expansion into a converging flow. A comparison with three-dimensional ideal magnetohydrodynamic simulation results supports the observation that the efficient lateral plasma confinement causes the plasma to converge on the axis and initiate a directed flow. The resulting collimated flow propagates across the magnetic field in a kinetic regime, which cannot be modeled within the same framework.

We continue to study the physics of wire arrays and to search for more efficient X-ray radiators from wire array Z-pinch plasmas. Two important questions are addressed in this talk. First, what are the main contributions in the total radiation yield? To consider this question, we divided the time interval of the Z-pinch dynamics where wire ablation, implosion, stagnation, and plasma expansion occur in three time zones and studied the radiative and implosion characteristics within each. The second question is how does the distribution of the radiated energy within these three time zones depend on the configuration of the load and wire materials. The experiments were performed on the 1 MA Zebra generator at UNR with a full set of diagnostics that included 10 beam lines. In particular, PCD, XRD, and EUV detectors, X-ray/EUV spectrometers and X-ray pinhole cameras, and laser shadowgraphy were utilized. We collected and analyzed the experimental results from single and nested cylindrical as well as single and double planar wire arrays. A broad range of wire materials were explored from low-Z (Al) to moderate mid-Z (Fe, Ni, and Cu) and higher mid-Z (Mo and Ag). The strong dependence of the distribution of radiation energy between the three zones on the load configuration was observed. Z-pinch dynamics and both X-ray and EUV radiative properties in all three time zones were considered. A precursor plasma in the first zone from cylindrical wire arrays as well as the plasma in the second zone at stagnation from cylindrical and planar wire arrays are discussed. A special emphasis was put on the time evolution of EUV radiation in all three zones and in particular on its substantial value in the third zone after stagnation. Future directions of this work are discussed.

Experiments with Ag, uniform and combined, single and multi-planar wire arrays (PWAs) are presented. Mo has been extensively studied in previous years and has shown to have high electron temperature (1400 eV) and energy output (25 kJ), which is of interest for future applications of inertial confinement fusion. Preliminary results on Ag single PWAs have shown promising results of even higher temperatures and energies, indicating the importance of future studies. All experiments were performed on the Zebra generator at UNR. A set of diagnostics included fast, filtered x-ray diodes; a Ni bolometer; laser shadowgraphy and streak setups; time-gated and time-integrated x-ray pinhole cameras; and time-integrated spatially resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectrometers. The acquisition of spatial and temporal gradients in L-shell plasmas of very high mid-Z elements is of interest. In particular, the understanding of the time evolution of these plasmas is a critical next step of research. Implosion dynamics were analyzed with the wire ablation dynamics model. Non-LTE kinetic modeling was utilized to derive plasma electron temperature and density of L-shell Mo and Ag, which includes a new updated model of Ag presented for the first time. Further applications to pulse shaping and possible changes in opacity effects with different mixtures of Ag and Mo are also discussed. In result, the first analysis and comparison of L-shell Ag uniform and combined PWAs was accomplished.

Previous experiments on the 1-MA “Zebra” generator at UNR studied precursor plasmas with Ni-60 alloy (96% Cu) wire arrays1. Those precursor plasmas were shown to consistently have electron temperatures > 400 eV 1. New recent experiments performed on Zebra using cylindrical wire arrays (CWA) continue to emphasize stronger radiating precursor plasma but from other mid-atomic number wire arrays. 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 during the recent mid-atomic number (Cu, brass, and alumel) CWA experiments. Preliminary results on these CWA are consistent with previous Ni-60 results. Non-LTE kinetic models of Cu, Zn, and Ni have been applied to account for the L-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.

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.

Triple Planar Wire Arrays (TPWA) consisted from three planes made of Mo (uniform) or from Mo and Al wire planes (combined). Each plane from Mo or Al wires was almost the same mass and therefore the total linear mass of both uniform and combined TPWAs was about 115 µg/cm. The previous results with 3 mm between planes are revisited. New experiments with the same TPWAs but with a reduced interwire gap of 1.5 mm that showed high yield of 25 kJ (close to the highest yield from Mo double planar wire arrays) are presented and analyzed. In addition, implosion dynamics and radiative properties of combined nested cylindrical wire arrays (NCWA) made of the same size Mo and Al wires (as in TPWAs) were compared with TPWAs, and similarity and differences are discussed. All experiments were performed on the Zebra generator at UNR. A set of diagnostics included fast, filtered x-ray diodes; a Ni bolometer; laser shadowgraphy and streak setups; time-gated and time-integrated x-ray pinhole cameras; and time-integrated spatially resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectrometers. Implosion dynamics were analyzed with the wire ablation dynamics model. Non-LTE kinetic modeling was utilized to derive plasma electron temperature and density and to estimate opacity effects in K-shell Al lines. In particular, spatially resolved L-shell Mo plasma parameters were determined from TISR spectra. Time-gated (from TGSI data) and spatially-resolved (from TISR data) K-shell Al and Mg plasma parameters were modeled. In result, a comprehensive analysis and comparison of L-shell and K-shell plasma parameters for TPWAs with different geometries as well as for NCWAs was accomplished. Future directions of this work are discussed.

EUV spectroscopy is a feasible tool for diagnosing of Z-pinch plasma before and after stagnation in complement to x-ray spectroscopy, which is conventionally used in studying of the stagnating hot dense plasmas. A collection of aluminum EUV spectra from implosions of traditional cylindrical wire arrays and new planar wire arrays (PWA) on the 1 MA Zebra generator at UNR are being analyzed in this work. Al PWA loads were uniform and combined with Mo wires. Also, the EUV spectra and images from experiments with combined aluminum and stainless steel alloy 304, nested wire arrays from the 1MA COBRA generator at Cornell University are presented. The loads consisted of 8 or 16 wires of different materials in outer and inner arrays: stainless steel or aluminum. Non-LTE Al model was used to identify EUV spectra and to estimate plasma parameters. The special emphasis was made on study of dependence of Al EUV spectra on the type and composition of the load. WADM and radiation MHD simulations have been performed to model the load implosion, analyze the EUV images and calculate the bulk temperature of the precursor plasma column. In addition, we study the aluminum EUV spectra, recorded at the laser plasma facility “Sparky” at UNR. These spectra help to benchmark our spectroscopic models and identify the Z-pinch spectra.

The study of characteristics of hard x-rays and generation of electron beams is very important for High Energy Density Physics (HEDP).The illustration of such a study is performed by using Z-pinch plasma. Specifically, we are studying implosions of different types wire array loads and materials that produce hard x-rays (HXRs) and e-beams. We analyze characteristics of HXR produced by in multi-planar wire arrays such as single planar, double planar, triple planar, cross planar, and compact cylindrical wire arrays. The study of HED plasma from such elements as Al, Cu, Mo, W is important for HEDP. Data from spatially resolved time-integrated and spatially integrated time- gated x-ray spectra recorded by LiF spectrometers, and pinhole images has been analyzed to study spatial distribution of HXR emission with different load and different material. The dependence of the HXR yield and power on the wire material, and geometry of the load is determined. The HXR yield increases with the rise of atomic number of material for all loads. For W plasma, the intensity of cold L-shell spectra (1.39-1.54.) correlates with corresponding amplitude of HXR peaks which may suggest the evidence of generation of electron beams. The HXR yield per unit length increases with the rise of load height (anodecathode gap). The presence of aluminum wires as a tracer to main material like Cu, Mo, W in combined wire arrays decreases HXR yield. It is found that HXRs are generated from different plasma regions by the interaction of e-beam with cooler trailing mass, material of anode and due to thermal radiation from plasma bright spots.

Plasma study at university level pulse power facilities as well as at the world's largest z-pinch machines serves as one of the major drivers in the emerging topic of High Energy Density Physics. Plasmas created by 1 MA discharge at the university laboratories are very interesting and unique XUV and x-ray radiation sources that have extreme densities (nearing the solid state densities) and temperatures (in some cases exceeding 1 keV level). At such parameter range the dynamics and radiation performance features of these plasma objects are strongly dominated by the radiation effects such as the radiative cooling and radiation transport. Considering as an example wire array z-pinch, one can estimate that near the current pulse maximum the radiation energy density becomes comparable with the specific internal energy of the precursor plasma column. At the same time the total radiation yield from z-pinch is several times higher than the plasma thermal energy. The main focus of this presentation is the study of radiation effects on the dynamics of high energy density plasma with the radiation MHD modeling. Dynamics of the precursor plasma column in imploding compact cylindrical wire arrays is discussed in greater details, while the simulation results are verified versus the experimental data. Particularly, the radiation MHD simulations demonstrate the transformation of precursor structure from uniform to heterogeneous as the precursor mass increases and radiation effects intensify. Novel Wire Ablation Dynamics Model has been used to account for wire material properties, such as the mass ablation rate. It is shown that by changing the wire materials one can control the parameters and radiation performance of the precursor column plasma. It is demonstrated that the flexibility of wire array load configuration provides the unique opportunities for a wide range of High Energy Density Physics experiments, particularly in the area of laboratory astrophysics.

A university-scale Z-pinch generator is able to produce HED plasmas within a broad range of temperatures and densities depending on the type of load configurations and wire materials. Experiments with very different Z-pinch loads were performed on 1 MA Zebra generator at UNR and analyzed during the last five years: X-pinch, Cylindrical, Nested, and various types of Planar Wire Arrays (PWA). X-pinches are very good sources of x-rays and can be used for studying radiative properties of high density (≫1022 cm−3) and temperature (≫ 2 keV) plasmas with scales from a few µm to several mm in size. They yield short (few nsec) x-ray bursts from one or several bright plasma spots near the wire cross point and produce strong electron beams as well as plasma jets. PWAs, a new type of the wire array load, were tested in single and multi-plane configurations. It was shown that PWAs may produce significant radiation yield in a ns-scale (up to 25 kJ) and generate bright spots of sub-mm size. The main focus of this presentation is the comparative analysis of the results from recent experiments with the X-pinch, Cylindrical and Nested Wire Arrays and PWAs made from stainless steel wires. The diagnostic set included XRD and PCD detectors, laser shadowgraphy, x-ray time-gated and time integrated imaging, time integrated spatially resolved spectrometers for harder (K-shell) and softer (L-shell) x-ray regions, and a time-gated spatially integrated spectrometer for the softer (L-shell) x-ray region. The non-LTE kinetic, WADM, and MHD codes are used to guide the analysis of the results. In particular, the new results for X-pinches with different angles between the wires are presented and their relevance to astrophysics is demonstrated. The new results from the experiments with above-mentioned wire array loads focus on the study of precursor formation as well as plasma evolution at the stagnation phase. The unique properties and HED features of such plasmas are summarized and discussed.

Previous experiments on the 1-MA “Zebra” generator at the University of Nevada, Reno studied L-shell Cu and Zn emissions from implosions of brass planar wire arrays1. Essentially the results of that study shows how the temporal variation in electron temperature (Te=320–450 eV) correlates with the x-ray burst during the stagnation stage. The purpose of the present study was to expand upon the previous knowledge by investigating the temporal behavior of cold K-shell characteristic lines that originate from the cooler plasma. This will provide understanding of the nature of cooler plasmas and also electron beams. In order to capture the cold Kα lines from the wire material (1.39–1.54 A), the time-gated spectrometer was used with a LiF crystal. The diagnostic suite also included time-integrated spatially resolved spectrometers capturing L-shell and K-shell spectra using a KAP crystal and a LiF crystal, respectively, time-integrated and time-gated pinhole imaging, PCD, XRD, Si-diode signals, and laser shadowgraphy. The WADM is used to analyze the array implosion. Radiation MHD is preformed to understand the origin of strong gradients in the plasma electron temperature. The usefulness of measuring cold Kα lines with time resolution for a better understanding of wire array plasma formation is demonstrated.

Summary form only given. The double-planar wire array (DPWA) is the most promising Z-pinch load studied on the Zebra generator for application to radiation physics and inertial confinement fusion studies. These loads radiate powers and energies up to ~1 TW and ~25 kJ, respectively, in a versatile pulse shape on the 1 MA Zebra generator. Recent studies show that radiation yields of this load scale near-quadratically with currents ranging 0.8-1.4 MA, and that maximum powers were obtained with the Mo DPWA at the minimum inter-planar gap studied: 1.5 mm (6 mm width). This scalability with current and dimension, along with superior yields, make the DPWA attractive for future studies on larger, pulsed-power generators. Initial investigation of this load revealed unique implosion dynamics: each plane implodes independently into two off-axis columns before coalescing into the final pinch. New results are presented exploring the formation of these off-axis plasma columns with laser shadowgraphy and time-gated X-ray imaging, each fielded along the radial line of symmetry. The wire ablation dynamics model (WADM) is used to show new details about the precursor formation and implosion of DPWA loads composed of Al, SS, and W. The WADM was used with combined-material nested cylindrical wire arrays to show an ablation dominated implosion regime. This technique is applied to combined Al/SS and Al/W DPWA to study ablation effects on the formation of the precursor and implosion. This analysis is then applied to the triple-planar wire array and the new, skewed-wire, DPWA with axial magnetic field. Finally, results from the analysis of Al/SS DPWA time-gated spectra are presented.

Summary form only given. Analysis of the results of tungsten single and double planar wire array (PWA) experiments performed on the UNR 1 MA generator "Zebra" is presented. Uniform as well as combined PWAs with Al wires arrays were investigated and compared. Previous spectroscopic work focused on analysis of time-integrated spatially resolved (TISR) stagnation phase data, while this presentation continues and expands that study to include the early implosion plasma stage and more data from recent shots. A full diagnostic set was utilized, including a pulse laser shadowgraphy system, a gated fast intensified CCD (ICCD) camera, an X-ray time-gated pinhole camera, a TISR spectrometer, and a time-gated spatially integrated (TGSI) spectrometer. Non-LTE kinetic modeling is used to describe spectroscopic data for combined PWAs in particular. wire ablation dynamics model is employed to model the implosion and interpret shadowgraphy data. Opacity effects in plasmas from uniform and combined PWAs with Al wires are discussed and compared. The advantage of using the tracer Al wires is emphasized. Applications to current fusion research are also considered.

International audience; Summary form only given. Experiments with different Z-pinch loads were performed on 1.4 MA, 100 ns Zebra generator at UNR. The program emphasized investigation of plasma formation, implosion and radiation features as a function of the load configuration: compact multi-planar and cylindrical wire arrays. The multi-planar wire arrays (PWA) were studied in open and closed configuration with Al, stainless steel, Cu, brass, Mo and W wires. In open configurations, for single, double, and triple PWAs, wire rows are parallel to each other and there is magnetic field inside the array from the beginning of the discharge. A new closed configuration, Prism-like triangle PWA, eliminates the global magnetic field inside (as cylindrical arrays) ahead of the plasma flow. Prism-like PWAs (that are not equal to compact cylindrical array) show high flexibility in control of implosion dynamics and precursor formation by switching of wire materials and changing mass ablation rates in prism rows and corners. The spectra modeling, MHD and WADM codes have been used for implosion and stagnation parameters simulation. Electron temperature and density in multiple bright spots reached ~1.4 keV and ~5x1021 cm-3, respectively. With current rise from 0.8 up to 1.3 MA, opacity effects became more pronounced, which may limit X-ray yields from PWA and especially from CCWA loads: much lower peak current yields scaling for CCWAs compared with PWAs; appearance of absorption lines in X-ray spectra, and anisotropy of sub-keV emission from SPWAs. The DPWA was found to be the best X-ray radiator at 1 MA tested at Zebra: it is characterized by combination of larger resistive energy and power gain (25 kJ and 1 TW), small mm-scale size, the possibility of radiation pulse shaping, and easy diagnostics access to plasmas. These characteristics may even be improved by application of a new tested load: DPWA with skewed wires. In this load, an initial axial magnetic filed is created that migh- mitigate the magneto-RT instabilities and provide more effective X-ray generation. The experimental results are summarized and discussed.

Summary form only given: The novel wire ablation dynamics model (WADM) has been recently developed and applied to simulate the implosion dynamics and to predict the implosion time of various wire array load configurations, including single- and multi-planar and nested cylindrical single material and combined material arrays. However, besides the accurate prediction of the implosion time the WADM provides the important information about the evolution of key dynamic parameters of array implosion, such as the wire mass ablation rate and the velocity of the inward streams of the ablated plasma. In its turn, these data allow the calculation of another important parameter such as the bulk temperature of the precursor plasma column. Yet, such calculation can only be considered as a rough estimation because the WADM does not resolve the spatial structure of the precursor column (thus, disregarding possible gradients of plasma temperature) and does not account for radiative cooling due to the radiation power losses. All these drawbacks, however, can be eliminated in radiation MHD modeling. This presentation focuses on the simulations of the precursor plasma column formation and its further evolution until the array implosion, using the combined WADM and MHD modeling. A substantial amount of experimental data from the diagnostic complex, including the laser probing, optical streak camera, XUV and X-ray imaging, X-ray detector signals, are used to provide the input for WADM and MHD simulations and to verify the modeling results. Study of precursor formation and evolution in the same compact cylindrical arrays but made of different wire materials shows a strong dependence of the precursor plasma parameters and radiation performance features on the material of array wires. The similarities and differences of the precursor formation and evolution in different wire array geometries, such as compact cylindrical and double planar arrays, have also been studied. The algorithm of combined WADM and MHD modeling has no limitation on wire array geometry, while the models of precursor dynamics provide critical information about z-pinch plasma parameters at pre-stagnation stage, which is particularly important to understand the dynamics and radiation performance of z-pinch plasma during the maximum X-ray output.

A general overview of Mo planar wire array implosions (single, double, and triple) from the past three years on the 1 MA, 100 ns UNR Zebra generator is presented. Mo planar wire arrays on Zebra have radiated high peak powers and yields (up to 1 TW and 24 kJ) as well as produced high-temperature L-shell plasmas (up to 1.4 keV), indicating the importance in connection with inertial confinement fusion. In addition, Mo planar wire arrays combined with Al are presented. Implosion dynamics in different load geometries is analyzed with a wire-ablation dynamics model (WADM). Bright spot dynamics is considered with RMHD simulations. Overall trends of various implosion results, such as energy output, as functions of various parameters, such as load type and linear mass, is discussed. Plasma parameters of electron density and temperature derived from non-LTE models of Mo and Al are also presented and discussed. The advantage of using Mo wires for future study of wire arrays is demonstrated.

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 $g1000\text{ }\text{ }\mathrm{eV}$ 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 $\ensuremath{\sim}450\text{ }\text{ }\mathrm{eV}$, after which the temperature decreases to $\ensuremath{\sim}220--320\text{ }\text{ }\mathrm{eV}$ until the main implosion.

The experiments studied the variation in the precursor with different numbers of wires and wire array diameters. Preliminary results show a single X-ray pulse that resembles the shape, duration, and timing of the precursor observed on earlier shots. Therefore not only time-gated spatially-integrated (TGSI) but also time-integrated spatially resolved (TISR) X-ray spectra were generated by the precursor plasmas in shots when the main X-ray burst was negligible. Non-LTE kinetic models of Cu and Ni have been applied to account for the L-shell radiation from the Cu and Ni ions. The advantage of using Cu alloys with a small percentage of Ni for diagnostics of Cu wire arrays are illustrated. 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.

The requirements on lossless power transport through vacuum interface and MITL's limit from above the physical volume and hence inductance of the vacuum part of pulse power generators. This in turn limits the generator-to-load energy coupling and hence the magnetic energy available in vacuum loads used in high energy density physics research. We obtained on Zebra generator (1.9 Ohm, 1 MA, 100 ns) an enhanced load magnetic energy corresponding to the load current increase from the nominal 0.95 MA to 1.65 (plusmn0.05) MA. This improvement was achieved without changing the generator architecture, but through better generator-to-load energy coupling using the new Load Current Multipliers (LCM) technique. The average experimental load-to-generator current amplitude ratio in LCM with both a 7 nH constant-inductance load and with z-pinch loads was 1.7plusmn0.2. We report on new generator electrotechnical parameters with LCM and on characterization of the plasma dynamics and radiative properties of planar wire-array z-pinches at the achieved enhanced load magnetic energy level.

Two different sets of new experiments with wire arrays made from Mo or W (primary material) wires together with a few Al wires (tracer material) are analyzed. The first set of experiments involved nested wire arrays and was performed on the 1 MA COBRA generator at Cornell University. These nested wire arrays consisted of 16 Mo or W wires with equal number of wires in outer and inner arrays and with two wires of the main material replaced by Al wires either in the outer or in the inner array. The second set of experiments was performed on 1 MA Zebra generator at University of Nevada, Reno, with planar wire arrays that consisted of 10-16 W wires with two periphery wires replaced by Al wires. Diagnostics included fast X-ray and EUV detectors, time-integrated X-ray and time-gated EUV/x-ray imaging pinhole cameras, time-gated and time-integrated X-ray/EUV spectrometers, an optical streak camera and laser imaging. The analysis of spatially resolved X-ray spectra shows that L-shell radiation (Mo wire arrays) and M-shell radiation (W wire arrays) originate from hot spots in the imploded plasma, whereas K-shell Al radiation comes from a plasma column that extended between the cathode and the anode. Using non-LTE kinetic modeling, location, sizes, and plasma parameters of various L-shell Mo and M-shell W hot spots are estimated and discussed. It is shown that the electron temperature of hot spots can be 1 keV or more, but it is much less (200-300 eV) in the Al plasma columns.

Summary form only given. The planar wire arrays with multi-plane configuration (triple- TPWA and cross-plane-CPWA) were studied and compared with single and double-planar wire arrays (SPWA and DPW A) on the 1 MA, 100 ns Zebra generator at the University of Nevada, Reno. The uniform arrays were made of Al or Mo, and the combined made of both materials. The DPWA and TPWA consist of two and three wire rows, respectively, placed in parallel between the anode and the cathode. In a cross-plane load two single wire arrays cross under 90deg. The maximum power (1 TW) and yields (23-24 kJ) in short ns-scale rise-time x-ray bursts were observed for DPWA. The scaling shows that DPWA and TPWA yields do not drop rapidly as their width is decreased, and these arrays can be very compact (3-5 mm). In DPWA and TPWA the precursor is formed in several steps and in several locations, some of which do not coincide with the position of the stagnated plasma. Specific features of DPWA and particularly TPWA may open new paths for controlling the shape of radiation pulses. The CPWA implosion occurs in one step and the precursor position coincides with the position of the stagnated plasma which is the most uniform among planar wire arrays. For all planar arrays, the significant part of radiation at stagnation is associated with hot spots and radiation yields exceed conversion of the kinetic energy. A comparison of DPWA and cylindrical compact arrays indicates the possibility of the existence of the same plasma heating mechanism during the last several ns of implosion. Despite their very different implosion dynamics that continue even until 10 ns before stagnation, they form plasmas with similar temperatures and densities as well as generate similar powers and yields at stagnation. The possible heating mechanisms are discussed. The novel Wire Dynamics Model and MHD calculations which replaced the conventional 0D model are successfully used to model implosions of these complex wire arrays.