Detailed spectroscopic diagnostics of the stagnating plasma in two disparate z pinches allow, for the first time, the examination of the plasma properties within a 1D shock wave picture, demonstrating a good agreement with this picture. The conclusion is that for a wide range of imploding-plasma masses and current amplitudes, in experiments optimizing non-Planckian hard radiation yields, contrary to previous descriptions the stagnating plasma pressure is balanced by the implosion pressure, and the radiation energy is provided by the imploding-plasma kinetic energy, rather than by the magnetic-field pressure and magnetic-field-energy dissipation, respectively.

The Z Accelerator has been used for many years as a research facility for high energy density plasmas, with applications ranging from astrophysics to inertial confinement fusion. The available current at the Z Accelerator (>15MA) has also allowed for experiments over a wide range of K-shell X-ray sources, including Al (∼1.6keV), Ar (∼3.1keV), Ti (∼4.8keV), stainless steel (SS, ∼6.7keV), and Cu (∼8.4keV). The K-shell sources provide excellent opportunities for studying the details of a z-pinch through radiated output in various photon energy regimes, imaging, and spectroscopy. Variations in initial load configurations illustrate the difficulty in achieving appropriate plasma conditions for K-shell emissions, particularly for stainless steel and Cu. The requirement for large diameter loads (>40mm) enhances the growth of the magnetic Rayleigh–Taylor instability during the implosion; evidence of this instability is presented in stagnated pinch data. Data from a variety of K-shell sources and load configurations are presented and discussed to illustrate the details of the imploding and stagnated z pinches. The application of existing and modified scaling theories to the K-shell data is also described, as are multi-dimensional calculations that can be directly compared to the experimental observations. The recent refurbishment of the Z Accelerator will ultimately increase the coupled energy available to a load, with an anticipated peak current of ∼26MA into a wire array. The current waveforms measured to date are presented, along with a brief discussion of the current status of K-shell work at the Z Accelerator.

A series of experiments at the Z Accelerator was performed with 40mm and 50mm diameter nested wire arrays to investigate the interaction of the arrays and assess radiative characteristics. These arrays were fielded with one array as Al:Mg (either the inner or the outer array) and the other array as Ni-clad Ti (the outer or inner array, with respect to location of the Al:Mg). In all the arrays, the mass and radius ratio of the outer:inner was 2:1. The wire number ratio was also 2:1 in some cases, but the Al:Mg wire number was increased in some loads. This presentation will focus on analysis of the emitted radiation (in multiple photon energy bins) and measured plasma conditions (as inferred from x-ray spectra). A discussion on what these results indicate about nested array dynamics will also be presented.

University-scale Z-pinch generators are able to produce plasmas with a broad range of temperatures, densities, and opacity properties depending on the type, size, and mass of wire-array loads and wire materials. Experiments with very different Z-pinch loads were performed on the 1 MA Zebra generator at UNR and analyzed during the last five years including Single and Nested Cylindrical, Conical, and various types of Planar Wire Arrays. It is shown that such wire arrays are good sources of x-rays and that they produce significant radiation yield (up to 25 kJ) on a ns time scale, and generate bright spots of sub-mm size. They can be used for studying radiative properties of moderate density (between 10(18) cm(-3) and 5x10(21) cm(-3)) and temperature (<= 1.5 keV) plasmas. In addition, X-pinches generated higher density (>10(22) cm(-3)) and temperature (>2 keV) plasmas on scales as small as a few to several mm in size. Wire materials with a broad range of nuclear charge Z were used, ranging from low-Z, such as alloyed Al wires with varying concentrations of Mg, to mid-Z, such as Stainless steel, Cu, Brass, and Mo. Uniform (made from one wire material) as well as combined (made from two wire materials with almost equal wire masses) wire arrays were considered. Uniform, combined, symmetric and asymmetric X-pinches (some of which included a small fraction of tracer Al wires) were also considered. Non-LTE kinetic models to account for K- and L-shell radiation were employed to understand radiative properties of Z-pinch and X-pinch plasmas. Implosion characteristics of such loads are discussed using the wire dynamics and MHD models. Opacity effects of Z-pinch plasmas are studied and benefits of using alloyed and tracer wires are highlighted.

Axially localized NaF dopants are coated onto Al cylindrical wire arrays in order to act as spectroscopic tracers in the stagnated z-pinch plasma. Non-local-thermodynamic-equilibrium kinetic models fit to Na K-shell lines provide an independent measurement of the density and temperature that is consistent with spectroscopic analysis of K-shell emissions from Al and an alloyed Mg dopant. Axial transport of the Na dopant is observed, enabling quantitative study of instabilities in dense z-pinch plasmas.

Low wire number nested array Z-pinch experiments have been carried out with wires made of aluminum, stainless steel (uniform), and combinations of these two materials (mixed) on the 1MA COBRA generator at Cornell University [J. D. Douglass, J. B. Greenly, D. A. Hammer et al., in Proceedings of the 15th IEEE International Pulsed Power Conference (IEEE, Piscataway, NJ, 2005)]. The outer array consisted of eight wires, whereas the inner array had four or eight wires. The 10μm Al wires were alloy 5056 and the 6.25μm stainless steel wires were alloy SS304. The diagnostic suite included fast-x-ray and extreme ultraviolet (EUV) detectors, a time-gated x-ray pinhole camera, x-ray spectrometers, and laser shadow imaging. The main focus was made on the spectroscopic study of plasma evolution after the main x-ray burst though the data from photoconducting detector (PCD) and EUV signals over the whole period of current, and in addition laser shadowgraphy images before the main x-ray burst were analyzed. Modeling of the time-gated spectra recorded after the main x-ray burst indicates that the electron temperature Te either follows the PCD signals and peaks at times of the second (and the third if present) x-ray burst or has the higher value at the first frame (closest to the main x-ray burst), then slightly changes and increases at the last frame, which coincides with the second maximum of the current. It was also found that the values of Te never drop below 150eV, and the EUV signal remains intense even when the PCD signal is almost zero.

Summary form only given. The time-history of the ion-kinetic energy Ek ion throughout the stagnation phase of a neon-puff, 500 ns, 600 kA, Z-pinch implosion was determined. The X-ray spectroscopic system provides a resolving power of 6700 and four consecutive time gated (~1 ns) spectra. A simultaneous axial imaging allows for studying the ion kinetic energy at 0.1-mm-resolution along the pinch column. Ek ion in the stagnating plasma is obtained from the Doppler contribution to the line shapes of the Lyalpha satellites, verified to be optically thin. The line shapes give the ion velocity distribution just before stagnation (non Gaussian) and throughout the 10-ns-long stagnation (Gaussian-like). Ek ion was found to be sime12 keV early at stagnation, dropping down during the stagnation to the electron thermal energy (sime300 eV). The time scale of ion-kinetic energy loss is longer (cong2 ns) than expected from the ion and electron collisional thermalization time (cong0.1 ns). A plausible explanation of the data is that upon reaching the pinch axis, the stagnating plasma develops a turbulent flow, in which most of the implosion energy is stored. The turbulent motion then dissipates into ion heat more slowly than the ion-electron energy equilibration time, which causes Tion to be low, resulting in a slowing down of the ion energy transfer to electrons and to radiation. Detailed study of the experimental line shapes is used to examine this explanation. Axially-resolved measurements of the time-dependent stagnating-plasma properties, and the absolute total neon K radiation show that, within the experimental uncertainties, the observed total ion-kinetic energy accounts for the total radiation emitted from this plasma. These findings, and assuming the explanation given above, can be used to discriminate between the thermal and the turbulent ion kinetic energies throughout the stagnation. - - Results on the thus-inferred Tion will be presented. Comparisons will be made to implosion velocities and time-resolved line-widths observed in wire-array implosions on the Z machine.

Experiments have been performed at the Z accelerator to elucidate the effects of initial load diameter on the radiated output of a 7keV wire array x-ray source. Nested wire arrays with initial outer diameters of 45–80mm were fielded, with the masses chosen to maintain a nominally constant coupling to the Z generator. The total radiated output decreased from ∼1.1MJ to <0.5MJ for the largest diameter arrays, while the >1keV and K-shell radiation decreased at both small and large diameters. The >1keV output peaked at ∼340kJ, while the K-shell yield peaked at ∼55kJ. The observed trends in radiated output and stagnated plasma parameters are consistent with a phenomenological K-shell scaling theory, and are reproduced in one-dimensional modeling, although multidimensional effects, such as, growth of the Rayleigh–Taylor instability, are observed in the experiments and appear to impact the stagnated plasma for the larger diameter arrays.

Summary form only given. It is well known that implosions of X-pinches and wire arrays produce the powerful laboratory X-ray sources. X-ray spectroscopy is a very useful tool for diagnostics of X-and Z-pinch plasmas at stagnation while EUV spectroscopy seems to be an appropriate tool of diagnosing the plasma before and after the stagnation. Though X-ray spectra that characterize the stagnating plasmas are intensively used for X-and Z-pinch plasma diagnostics the EUV spectra are not vet studied in detail. In the present work a collection of EUV spectra from implosions of very different X-and Z-pinch loads on the 1 MA Zebra generator at UNR is presented for the first time. Specifically, the loads were Al X-pinches with 2 and 4 wires, cylindrical wire arrays (with and without a small portion of NaF coating), and planar wire arrays. The Al wires were from Al 5056 (95% Al and 5% Mg) and Al 1100 (99% Al) alloys. Non-LTE kinetic models of Al as well as Mg that were recently used to model X-ray K-shell Al and Mg spectra from the same loads were applied here to calibrate and identity the EUV spectra. Preliminary plasma parameters were computed. Similar and different features of the EUV spectra as well as corresponding plasma parameters from the above-mentioned loads were identified and analyzed. Modification of the existing models for the best fit of EUV spectra is discussed.

Summary form only given. The study of radiative properties of L-shell radiators from the new mid-Z wire materials is important for the development of diagnostics on the future SNL ZR. In the present work we extend our previous study of L-shell radiators of copper and stainless steel wire arrays to brass arrays. Modeling of X-ray spectra from the implosion of brass wire array loads is challenging because of the overlapping contributions of L-shell spectra from the Zn and Cu ions. L-and K-shell X-ray spectra have been accumulated from brass planar wire array experiments on the 1MA Zebra generator. They have been analyzed in detail and compared with the similar spectra from implosions of copper planar and cylindrical wire arrays. In particular, L-shell spectra recorded bv a KAP crystal cover the spectral range from 8 to 17 Aring for axially-resolved time integrated spectra and the narrower spectral interval from 6 to 11 Aring for time-gated spatially integrated spectra. Non-LTE Cu and Zn kinetic models have been applied to account for the K-and L-shell radiation from these ions. The Cu model was successfully applied before to study radiation from X-pinches and wire arrays from Zebra and the Zn model is a new model developed in this work. In general, modeling of x-ray spectra from mid-Z wire materials indicates a moderate Te of 200-400 eV for L-shell plasmas and much higher Te of 1.5-2.5 keV for K-shell plasmas. Though both L-and K-shell spectra from brass arrays were modeled the emphasis of this work was on L-shell modeling. Modeling of high quality time-gated spectra collected in these experiments provided valuable information on temporal plasma evolution during the stagnation phase. Modeling of the EUV spectra from these shots will also be presented and compared with similar spectra from copper shots.

Over the last several years, the Z Accelerator has been engaged in research on near-Planckian x-ray sources for inertial confinement fusion and on K-shell emitting sources for radiation-material interaction studies. These radiating z pinches exhibit complex dynamics that have been, and continue to be, studied over a wide range of configurations. In this paper, the progress to date for the production of tens to hundreds of kilojoules of K-shell emission from 8 keV to 3 keV will be presented. Nested wire arrays and multi-shell gas puffs have been employed to help mitigate implosion instabilities and asymmetries to produce x-ray powers of tens of terawatts with emitted x-ray risetimes of a few nanoseconds. Spectroscopy and modeling of these pinches are providing insight into the role of temperature and density gradients and other plasma phenomena in the production of the radiation. Future directions will also be discussed.

Planar wire array plasmas created by a 1MA current discharge provide a novel method to investigate dense plasma dynamics, heating mechanism and radiation properties. This is a strongly inhomogeneous on small scale plasma, which has high resistivity with density and magnetic field dependence. The planar array dynamics leads to generation of a plasma with a chain of hot spots in a dense, high-opacity column. The single planar array consists of a number of wires with inter-wire separation of ≤1mm in a linear row. It has been shown that a strongly inhomogeneous on small scale planar wire array plasma can radiate much more energy than the kinetic energy of imploding plasma. The results of recent experiments on scaling of radiation yields and powers with array masses, materials, inter-wire gaps and array width at 1MA, 1.5TW power Z-pinch Zebra generator at University of Nevada at Reno are presented. Radiation properties of planar arrays were compared with low-number cylindrical arrays and compact cylindrical array loads. Data on the generation of the hot spots during implosion of planar array plasma and its impact on the radiation pulse are reported.

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9×1013(±20%) neutrons at 2.34 MeV (±0.10MeV). Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic (MHD) calculations have indicated that thermonuclear outputs from Z could be expected to be in the (0.3–1.0)×1014 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2keV and ion densities of 2×1020cm−3, in agreement with the MHD calculations.

A variety of wire array experiments the last few years at the 20 MA Z Accelerator have been performed to assess the impact of initial load mass, initial load diameter, and variations of the nested array configuration on the K-shell output. Nominally, optimized configurations have been identified, with optimization determined by the highest K-shell output with the fastest rising, narrowest x-ray pulse. In this paper, the results of experiments performed to evaluate additional load configuration variations such as increased radial anode-cathode (RAK) gap, increases in wire number on nested arrays, and orientation of the nested arrays are presented. For stainless steel wire arrays (K-shell emission ∼6.7 keV), increasing the wire number on the nested arrays resulted in increased K-shell yield and K-shell power. Increasing the RAK gap from 6 mm to 10 mm resulted in changes to both the soft x-ray emission and stainless steel K-shell emission. The orientation of the wires on the inner and outer arrays of copper (8.4 keV) wire arrays was not observed to impact the radiated output, although calculations suggest that the effect of wire orientation will be overwhelmed by magnetic field asymmetries induced by the wire location relative to the openings in the return current can.

The Z machine at Sandia National Laboratories drives 20 MA in 100 ns through a cylindrical array of fine wires which implodes due to the strong j x B force, generating up to 250 TW of soft x-ray radiation when the z-pinch plasma stagnates on axis. The copious broadband self-emission makes the dynamics of the implosion well suited to diagnosis with soft x-ray imaging and spectroscopy. A monochromatic self-emission imaging instrument has recently been developed on Z which reflects pinhole images from a multilayer mirror onto a I ns gated microchannel plate detector. The multilayer can be designed to provide narrowband (similar to 10 eV) reflection in the 100-700 eV photon energy range, allowing observation of the soft emission from acereting mass as it assembles into a hot, dense plasma column on the array axis. In the present instrument configuration, data at 277 eV photon energy have been obtained for plasmas ranging from Al to W, and the z-pinch implosion and stagnation will be discussed along with > I keV self-emission imaging and spectroscopy. Collisional-radiative simulations are currently being pursued in order to link the imaged emissivity to plasma temperature and density profiles and address the role of opacity in interpreting the data.