N. Ouart

Radiation properties and implosion dynamics of planar and cylindrical wire arrays, asymmetric and symmetric, uniform and combined X-pinches on the UNR 1-MA zebra generator

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.

Energetic electron beam generation and anisotropy of hard x-ray emission from 0.9 to 1.0 MA high-Z X pinches

An analysis of the energy, spatial character, and temporal evolution of electron beams and hard x rays from 0.9 to 1.0 MA high-Z X pinches is presented. Experimental results from Ti, Fe, Mo, and W X pinches show that X pinches are an effective source of hard x rays with energies greater than 30 keV. Electron beams with energies up to 2 MeV higher than the applied anode-cathode voltage are generated along the pinch axis before the maximum current is reached. The beams have diameters of about 3 mm and generate bursts of hard x rays with sizes between 1 and 2 mm and total time durations of up to 150 ns. The measured 100–500 keV x-ray distribution is spectrally anisotropic. Hard x-ray synchrotron radiation in the side-on direction is proposed as a possible explanation of this anisotropy.

Planar Wire Array as Powerful Radiation Source

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

X-ray spectropolarimetry of high-temperature plasmas

X-ray spectropolarimetry is a powerful new tool for investigating the anisotropy of high-temperature plasmas. It is sensitive to the energetic electron distribution function and magnetic field, which, in general, have not been measured adequately. This diagnostic is based on the theoretical modeling of x-ray line polarization-dependent spectra measured simultaneously by spectrometers with different sensitivities to polarization. The increasing sophistication of spectroscopic monitoring of high-temperature plasmas has led to the necessity to account for both non-Maxwellian electron distribution functions and polarization properties of lines. This diagnostic can be applied to a broad range of plasmas, from low-density astrophysical plasmas to dense z-pinch plasmas. This work shows how x-ray spectropolarimetry complements the usual spectroscopic monitoring of hot plasmas and demonstrates the importance of accounting for x-ray line polarization in plasma diagnostics.

Opacity and gradients in aluminum wire array z-pinch implosions on the Z pulsed power facility

Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.

Advanced x-ray and extreme ultraviolet diagnostics and first applications to x-pinch plasma experiments at the Nevada Terawatt Facility

A wide variety of x-ray and extreme ultraviolet (EUV) diagnostics are being developed to study z-pinch plasmas at the Nevada Terawatt Facility at the University of Nevada, Reno. Time-resolved x-ray/EUV imaging and spectroscopy, x-ray polarization spectroscopy, and backlighting will be employed to measure profiles of plasma temperature, density, flow, and charge state, and to investigate electron distribution functions and magnetic fields. The instruments are state-of-the-art applications of glass capillary converters (GCC), multilayer mirrors (MLM), and crystals. New devices include: a novel GCC-based two-dimensional imaging spectrometer, a six-channel crystal/MLM spectrometer (“polychromator”) with a transmission grating spectrometer, and two sets of x-ray/EUV polarimeters/spectrometers. An x-pinch backlighter is under development. X-ray polarimeter/spectrometer, a survey spectrometer, a multichannel time-gated x-ray pinhole camera, and filtered fast x-ray diodes have observed the structure of Ti and Fe ...

X-ray spectroscopy and spectropolarimetry of high energy density plasma complemented by LLNL electron beam ion trap experiments

X-ray spectra of high energy density (HED) Z-pinch plasmas have been analyzed by means of a comprehensive kinetic modeling. A new diagnostic, x-ray spectropolarimetry, was applied to study anisotropy of Z-pinch plasma. This diagnostic is based on theoretical modeling of polarization-dependent spectra measured simultaneously by spectrometers with different sensitivity to polarization. Specifically, K-shell emission from Ti X-pinches was recorded simultaneously with identical LiF crystal spectrometers with the dispersion plane perpendicular and parallel to the discharge axis. Spectroscopic results from seven Ti X-pinch shots have been analyzed. Similar K-shell Ti polarization-dependent spectra generated by a quasi-Maxwellian electron beam at the LLNL EBIT-II electron beam ion trap have been studied. Further, the EBIT-II M-shell W spectra have proved to be important in the development of M-shell diagnostics of HED Z-pinch plasma. The advantages provided by electron beam ion trap data in the interpretation of...

Signatures of hot electrons and fluorescence in Mo Kα emission on Z

Recent experiments on the Z accelerator have produced high-energy (17 keV) inner-shell K-alpha emission from molybdenum wire array z-pinches. Extensive absolute power and spectroscopic diagnostics along with collisional-radiative modeling enable detailed investigation into the roles of thermal, hot electron, and fluorescence processes in the production of high-energy x-rays. We show that changing the dimensions of the arrays can impact the proportion of thermal and non-thermal K-shell x-rays.

Implosion dynamics in double planar wire array Z pinches

The double planar wire array (DPWA) Z pinch is a highly efficient radiation source with unique implosion dynamics and precursor formations. The inductively divided current successively ablates the wires and injects the material to the interior of the array. Three uniquely imploding DPWA load types were identified and classified according to the critical load parameter: low, intermediate, or high aspect ratio. Radiation pulse shaping was obtained from secondary precursors: off-axis mass carrying high current densities during the implosion phase. Time-gated EUV spectroscopy of off-axis mass accumulations is used to assess a 60 eV electron temperature prior to the implosion phase. These structures are shown to form by a different mechanism than the secondary precursors. High yields, compact size, and shaped radiation pulses merit further consideration of the DPWA as a candidate for inertial confinement fusion research.

Dynamics of Copper Wire Arrays at 1 MA and 20 MA Generators

Experiments to study the implosion dynamics and radiation characteristics of copper z-pinches have been fielded at the 1 MA Zebra facility and the 20 MA Z facility. The impact of initial load mass, initial load diameter, and nesting of wire arrays on the precursor and the stagnated plasma has been evaluated through spectroscopy, shadowgraphy, and fluence measurements. Plasma parameters extracted from modeling of the time-integrated L-shell spectra indicate the presence of more than one plasma source contributing to the radiation, likely due to non-uniform hot spot x-ray emission or temporal gradients.