Paul David LePell

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.

Assessing the ZR Machine's Potential for Producing Multi-keV X-Ray Yields in K-Shell Line and Free-Bound Continuum Radiation

This paper presents theoretical extrapolations for the multi-keV X-ray radiation production capability of the 26-MA ZR accelerator at Sandia National Laboratories, which is scheduled to become available for experiments in 2007. These extrapolations are based on scaling models and ideas that have been developed over the years. These models and ideas have evolved and been refined through the process of benchmarking one-dimensional nonlocal-thermodynamic equilibrium magnetohydrodynamic model results to experimental K-shell yields and powers as well as inferred temperatures and densities. For this ZR assessment, the models are first benchmarked to K-shell yields obtained from argon, titanium, stainless-steel, and copper Z experiments and then they are applied to extrapolate yield predictions to the ZR machine. Extrapolations are based on 2-cm-length loads and similar wire configurations and nozzle designs as those employed in Z experiments. Projected K-shell yields for Ar (photon energy ~3 keV), Ti (~5 keV), stainless steel (~7 keV), and Cu (~8.6 keV) are 520, 300, 200, and 80 kJ, respectively. In addition, the high-energy free-bound continuum emission above 10 keV is calculated to be 40 kJ on ZR

Scaling of K-Shell Emission From

Experiments in the last few years at the 20-MA Z Accelerator have produced significant K-shell X-ray output from a variety of initial load materials, including aluminum (1.7-keV photons, 400-kJ yield), argon (3.1-keV photons, 300-kJ yield), titanium (4.8-keV photons, 100-kJ yield), stainless steel (6.7-keV photons, 50-kJ yield), and copper (8.4-keV photons, 20-kJ yield). K-shell scaling theories developed at the Naval Research Laboratory [K. G. Whitney , Phys. Rev. E 50, 2166 (1994)] in the 1990s were benchmarked against the Al K-shell emission data from 10-MA facilities. The experiments at Z have not only led to a heuristic validation of this original theory but have also provided the data to fine tune the models for application to higher photon energies and for extension to higher current generators. The upgrade of the Z Accelerator to ZR, which will provide 26 MA to a -pinch load, should increase the radiated K-shell output for sources previously fielded at Z and will extend the range of photon energies where measurable radiation can be observed, which is likely up to 13 keV. A summary of the K-shell experiments at Z is presented, as well as an overview of the modified empirical-scaling theory. Proposed load configurations for ZR are discussed, as well as predictions for K-shell output.

Z

Summary form only given. Tests of nested aluminum (A15056) arrays at the Z accelerator have produced X-ray outputs of >350 kJ. The measured X-rays consist of the K-shell emission of aluminum and magnesium ions, stripped to the He- and H-like states, as well as free-bound continuum. For these tests we recorded these emissions with a variety of spectrometers: time-integrated, spatially resolved and time-resolved, spatially integrated spectrometers. These instruments were configured to capture the broad range of K-shell emissions from the plasma ions and as such were survey instruments, with spectral resolutions of approximately 300 (E/DeltaE). In this paper, details of the two Z shots, Z1519 and Z1520, will be presented. For these shots, each spectrometer showed intense Mg K-shell lines as well as intense Al K-shell lines; however, the intensity of the Mg lines exceeds the 5% Mg mass content of the 5056 alloy. In particular, the ratio of the Mg and Al Lyman-alpha lines approaches 25% (Mg/Al), suggestive of substantial line opacity. The time-resolved measurements show that the most intense lines (Al Lyman-alpha) persist for the cameras entire detection time of 21 ns, exceeding that of the lower energy Al He-alpha line. Furthermore, the Al Lyman-alpha lines extend out to a radius of 4-5 mm, and uniformly emit along the plasmas visible length; these observations are consistent with pinhole imaging data collected on the same shots. Measurements of the X-ray yields and pulse shapes will also be presented

-Pinches: Z to ZR

Summary form only given, as follows. The Z machine at Sandia National Laboratories is a z-pinch facility capable of producing a 1 MJ, 200 eV radiation source. At present, this source is being investigated as a potential driver for inertial confinement fusion (ICF) experiments. For optimum ICF capsule implosions, however, it will be necessary to generate shaped radiation pulses. Specifically, it will be necessary to generate a 10-20 ns foot prior to the main radiation pulse, with about 1/14 the amplitude of the peak radiation. One possible method of generating an appropriately shaped radiation pulse is to use nested wire arrays. The array parameters can in principle be chosen to allow different arrival times on axis, thereby producing a shaped radiation pulse. To investigate this possibility, we utilized a simple formulation for self and mutual inductances for the nested arrays. We added to this formulation a modification of the equations of motion by introducing instantaneous momentum transfer when the position of the two arrays coincides.

Spectroscopic measurements of recent aluminum wire array Z-pinch implosions on the Z-accelerator

Summary form only given, as follows. Over the last 25 years, Z-pinch loads as sources of X-ray radiation have been pursued on pulsed power machines. K-shell radiation has been of particular interest, with common sources that include Ne (/spl sim/1 keV), Ar (3.1 keV), and Al (1.8 keV). The current level (20 MA) available on the Z Accelerator at Sandia National Labs has made possible the study of higher photon energy K-shell radiation that was previously inaccessible due to current limitations. Experiments on Z using single wire arrays and nested wire arrays have resulted in the production of up to 130 kJ of Ti K-shell (4.8 keV), 50 kJ of K-shell emissions from stainless steel (Fe, Cr, and Ni at 6.7 keV, 5.6 keV, and 7.7 keV respectively), and >10 kJ of Cu K-shell (8.4 keV). With the variety of experiments that have been performed, valuable information about the scaling of K-shell emissions has been obtained. In this paper, experimental measurements from the various K-shell radiators on Z will be presented. The data will be compared with theoretical K-shell scaling law predictions as well as measurements from other, smaller, facilities with a goal of providing insight into parameters impacting the agreement between theory and measurement.

Array on array circuit studies for ICF radiation pulse tailoring

Mixed, nested wire arrays with 40mm and 50mm outer array diameters were used 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.

K-Shell scaling of high photon energies on the Z accelerator

Summary form only given. Planar wire arrays were shown to produce the strong power and yield in EUV/X-ray region. The new results of the total radiation yield Et, time-resolved sub-keV and keV outputs, X-ray spectra and images from ten wire planar arrays from low-to high-Za materials were collected recently on Zebra at UNR. Data were compared with the similar mass cylindrical wire arrays results. Planar arrays were characterized by a short rise-time of a single radiation peak: up to 8 ns for sub-keV radiation and near 2 ns in keV and harder X-ray regions, and much higher peak power compared to cylindrical arrays usually having precursor and main peak with a fastest rise-time >10-15 ns. The largest Et>18 kJ was found for Cu and Mo planar arrays (up to 25% from an energy delivered to a load) that is higher than a maximum Etap15-16 kJ for Cu or 10 kJ for Alumel (Ni) cylindrical arrays. The radiating imploding plasma is strongly inhomogeneous. The plasma (few-mm thick in the EUV/sub-keV region) has within itself hundred-mum scale structures on the axis (in several keV quanta). The electron temperatures up to 800-1000 eV were estimated for Mo planar arrays that are much higher than for cylindrical arrays and comparable with the values for X-pinches. The implosion dynamics of wire arrays were studied using spectral and imaging techniques. The results were compared with radiation and MHD modeling

Nested array dynamics from Ni-clad Ti - Al wire array Z pinches

Summary form only given. Planar wire arrays were shown to produce the record-high peak radiation powers and yields in sub-keV and keV among all other tested loads on the 1 MA pulsed power Zebra generator at UNR. Spectroscopic modeling of radiation from such wire arrays is essential for understanding of the plasma parameters achieved in their implosions. X-ray spectra and images from 10 wire planar arrays from Alumel (alloy with 95% of Ni and 5% Al), Cu (alloy with 4% of Ni), and Al/Cu (2 Al and 8 Cu wires) were accumulated in recent experiments on Zebra. In particular, axially resolved time integrated X-ray K-shell spectra of Al and L-shell spectra of Cu and Ni were recorded by a KAP crystal (in a spectral region from 6 to 15 Aring) through different slits. The same spectra were recorded by a time-gated spectrometer in a spectral region from 6 to 10.5 Aring. In addition, spatially integrated harder X-ray spectra were monitored by a LiF crystal. Non-LTE kinetic models of Al, Ni, and Cu provided spatially and time resolved plasma parameters. The comparison with X-pinch and cylindrical wire array data is presented. Advantages of using wire array loads from alloys with small concentrations of tracer or composed from wires of a different material for spectroscopic plasma diagnostics will be highlighted. The distinct features of wire arrays plasma dynamics, magnetic energy dissipation and radiation from planar wire arrays will be discussed

New results on planar wire array implosions and their comparison with cylindrical wire arrays on the 1 MA zebra generator

Summary form only given. Experiments in the last few years at the 20 MA Z Accelerator have produced significant K-shell X-ray output from a variety of initial load materials, including argon (3 keV, >300 kJ), titanium (4.8 keV, 100 kJ), stainless steel (7 keV, >50 kJ), and copper (8.4 keV, 20 kJ). K-shell scaling theories, developed at NRL, were benchmarked against the K-shell emission data from lower current facilities and photon energies up to 3 keV. This theory identifies the initial load conditions necessary to get enough mass to implode at a high enough velocity to achieve the high plasma temperatures and densities required for K-shell emission. For the conditions available at Z, efficient K-shell emission up to 5 keV is expected. The experimental results from the various sources at Z have not only demonstrated the heuristic validity of K-shell scaling models, but have provided the data to fine tune the models for higher photon energies. The upgrade of the Z accelerator to ZR, which will provide 26 MA to a Z-pinch load, should extend the region of efficient K-shell production to include photon energies up to 7 keV. This will increase the radiated K-shell output for sources previously fielded at Z, but will also extend the range of photon energies where measurable radiation can be observed, likely up to 13 keV. In this paper, the results of K-shell scaling experiments from Z will be presented, as well as the expected scaling of these sources to ZR. Plasma conditions from the Z experiments will be discussed within the context of efficient K-shell production, which helps identify the appropriate plasma conditions necessary for efficient production at ZR. Calculated results for expected ZR outputs will be presented