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Title: X-ray emission from a high-atomic-number z-pinch plasma created from compact wire arrays

Abstract

Thermal and nonthermal x-ray emission from the implosion of compact tungsten wire arrays, driven by 5 MA from the Saturn accelerator, are measured and compared with LLNL Radiation-Hydro-Code (RHC) and SNL Hydro-Code (HC) numerical models. Multiple implosions, due to sequential compressions and expansions of the plasma, are inferred from the measured multiple x-radiation bursts. Timing of the multiple implosions and the thermal x-ray spectra measured between 1 and 10 keV are consistent with the RHC simulations. The magnitude of the nonthermal x-ray emission measured from 10 to 100 keV ranges from 0.02 to 0.08% of the total energy radiated and is correlated with bright-spot emission along the z-axis, as observed in earlier Gamble-11 single exploding-wire experiments. The similarities of the measured nonthermal spectrum and bright-spot emission with those measured at 0.8 MA on Gamble-II suggest a common production mechanism for this process. A model of electron acceleration across magnetic fields in highly-collisional, high-atomic-number plasmas is developed, which shows the existence of a critical electric field, E{sub c}, below which strong nonthermal electron creation (and the associated nonthermal x rays) do not occur. HC simulations show that significant nonthermal electrons are not expected in this experiment (as observed) because the calculatedmore » electric fields are at least one to two orders-of-magnitude below E{sub c}. These negative nonthermal results are confirmed by RHC simulations using a nonthermal model based on a Fokker-Plank analysis. Lastly, the lower production efficiency and the larger, more irregular pinch spots formed in this experiment relative to those measured on Gamble II suggest that implosion geometries are not as efficient as single exploding-wire geometries for warm x-ray production.« less

Authors:
; ;  [1]
  1. and others
Publication Date:
Research Org.:
Sandia National Labs., Albuquerque, NM (United States)
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
211368
Report Number(s):
SAND-96-0222
ON: DE96008072; TRN: 96:009595
DOE Contract Number:
AC04-94AL85000
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: Mar 1996
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPE AND RADIATION SOURCE TECHNOLOGY; 66 PHYSICS; X-RAY SOURCES; EXPLODING WIRES; IMPLOSIONS; PINCH EFFECT; ACCELERATORS; X-RAY SPECTRA; TUNGSTEN

Citation Formats

Sanford, T.W.L., Nash, T.J., and Marder, B.M.. X-ray emission from a high-atomic-number z-pinch plasma created from compact wire arrays. United States: N. p., 1996. Web. doi:10.2172/211368.
Sanford, T.W.L., Nash, T.J., & Marder, B.M.. X-ray emission from a high-atomic-number z-pinch plasma created from compact wire arrays. United States. doi:10.2172/211368.
Sanford, T.W.L., Nash, T.J., and Marder, B.M.. 1996. "X-ray emission from a high-atomic-number z-pinch plasma created from compact wire arrays". United States. doi:10.2172/211368. https://www.osti.gov/servlets/purl/211368.
@article{osti_211368,
title = {X-ray emission from a high-atomic-number z-pinch plasma created from compact wire arrays},
author = {Sanford, T.W.L. and Nash, T.J. and Marder, B.M.},
abstractNote = {Thermal and nonthermal x-ray emission from the implosion of compact tungsten wire arrays, driven by 5 MA from the Saturn accelerator, are measured and compared with LLNL Radiation-Hydro-Code (RHC) and SNL Hydro-Code (HC) numerical models. Multiple implosions, due to sequential compressions and expansions of the plasma, are inferred from the measured multiple x-radiation bursts. Timing of the multiple implosions and the thermal x-ray spectra measured between 1 and 10 keV are consistent with the RHC simulations. The magnitude of the nonthermal x-ray emission measured from 10 to 100 keV ranges from 0.02 to 0.08% of the total energy radiated and is correlated with bright-spot emission along the z-axis, as observed in earlier Gamble-11 single exploding-wire experiments. The similarities of the measured nonthermal spectrum and bright-spot emission with those measured at 0.8 MA on Gamble-II suggest a common production mechanism for this process. A model of electron acceleration across magnetic fields in highly-collisional, high-atomic-number plasmas is developed, which shows the existence of a critical electric field, E{sub c}, below which strong nonthermal electron creation (and the associated nonthermal x rays) do not occur. HC simulations show that significant nonthermal electrons are not expected in this experiment (as observed) because the calculated electric fields are at least one to two orders-of-magnitude below E{sub c}. These negative nonthermal results are confirmed by RHC simulations using a nonthermal model based on a Fokker-Plank analysis. Lastly, the lower production efficiency and the larger, more irregular pinch spots formed in this experiment relative to those measured on Gamble II suggest that implosion geometries are not as efficient as single exploding-wire geometries for warm x-ray production.},
doi = {10.2172/211368},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1996,
month = 3
}

Technical Report:

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  • Thermal and nonthermal x-ray emission from the implosion of compact tungsten wire arrays in 5-MA Saturn discharges is reported. The timing of multiple implosions and the thermal x-ray spectra (1 to 10 keV) agree with 2D radiation-hydrocode simulations. Nonthermal x-ray emission (10 to 100 keV) correlates with pinch spots distributed along the z-axis. The similarities of the measured nonthermal spectrum, yield, and pinch-spot emission with those of 0.8-MA, single- exploded-wire discharges on Gamble-II suggest a common nonthermal- production mechanism. Nonthermal x-ray yields are lower than expected from current scaling of Gamble II results, suggesting that implosion geometries are not asmore » efficient as single-wire geometries for nonthermal x-ray production. The instabilities, azimuthal asymmetries, and inferred multiple implosions that accompany the implosion geometry lead to larger, more irregular pinch spots, a likely reason for reduced nonthermal efficiency. A model for nonthermal-electron acceleration across magnetic fields in highly- collisional, high-atomic-number plasmas combined with 1D hydrocode simulations of Saturn compact loads predicts weak nonthermal x-ray emission.« less
  • A major goal in using z-pinch plasmas as high brightness laboratory sources of x rays is to increase the radiation emission above about 1keV through the use of moderate atomic number (Z = 10 to 36) materials. Two kinds of configuration, single wires and wire arrays or foils, have been investigated in the past. In both cases, it has been observed that optimal x-ray emission decreases on a given machine as the atomic number of the z-pitch material is increased. This paper describes how a previously developed model can be modified and extended to provide predictions of how machine andmore » diode designs must be scaled in order to maintain a given K-shell x-ray emission as the plasma atomic number increases in z-pinch array implosions. Simple scaling arguments are developed to show how specification of a desired total K-shell emission can be used to determine the parameters of an imploding array z-pinch plasma as a function of the atomic number of the pinch. The model we use ignores all hydrodynamic details of the z-pinch implosion and thermalization processes and assumes that all the plasma mass is imploded and thermalized. The pinch is also assumed to be driven by a prescribed linearly rising current. Scaling laws that are derived from these assumptions, in effect, contain phenomenological scaling parameters whose values will need to be more accurately determined by more detailed theoretical calculations and/or by z-pinch experiments on a variety of different machines.« less
  • No abstract prepared.
  • An analysis of x-ray data from two series of Z-pinch shots taken on the short current-risetime Saturn accelerator at Sandia National Laboratories [Proceedings of 6th International IEEE Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] is presented. In one series, the array radius was held constant and the array mass was varied; in the other series, the array mass was held constant and its radius varied. In both sets of experiments, large wire-number loads (N{>=}93) of aluminum were used in contrast to earlier small wire-number aluminum array experimentsmore » on Saturn where N{<=}42. Average electron temperatures and ion densities were inferred from the data. In addition, from the measured size of the emission region of K-shell x rays and from the inferred ion density, a fraction of the total array mass that participated in the K-shell emission was inferred and found to be directly correlated to the K-shell yields that were measured. This paper also demonstrates that the yields varied as a function of array mass and radius in much closer agreement with predictions [J. Appl. Phys. 67, 1725 (1990)] than had been observed in the earlier small wire-number experiments. Thus, a serious misperception that the reason for the early disagreement was in the calculations and not in the experiments is corrected. These predictions were made using one-dimensional (1D) magnetohydrodynamics calculations. The density and temperature trends inferred from the data analysis are well-behaved and consistent with the 1D calculations. This data analysis confirms the importance of achieving uniform plasma initial conditions and implosion symmetry when comparing computer code calculations with experiment. When the wire number of an array load is increased, a more uniform shell of plasma is calculated initially as the wires explode and, as the plasma stagnates on axis, the x-ray powers and yields are found experimentally to approach the powers and yields predicted by 1D calculations.« less
  • Microfabrication methods have been applied to the fabrication of wire arrays suitable for use in Z. Self-curling GaAs/AlGaAs supports were fabricated as an initial route to make small wire arrays (4mm diameter). A strain relief structure that could be integrated with the wire was designed to allow displacements of the anode/cathode connections in Z. Electroplated gold wire arrays with integrated anode/cathode bus connections were found to be sufficiently robust to allow direct handling. Platinum and copper plating processes were also investigated. A process to fabricate wire arrays on any substrate with wire thickness up to 35 microns was developed. Methodsmore » to handle and mount these arrays were developed. Fabrication of wire arrays of 20mm diameter was demonstrated, and the path to 40mm array fabrication is clear. With some final investment to show array mounting into Z hardware, the entire process to produce a microfabricated wire array will have been demonstrated.« less