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Title: Recent Results for Large Diameter (12 cm) Gas Puff Z-Pinches at Peak Currents of >3 to <6 MA

Abstract

There is strong interest in many laboratories worldwide in utilizing less expensive, longer rise-time (> 200 ns) pulsed power to drive x-ray producing z-pinches. Based on the idea of a magnetically-driven annular implosion, the emission of K-shell photons requires high energy per ion (implosion velocity above 43 cm/{mu}s for argon) to strip the atoms to the helium-like and hydrogen-like states. This high velocity must be combined with high density in the final hot plasma to produce significant x-ray yield. To first order, implosion velocity correlates with the initial diameter of the z-pinch load in proportion to the implosion time. Thus some effort has been made in the last few years to develop larger diameter z-pinch loads suitable for use with the longer rise-time drivers. Advancing from the <4 cm diameter loads (used for 100 ns implosions) of a decade ago, progress with 8 cm loads was reported at the last DZP meeting. Here we review further progress with 12 cm loads as used to date at peak currents of 3.5 MA to almost 6 MA with >200 ns implosion times. The most interesting result is that implosions from 12 cm diameter have not proven hopelessly unstable. High quality pinches withmore » few millimeter K-shell emitting diameters, <5 ns pulse widths, electron temperatures above 1.7 keV and ion densities >4*1019/cm{sup 3} have been achieved. The observed argon K yield has equaled simple scaling estimates that ignore the expected increase in instabilities for large initial diameters. This more stable result probably occurs because we are using radial mass distributions that are 'snowplow' stabilized, i.e., they are not shell-like but rather have smoothly varying mass with the radial density gradient, d{rho}/dr small or negative over much of the gas flow. Data on yield as a function of the radial distribution suggest that a near or on-axis peak in the initial gas density is probably optimal. Work remains to be done to establish the details of the 'best' mass distribution.« less

Authors:
; ;  [1]; ; ; ; ;  [2]; ; ; ;  [3];  [4];  [5]
  1. Alameda Applied Sciences Corp, 626 Whitney St, San Leandro CA, 94577 (United States)
  2. Titan Pulsed Sciences Division, 2700 Merced St., San Leandro, CA 94577 (United States)
  3. Naval Reearch Laboratory, Code 6720, 4555 Overlook Ave SW, Washington DC 20375 (United States)
  4. Naval Reearch Laboratory, Code 6770, 4555 Overlook Ave SW, Washington DC 20375 (United States)
  5. Defense Threat Reduction Agency, Kirtland AFB, Albuquerque, NM 87117 (United States)
Publication Date:
OSTI Identifier:
20729240
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 808; Journal Issue: 1; Conference: 6. international conference on dense Z-pinches, Oxford (United Kingdom), 25-28 Jul 2005; Other Information: DOI: 10.1063/1.2159344; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ARGON; ELECTRIC CURRENTS; ELECTRON TEMPERATURE; GAS FLOW; HELIUM; HOT PLASMA; HYDROGEN; IMPLOSIONS; ION DENSITY; K SHELL; KEV RANGE; LINEAR Z PINCH DEVICES; LONGITUDINAL PINCH; MASS DISTRIBUTION; PHOTONS; PLASMA DENSITY; PLASMA INSTABILITY; PULSE RISE TIME; X RADIATION

Citation Formats

Coleman, P.L., Krishnan, M., Thompson, J.R., Banister, J.W., Failor, B.H., Levine, J.S., Qi, N., Sze, H.M., Apruzese, J.P., Davis, J., Thornhill, J.W., Velikovich, A.L., Commisso, R.J., and Verma, A. Recent Results for Large Diameter (12 cm) Gas Puff Z-Pinches at Peak Currents of >3 to <6 MA. United States: N. p., 2006. Web. doi:10.1063/1.2159344.
Coleman, P.L., Krishnan, M., Thompson, J.R., Banister, J.W., Failor, B.H., Levine, J.S., Qi, N., Sze, H.M., Apruzese, J.P., Davis, J., Thornhill, J.W., Velikovich, A.L., Commisso, R.J., & Verma, A. Recent Results for Large Diameter (12 cm) Gas Puff Z-Pinches at Peak Currents of >3 to <6 MA. United States. doi:10.1063/1.2159344.
Coleman, P.L., Krishnan, M., Thompson, J.R., Banister, J.W., Failor, B.H., Levine, J.S., Qi, N., Sze, H.M., Apruzese, J.P., Davis, J., Thornhill, J.W., Velikovich, A.L., Commisso, R.J., and Verma, A. Thu . "Recent Results for Large Diameter (12 cm) Gas Puff Z-Pinches at Peak Currents of >3 to <6 MA". United States. doi:10.1063/1.2159344.
@article{osti_20729240,
title = {Recent Results for Large Diameter (12 cm) Gas Puff Z-Pinches at Peak Currents of >3 to <6 MA},
author = {Coleman, P.L. and Krishnan, M. and Thompson, J.R. and Banister, J.W. and Failor, B.H. and Levine, J.S. and Qi, N. and Sze, H.M. and Apruzese, J.P. and Davis, J. and Thornhill, J.W. and Velikovich, A.L. and Commisso, R.J. and Verma, A.},
abstractNote = {There is strong interest in many laboratories worldwide in utilizing less expensive, longer rise-time (> 200 ns) pulsed power to drive x-ray producing z-pinches. Based on the idea of a magnetically-driven annular implosion, the emission of K-shell photons requires high energy per ion (implosion velocity above 43 cm/{mu}s for argon) to strip the atoms to the helium-like and hydrogen-like states. This high velocity must be combined with high density in the final hot plasma to produce significant x-ray yield. To first order, implosion velocity correlates with the initial diameter of the z-pinch load in proportion to the implosion time. Thus some effort has been made in the last few years to develop larger diameter z-pinch loads suitable for use with the longer rise-time drivers. Advancing from the <4 cm diameter loads (used for 100 ns implosions) of a decade ago, progress with 8 cm loads was reported at the last DZP meeting. Here we review further progress with 12 cm loads as used to date at peak currents of 3.5 MA to almost 6 MA with >200 ns implosion times. The most interesting result is that implosions from 12 cm diameter have not proven hopelessly unstable. High quality pinches with few millimeter K-shell emitting diameters, <5 ns pulse widths, electron temperatures above 1.7 keV and ion densities >4*1019/cm{sup 3} have been achieved. The observed argon K yield has equaled simple scaling estimates that ignore the expected increase in instabilities for large initial diameters. This more stable result probably occurs because we are using radial mass distributions that are 'snowplow' stabilized, i.e., they are not shell-like but rather have smoothly varying mass with the radial density gradient, d{rho}/dr small or negative over much of the gas flow. Data on yield as a function of the radial distribution suggest that a near or on-axis peak in the initial gas density is probably optimal. Work remains to be done to establish the details of the 'best' mass distribution.},
doi = {10.1063/1.2159344},
journal = {AIP Conference Proceedings},
number = 1,
volume = 808,
place = {United States},
year = {Thu Jan 05 00:00:00 EST 2006},
month = {Thu Jan 05 00:00:00 EST 2006}
}
  • This paper reviews the motivation for, results from, and analyses of 12-cm-diameter argon gas-puff experiments carried out over the last four years on three generators at 3.2- to 6.5-MA peak currents, all with implosion times {>=}200 ns. Using the argon K-shell yield as a metric of implosion quality, high-quality implosions are obtained for an appropriate initial radial mass distribution, i.e., a distribution that is peaked on axis. Higher compressed densities and smaller final radii are observed compared to shell-like initial mass distributions. Theory and data suggest that these distributions mitigate the magnetic Rayleigh-Taylor instability. An energy analysis shows that (1)more » significant electrical energy is directly coupled to the pinch during the K-shell radiation pulse and (2) conversion of radially-directed kinetic energy into thermal energy is not the dominant mechanism responsible for the pinch K-shell radiation.« less
  • Recently, a new approach for efficiently generating K-shell x-rays in large-diameter, long-implosion time, structured argon gas Z-pinches has been demonstrated based on a 'pusher-stabilizer-radiator' model. In this paper, direct observations of the Rayleigh-Taylor instability mitigation of a 12-cm diameter, 200-ns implosion time argon Z-pinch using a laser shearing interferometer (LSI) and a laser wavefront analyzer (LWA) are presented. Using a zero-dimensional snowplow model, the imploding plasma trajectories are calculated with the driver current waveforms and the initial mass distributions measured using the planar laser induced fluorescence method. From the LSI and LWA images, the plasma density and trajectory during themore » implosion are measured. The measured trajectory agrees with the snowplow calculations. The suppression of hydromagnetic instabilities in the ''pusher-stabilizer-radiator'' structured loads, leading to a high-compression ratio, high-yield Z-pinch, is discussed. For comparison, the LSI and LWA images of an alternative load (without stabilizer) show the evolution of a highly unstable Z-pinch.« less
  • Star-like wire arrays and small-diameter (1-3 mm in diameter) cylindrical loads were tested in the 1-MA Zebra generator. Mitigation of plasma inhomogeneity was observed in the implosions of star-like loads, which consisted of multiple nested, cylindrical arrays aligned azimuthally such that the wires appear as linear array 'rays' extending from the axis of symmetry. The implosion in these loads is directed along the 'rays' of the star and cascades from wire to wire to the center to form moving plasma columns with smooth leading edges. Despite the low azimuthal symmetry, a star-like wire array produces a stable x-ray pulse withmore » a high peak power and a short duration of 8-12-ns. This can be linked to the stabilization of instabilities due to the multiple nesting. X-ray generation and implosion dynamics in wire arrays 1-16 mm in diameter were investigated to find a transition between the regime with prevailing kinetic energy and 'non-kinetic' plasma heating. Loads 3-8 mm in diameter generate the highest x-ray power at the Zebra generator. The fall of x-ray power in 1-2-mm loads can be linked to the lack of kinetic energy. Laser probing diagnostics show the formation of 'necks' on the pinch during the bubble-like implosion. The energy balance provides the evidence of the enhanced plasma heating in z-pinches. Features of the implosions in small-diameter wire-arrays can help to identify the mechanisms of energy dissipation.« less
  • A multicolor, time-gated, soft x-ray pinhole imaging instrument is fielded as part of the core diagnostic set on the 25 MA Z machine [M. E. Savage et al., in Proceedings of the Pulsed Power Plasma Sciences Conference (IEEE, New York, 2007), p. 979] for studying intense wire array and gas puff Z-pinch soft x-ray sources. Pinhole images are reflected from a planar multilayer mirror, passing 277 eV photons with <10 eV bandwidth. An adjacent pinhole camera uses filtration alone to view 1-10 keV photons simultaneously. Overlaying these data provides composite images that contain both spectral as well as spatial information,more » allowing for the study of radiation production in dense Z-pinch plasmas. Cu wire arrays at 20 MA on Z show the implosion of a colder cloud of material onto a hot dense core where K-shell photons are excited. A 528 eV imaging configuration has been developed on the 8 MA Saturn generator [R. B. Spielman et al., and A. I. P. Conf, Proc. 195, 3 (1989)] for imaging a bright Li-like Ar L-shell line. Ar gas puff Z pinches show an intense K-shell emission from a zippering stagnation front with L-shell emission dominating as the plasma cools.« less
  • Double-shell Ar gas puff implosions driven by 16.5±0.5 MA on the Z generator at Sandia National Laboratories are very effective emitters of Ar K-shell radiation (photon energy >3 keV), producing yields of 330 ± 9% kJ (B. Jones et al., Phys. Plasmas, 22, 020706, 2015). In addition, previous simulations and experiments have reported dramatic increases in K-shell yields when adding an on-axis jet to double shell gas puffs for some configurations.