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Title: Hard x-ray and hot electron environment in vacuum hohlraums at the National Ignition Facility

Abstract

Time resolved hard x-ray images (hv>9 keV) and time integrated hard x-ray spectra (hv=18-150 keV) from vacuum hohlraums irradiated with four 351 nm wavelength National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] laser beams are presented as a function of hohlraum size, laser power, and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (F{sub hot}) shows a correlation with laser intensity and with an empirical hohlraum plasma filling model. In addition, the significance of Au K-alpha emission and Au K-shell reabsorption observed in some of the bremsstrahlung dominated spectra is discussed.

Authors:
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;  [1]
  1. Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94550 (United States) (and others)
Publication Date:
OSTI Identifier:
20782563
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 3; Other Information: DOI: 10.1063/1.2186927; (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; BREMSSTRAHLUNG; CORRELATIONS; ELECTRONS; GOLD; HARD X RADIATION; ICF DEVICES; IRRADIATION; K SHELL; KEV RANGE; LASERS; LIGHT TRANSMISSION; PLASMA; PLASMA DIAGNOSTICS; PLASMA HEATING; TIME RESOLUTION; US NATIONAL IGNITION FACILITY; X-RAY SOURCES; X-RAY SPECTRA

Citation Formats

McDonald, J.W., Suter, L.J., Landen, O.L., Foster, J.M., Celeste, J.R., Holder, J.P., Dewald, E.L., Schneider, M.B., Hinkel, D.E., Kauffman, R.L., Atherton, L.J., Bonanno, R.E., Dixit, S.N., Eder, D.C., Haynam, C.A., Kalantar, D.H., Koniges, A.E., Lee, F.D., MacGowan, B.J., and Manes, K.R. Hard x-ray and hot electron environment in vacuum hohlraums at the National Ignition Facility. United States: N. p., 2006. Web. doi:10.1063/1.2186927.
McDonald, J.W., Suter, L.J., Landen, O.L., Foster, J.M., Celeste, J.R., Holder, J.P., Dewald, E.L., Schneider, M.B., Hinkel, D.E., Kauffman, R.L., Atherton, L.J., Bonanno, R.E., Dixit, S.N., Eder, D.C., Haynam, C.A., Kalantar, D.H., Koniges, A.E., Lee, F.D., MacGowan, B.J., & Manes, K.R. Hard x-ray and hot electron environment in vacuum hohlraums at the National Ignition Facility. United States. doi:10.1063/1.2186927.
McDonald, J.W., Suter, L.J., Landen, O.L., Foster, J.M., Celeste, J.R., Holder, J.P., Dewald, E.L., Schneider, M.B., Hinkel, D.E., Kauffman, R.L., Atherton, L.J., Bonanno, R.E., Dixit, S.N., Eder, D.C., Haynam, C.A., Kalantar, D.H., Koniges, A.E., Lee, F.D., MacGowan, B.J., and Manes, K.R. Wed . "Hard x-ray and hot electron environment in vacuum hohlraums at the National Ignition Facility". United States. doi:10.1063/1.2186927.
@article{osti_20782563,
title = {Hard x-ray and hot electron environment in vacuum hohlraums at the National Ignition Facility},
author = {McDonald, J.W. and Suter, L.J. and Landen, O.L. and Foster, J.M. and Celeste, J.R. and Holder, J.P. and Dewald, E.L. and Schneider, M.B. and Hinkel, D.E. and Kauffman, R.L. and Atherton, L.J. and Bonanno, R.E. and Dixit, S.N. and Eder, D.C. and Haynam, C.A. and Kalantar, D.H. and Koniges, A.E. and Lee, F.D. and MacGowan, B.J. and Manes, K.R.},
abstractNote = {Time resolved hard x-ray images (hv>9 keV) and time integrated hard x-ray spectra (hv=18-150 keV) from vacuum hohlraums irradiated with four 351 nm wavelength National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] laser beams are presented as a function of hohlraum size, laser power, and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (F{sub hot}) shows a correlation with laser intensity and with an empirical hohlraum plasma filling model. In addition, the significance of Au K-alpha emission and Au K-shell reabsorption observed in some of the bremsstrahlung dominated spectra is discussed.},
doi = {10.1063/1.2186927},
journal = {Physics of Plasmas},
number = 3,
volume = 13,
place = {United States},
year = {Wed Mar 15 00:00:00 EST 2006},
month = {Wed Mar 15 00:00:00 EST 2006}
}
  • Time resolved hard x-ray images (hv > 9 keV) and time integrated hard x-ray spectra (hv = 18-150 keV) from vacuum hohlraums irradiated with four 351 nm wavelength NIF laser beams are presented as a function of hohlraum size and laser power and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (f{sub hot}) and a comparison to a filling model are presented.
  • On the National Ignition Facility (NIF), hot electrons generated in laser heated Hohlraums are inferred from the >20 keV bremsstrahlung emission measured with the time integrated FFLEX broadband spectrometer. New high energy (>200 keV) time resolved channels were added to infer the generated >170 keV hot electrons that can cause ignition capsule preheat. First hot electron measurements in near ignition scaled Hohlraums heated by 96-192 NIF laser beams are presented.
  • The first 96 and 192 beam vacuum hohlraum have been fielded at the National Ignition Facility demonstrating radiation temperatures up to 340 eV and fluxes of 20 TW/sr representing a 20 times flux increase over NOVA/Omega scale hohlraums. The vacuum hohlraums were irradiated with 2 ns square pulses with energies between 150 - 635 kJ. They produced nearly Planckian spectra with about 30 {+-} 10% more flux than predicted by the current radiation hydrodynamic simulations after careful verification of all component calibrations (which included an {approx} 10% downward correction to Center X-Ray Optics opacities just below the Cu L edgemore » at 50-750 eV), cable deconvolution, and analysis software routines. To corroborate these results, first a half hohlraum experiment was conducted using a single 2 ns-long axial quad with an irradiance of {approx} 1-2 x 10{sup 15} W/cm{sup 2} for comparison with NIF Early Light experiments completed in 2004. Second, we completed a conversion efficiency test using a 128-beam nearly uniformly illuminated gold sphere with intensities kept low (at 1 x 10{sup 14} W/cm{sup 2} over 5 ns) to avoid sensitivity to modeling uncertainties for non-local heat conduction and non-linear absorption mechanisms, to compare with similar intensity, 3 ns OMEGA sphere results. The 2004 and 2009 NIF half-hohlraums agreed to 10% in flux, but more importantly, the 2006 OMEGA Au Sphere, the 2009 NIF Au sphere and the calculated Au conversion efficiency agree to {+-}5% in flux, which is estimated to be the absolute calibration accuracy of the DANTEs. Hence we concluded the 30 {+-} 10% higher than expected radiation fluxes from the 96 and 192 beam vacuum hohlraums are attributable to differences in physics when we transitioned to large hot hohlraums. Specifically, using variants in the atomic physics models and electron heat conduction, newer simulations show that nonlocalization of energy deposition leads to less energy being stored in the coronal plasma leading to higher x-ray conversion efficiency. Since the larger volume-to-area ratio hohlraums have large coronal plasmas which scale volumetrically, the reduction in energy losses to the corona become more pronounced than for smaller NOVA/Omega scale hohlraums. The higher conversion efficiencies are also consistent with observations from other 1 ns gold sphere experiments conducted at Omega with 1 x 10{sup 15} W/cm{sup 2} laser irradiances.« less
  • The first 96 and 192 beam vacuum Hohlraum target experiments have been fielded at the National Ignition Facility demonstrating radiation temperatures up to 340 eV and fluxes of 20 TW/sr as viewed by DANTE representing an {approx}20 times flux increase over NOVA/Omega scale Hohlraums. The vacuum Hohlraums were irradiated with 2 ns square laser pulses with energies between 150 and 635 kJ. They produced nearly Planckian spectra with about 30{+-}10% more flux than predicted by the preshot radiation hydrodynamic simulations. To validate these results, careful verification of all component calibrations, cable deconvolution, and software analysis routines has been conducted. Inmore » addition, a half Hohlraum experiment was conducted using a single 2 ns long axial quad with an irradiance of {approx}2x10{sup 15} W/cm{sup 2} for comparison with NIF Early Light experiments completed in 2004. We have also completed a conversion efficiency test using a 128-beam nearly uniformly illuminated gold sphere with intensities kept low (at 1x10{sup 14} W/cm{sup 2} over 5 ns) to avoid sensitivity to modeling uncertainties for nonlocal heat conduction and nonlinear absorption mechanisms, to compare with similar intensity, 3 ns OMEGA sphere results. The 2004 and 2009 NIF half-Hohlraums agreed to 10% in flux, but more importantly, the 2006 OMEGA Au Sphere, the 2009 NIF Au sphere, and the calculated Au conversion efficiency agree to {+-}5% in flux, which is estimated to be the absolute calibration accuracy of the DANTEs. Hence we conclude that the 30{+-}10% higher than expected radiation fluxes from the 96 and 192 beam vacuum Hohlraums are attributable to differences in physics of the larger Hohlraums.« less