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Title: Measurements of the Effects of the Intensity Pickers on Laser Imprinting for Direct-Drive, Adiabat-Shaping Designs on OMEGA

Abstract

Effects of the intensity pickets on laser imprinting were investigated using laser-driven, planar plastic and foam targets on the OMEGA Laser System. Intensity pickets are used in adiabat-shaping techniques, designed to improve stability of inertial confinement fusion targets. The measurements were performed in planar foam targets using the decaying shock (DS) method of adiabat shaping and in planar plastic targets using the relaxation (RX) method.

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Laboratory for Laser Energetics, University of Rochester
Sponsoring Org.:
USDOE
OSTI Identifier:
901858
Report Number(s):
DOE/SF/19460-740
Journal ID: ISSN 1070-664X; PHPAEN; 1701; 2006-195; TRN: US0703251
DOE Contract Number:
FC52-92SF19460
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; OMEGA FACILITY; LASERS; DIRECT DRIVE ICF; LASER TARGETS

Citation Formats

Smalyuk, V.A., Goncharov, V.N., Anderson, K.S., Betti, R., Craxton, R.S., Delettrez, J.A., Meyerhofer, D.D., Regan, S.P., and Sangster, T.C. Measurements of the Effects of the Intensity Pickers on Laser Imprinting for Direct-Drive, Adiabat-Shaping Designs on OMEGA. United States: N. p., 2007. Web. doi:10.1063/1.2715550.
Smalyuk, V.A., Goncharov, V.N., Anderson, K.S., Betti, R., Craxton, R.S., Delettrez, J.A., Meyerhofer, D.D., Regan, S.P., & Sangster, T.C. Measurements of the Effects of the Intensity Pickers on Laser Imprinting for Direct-Drive, Adiabat-Shaping Designs on OMEGA. United States. doi:10.1063/1.2715550.
Smalyuk, V.A., Goncharov, V.N., Anderson, K.S., Betti, R., Craxton, R.S., Delettrez, J.A., Meyerhofer, D.D., Regan, S.P., and Sangster, T.C. Mon . "Measurements of the Effects of the Intensity Pickers on Laser Imprinting for Direct-Drive, Adiabat-Shaping Designs on OMEGA". United States. doi:10.1063/1.2715550.
@article{osti_901858,
title = {Measurements of the Effects of the Intensity Pickers on Laser Imprinting for Direct-Drive, Adiabat-Shaping Designs on OMEGA},
author = {Smalyuk, V.A. and Goncharov, V.N. and Anderson, K.S. and Betti, R. and Craxton, R.S. and Delettrez, J.A. and Meyerhofer, D.D. and Regan, S.P. and Sangster, T.C.},
abstractNote = {Effects of the intensity pickets on laser imprinting were investigated using laser-driven, planar plastic and foam targets on the OMEGA Laser System. Intensity pickets are used in adiabat-shaping techniques, designed to improve stability of inertial confinement fusion targets. The measurements were performed in planar foam targets using the decaying shock (DS) method of adiabat shaping and in planar plastic targets using the relaxation (RX) method.},
doi = {10.1063/1.2715550},
journal = {Physics of Plasmas},
number = ,
volume = 14,
place = {United States},
year = {Mon Apr 09 00:00:00 EDT 2007},
month = {Mon Apr 09 00:00:00 EDT 2007}
}
  • Effects of the intensity pickets on laser imprinting were investigated using laser-driven, planar plastic and foam targets on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Intensity pickets are used in adiabat-shaping techniques designed to improve stability of inertial confinement fusion targets. The measurements were performed in planar foam targets using the decaying shock method of adiabat shaping and in planar plastic targets using the relaxation (RX) method. In foam targets, the picket reduced the target areal-density modulations by {approx}6 times at shorter spatial wavelengths of 30 and 60 {mu}m, while at a longermore » wavelength of 120 {mu}m there was no reduction. The ''imprint efficiency,'' the equivalent surface amplitude produced by imprinting, was reduced by the intensity picket by a factor of {approx}3 at a spatial wavelength of 60 {mu}m, while it was increased by a factor of {approx}2 at a 120-{mu}m spatial wavelength. In plastic targets, used to test the RX method, the imprint efficiency was reduced by the intensity picket by a factor of {approx}2 at spatial wavelengths of 30 and 60 {mu}m, while it was about the same at a 120-{mu}m spatial wavelength.« less
  • (B204)Hydrodynamic instabilities seeded by laser imprint and surface roughness limit the compression ratio and neutron yield in the direct-drive inertial confinement fusion target designs. New improved-performance designs use adiabat shaping to increase the entropy of only the outer portion of the shell, reducing the instability growth. The inner portion of the shell is kept on a lower entropy to maximize shell compressibility. The adiabat shaping is implemented using a high-intensity picket in front of the main-drive pulse. The picket launches a strong shock that decays as it propagates through the shell. This increases the ablation velocity and reduces the Rayleigh-Taylormore » growth rates. In addition, as shown earlier [T.J.B. Collis and S. Skupsky, Phys. Plasmas 9 275 (2002)], the picket reduces the instability seed due to the laser imprint. To test the results of calculations, a series of the picket pulse implosions of CH capsules were performed on the OMEGA laser system [T.R. Boehly, D .L. Brown, R.S. Craxton, et al., Opt. Commun. 133, 495 (1997)]. The experiments demonstrated a significant improvement in target yields for the pulses with the picket compared to the pulses without the picket. Results of the theory and experiments with adiabat shaping are being extended to future OMEGA and the National Ignition Facility's [J.A. Paisner, J.D. Boyes, S.A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994)] cryogenic target designs.« less
  • Low-adiabat (high-compressibility) implosions were performed with shells filled with 15 atm of D2 gas driven with ~12.5 kJ, 3-ns-square pulse shapes, and a laser intensity of ~2 x 10^14 W/cm^2. Initial target modulations, imprinted on the target surface by laser nonuniformities, were varied by using drive with and without smoothing by spectral dispersion. The performance of the spherical implosions was correlated with measured target modulations at the end of the acceleration pulse performed in a planar target with similar conditions using through-foil radiography. The neutron yield decreases sharply when target modulations become comparable with the target thickness at the endmore » of the acceleration phase. The measured target areal densities are less sensitive to target modulations.« less
  • Low-adiabat (high-compressibility) implosions were performed with {approx}860 {mu}m diam, 24- and 35-{mu}m-thick shells filled with 15 atm of D{sub 2} gas driven with {approx}12.5 kJ, 3-ns-square pulse shapes, and a laser intensity of {approx}2x10{sup 14} W/cm{sup 2}. Initial target modulations, imprinted on the target surface by laser nonuniformities, were varied by using drive with and without smoothing by spectral dispersion. The performance of the spherical implosions (with a gas fuel convergence of {approx}16) was correlated with measured target modulations at the end of the acceleration phase performed in a planar target with similar conditions using through-foil radiography. The neutron yieldmore » decreases sharply when target modulations become comparable with the target thickness at the end of the acceleration phase. The measured target areal densities are less sensitive to target modulations. Implosions with 35-{mu}m-thick shells produced neutron-averaged areal densities that are among the highest on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)].« less
  • Neutron yields of direct-drive, low-adiabat (alpha ~~ 2 to 3) cryogenic D2 target implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have been systematically investigated using the two-dimensional (2D) radiation hydrodynamics code DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)]. Low-mode (ell <- 12) perturbations, including initial target offset, ice-layer roughness, and laser-beam power imbalance, were found to be the primary source of yield reduction for thin-shell (5 um), low-alpha, cryogenic targets. The 2D simulations of thin-shell implosions track experimental measurements for different target conditions and peak laser intensitiesmore » ranging from 2.5 x 10^14–6 x 10^14 W/cm^2. Simulations indicate that the fusion yield is sensitive to the relative phases between the target offset and the ice-layer perturbations. The results provide a reasonable good guide to understanding the yield degradation in direct-drive, low-adiabat, cryogenic, thin-shell-target implosions. Thick-shell (10 um) implosions generally give lower yield over clean than low-ell-mode DRACO simulation predictions. Simulations including the effect of laser-beam nonuniformities indicate that high-ell-mode perturbations caused by laser imprinting further degrade the neutron yield of thick-shell implosions. To study ICF compression physics, these results suggest a target specification with a <-30 um offset and ice-roughness of sigma_rms < 3 um are required.« less