Longer-scalelength plasma perturbations by an intense laser beam

Technical Report · Fri Sep 28 04:00:00 EDT 1984

OSTI ID:6401472

Stamper, J A; Young, F C; Herbst, M J; Obenschain, S P; Gardner, J H; Lehmberg, R H; McLean, E A; Grun, J; Kearney, K J; Ripin, B H

Plasmas with long (up to 400 microns at 0.1 critical density) density scalelengths are used in laser-plasma interaction experiments in order to more closely approximate the blowoff conditions expected for a high-gain ICF pellet. These long-scalelength plasmas are produced by focusing one beam (approx. 150 J, 4 nsec FWHM) of the Pharos II Nd laser (lambda = 1.054 micron) to a large focal spot (approx. 1 mm). A second beam is tightly focused within this spot to produce intensities up to 10/sup 15/ W/cm/sup 2/. The pulsewidth of the second beam is only 0.3 nsec to minimize hydrodynamic perturbation o the long-scalelength plasma. The peak intensities of the two beams occur at the same time. Plasma density perturbations are monitored by side-on imaging a second-harmonic emission. At low energy in the short-pulse beam, this emission occurs only at distances from the target surface corresponding to n/sub c/ less than or equal to n < n/sub c/, showing no evidence for perturbation of the long-scalelength low density plasma. However, at higher energy, emission is seen well in front of the target, in highly-localized, filamentary-appearing regions. This observation is consistent with the formation of a low density channel along the high-intensity beam path. The tendency to form a channel is predicted by a 2-D hydrodynamics code. The structure, intensity dependence, and polarization of the second-harmonic emission are consistent with self-focusing. Self-focusing of the incident beam in this experiment is shown to be consistent with a steady-state, self-focusing theory.

• Research Organization: Naval Research Lab., Washington, DC (USA)
• DOE Contract Number: AI08-79DP40092
• OSTI ID: 6401472
• Report Number(s): NRL-MR-5173; ON: DE85001392
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
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ELECTROMAGNETIC RADIATION
FOCUSING
LASER RADIATION
LASER TARGETS
LASER-PRODUCED PLASMA
LASERS
NEODYMIUM LASERS
PLASMA
PLASMA DENSITY
PLASMA WAVES
RADIATIONS
SOLID STATE LASERS
TARGETS