Longer-Scalelength Plasma Perturbations by an Intense Laser Beam.
Abstract
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 to the 15th power W/sq cm. The pulsewidth of the second beam is only 0.3 nsec to minimize hydrodynamic perturbation to 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 < or = 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 28, 1984
- Accession Number
- ADA147005
Entities
People
- Frank C. Young
- John A. Stamper
- John H. Gardner
- M. J. Herbst
- Stephen P.P. Obenschain
Organizations
- United States Naval Research Laboratory