San Diego, California, 17-19 June 2002

On Stimulated Electron-Acoustic Wave Scattering in Laser Driven Fusion Targets

M. M. Skoric* and Lj. Nikolic⁺

Vinca Institute of Nuclear Sciences,

POB 522, 11001 Belgrade, Yugoslavia

 

The propagation of laser light through regions of underdense plasma is an active topic. In particular, large effort was put into studies of stimulated Raman and Brillouin scattering which can reflect a significant part of the laser light and produce energetic particles to preheat a fusion pellet. Experiments and simulations also point out to a possible complex nonlinear interplay between various plasma instabilities. More recently, a new phenomenon of stimulated electron-acoustic wave scattering (SEAS) was proposed by Montgomery et al. [1] to reinterpret underderdense plasma data from the Trident laser facility. It was shown, that among electronic instabilities, stimulated scattering of laser light from the so-called, electron-acoustic wave (w < w _p) can possibly explain backscatter data at I >10¹⁶ W/cm²; previously attributed to stimulated Raman scattering (SRS) from unrealistically low plasma density. While in a Maxwellian plasma, the low-phase velocity linear electron-acoustic mode is strongly Landau damped; at finite wave amplitudes, electron trapping can give rise to undamped travelling wave solutions (BGK alike).

In first particle-in cell (1d3v) simulations of isolated SEAS, in a moderately underdense plasma not accessible to SRS (density ~ 0.5 N_cr), we observe a strong backscattered light near the electron plasma frequency. Intense transient reflectivity pulsations [2] due to SEAS are later supressed by huge relativistic heating of electrons. We propose a 3-wave model of parametric resonant coupling between laser light, slow backscattered wave (standing-like) and trapped electron-acoustic mode (w < < w _p) to explain the onset of an absolute SEAS instability in a uniform plasma layer. Spectral signatures, physical conditions and parameter dependence are studied in detail. In reported experiments SEAS to SRS signal ratio was smaller than 10^-3; however, we find hot plasma conditions such that SEAS can dominate over standard SRS. Further implications on relativistic-laser beam scattering and related intense electron heating are discussed.

 

* mskoricvin.bg.ac.yu

+ current address, GUAS, National Institute for Fusion Science, Toki-shi, Japan

[1] D.S. Montgomery et al., Phys. Rev. Lett. 87 (2001) 155001-1

[2] S. Miyamoto et al., J. Phys. Soc. Japan 67 (1998) 1281