High Frequency Instabilities in Underdense Plasmas Produced by a 0.35 µm Laser Beam

Abstract

High frequency parametric instabilities excited when an intense electromagnetic pulse (ω₀, k ₀) propagates through an underdense plasma are currently under active investigation.^1,2 Energy and momentum conservation give

$$ {\omega _{o}} = {\omega _{1}} + {\omega _{2}}$$

and,

$$ {\omega _{o}} = {\omega _{1}} + {\omega _{2}} $$

where at least one of the decay waves is an electron plasma wave (epw) with the dispersion relation

$$ {\underline k _{o}} = {\underline k _{1}} + {\underline k _{2}} $$

In the underdense corona of a laser fusion pellet there are two such instabilities of particular concern; the two-plasmon decay (2ω_p) instability and the stimulated Raman scattering (SRS) instability. This is because the epw’s associated with these instabilities can have phase velocities on the order of the speed of light and therefore produce very energetic electrons when they damp.³ Since such electrons can preheat the fuel in a laser fusion pellet before significant compression the 2ω_p and SRS instabilities are of particular concern. In the 2ω_p decay instability, the two decay waves are both epw’s so that frequency matching can occur only if ω₀ ≃ 2ω_p. This instability can therefore be expected to occur only near the quarter-critical layer of an inhomogeneous plasma.