Parametric Effects of Radiation on a Plasma

The parametric excitation of the modes of an infinite plasma by intense incident radiation is studied on the basis of the Vlasov equation. It is found that the modes can be driven into unstable oscillations for incident frequencies in the three regions ω₀ωₚₑ, ωₚₑ+ωᵢ, and 2ωₚₑ, where ωₚₑ is the electron plasma frequency, and ωᵢ is the ion acoustic frequency. In the limit of weak intensities, the features of the two resonances ω₀ωₚₑ+ωᵢ and 2ωₚₑ are found to be in substantial agreement with the results of DuBois and Goldman. For larger intensities it is found that the resonance ω₀ωₚₑ+ωᵢ is restricted to frequencies ω₀ which are not more than 4ωₚᵢ above or ωᵢ below this value, and has a maximum growth rate of 0.05ωₚₑ. The resonance near ω₀ωₚₑ is found to be dominated by collisional damping if γωₚₑ>10^-4, and limited to a range of frequencies ω₀ of only ωₚᵢ100. The present results do not generally agree with the results obtained by Silin. These results indicate that the usual harmonic approximation for the plasma is justified except in the above-mentioned frequency regions.