Forward Scattering of Light from a Laboratory Plasma

Abstract

THE scattering of electromagnetic radiation by a plasma in thermal equilibrium has been investigated by several authors^1–3 following the remarkable result of Bowles⁴, who found that the spectrum of the radiation back-scattered from the ionosphere has a breadth comparable to the Doppler shift at the thermal velocity even though the electrons do the scattering. Rosenbluth and Rostoker⁵ have extended those calculations to certain non-equilibrium cases. These calculations all show that the form and breadth of the spectrum of the scattered radiation are determined principally by the parameter denned as follows: where λ is the wave-length of the incident radiation, λ_D the Debye length of the plasma and θ the scattering angle measured with respect to the forward direction. If α ≪ 1 the electrons scatter as independent particles and the resulting spectrum is gaussian with a half-breadth given by the Doppler shift corresponding to the electron thermal velocity. If α > 1 the spectrum is dominated by collective effects and consists of a central peak broadened at the Doppler shift corresponding to the ion thermal velocity which, however, is gaussian in shape only for large frequency shifts and has two weak maxima at the half-breadth positions and two electron satellites at ± ω_p, the electron plasma frequency. These electron satellites each contain only 1/α² of the total scattered energy.