From incoherent to collective scattering

Electromagnetic wave scattering is a classical diagnostic of space as well as of fusion plasmas. Two different classes of scattering mechanisms are manifested by different spectra, and receive different theoretical interpretations: Namely, the ''incoherent scattering'', that results from the statistical addition of independently scattered fields from each plasma particles, and the ''collective scattering'', as obtained when the dielectric plasma susceptibility is macroscopically non-uniform. There exist however conditions for which neither of these two models are fully appropriate, when simultaneously particle motion and macroscopic dielectric susceptibility fluctuations must be taken into account. This could be the case of scattering experiments on magnetized, collisionless, randomly agitated, non-uniform plasmas, in conditions occurring sometimes in the magnetosphere, or in tokamaks. These conditions are met when the particle motion r(t), and the macroscopic motion R(t) result in two Doppler frequency shifts of similar order of magnitude. Specifically, if T is a macroscopic time scale, and k the scattering wavevector, both motions contribute to shape the scattering spectrum when k·r(T)∼k·R(T). In the present study, we investigate these conditions and calculate the collective scattering spectrum resulting from the motions of both a macroscopically non-uniform, externally agitated plasma, and of its individual particles. This effect will be illustrated by a numerical result. (author) 1 ref., 5 figs