Collective scattering of electromagnetic waves and cross-B plasma diffusion

Abstract

Magnetized plasmas occurring in nature as well as in fusion laboratories are often irregularly shaken by magnetic field fluctuations. The so-called 'coherent scattering' of electromagnetic waves from nonuniform, irregularly moving plasmas is investigated in the case where the scattering wavelength is large compared to the Debye length, but of the order of the irregularities correlation length. The scattered signal frequency spectrum is shown to be a transform of the plasma motion statistical 'characteristics'. When the scattering wavelength is larger than the plasma motion correlation length, the frequency spectrum is shown to be of a Lorentzian shape, with a frequency width that provides a direct measurement of the cross-B particle diffusion coefficient. This is illustrated by two series of experimental results: radar coherent backscattering observations of the auroral plasma, and far infrared scattering from tokamak fusion plasma. Radar coherent backscattering shows the transition from Gauss to Lorentz scattered frequency spectra. In infrared laser coherent scattering experiments from the Tore-Supra tokamak, a particular frequency line is observed to present a Lorentzian shape, that directly provides an electron cross-field diffusion coefficient. This diffusion coefficient agrees with the electron heat conductivity coefficient that is obtained from the observation of temperature profiles and energy balance.