Spectral line shapes in low frequency turbulent tokamak plasmas

Abstract

In this paper we investigate the influence of low frequency turbulence on Doppler spectral line shapes in magnetized plasmas. Low frequency refers here to fluctuations whose typical time scale is much larger than those characterizing the atomic processes, such as radiative decay, collisions and charge exchange. This ordering is in particular relevant for drift wave turbulence, ubiquitous in edge plasmas of fusion devices. Turbulent fluctuations are found to affect line shapes through both the spatial and time averages introduced by the measurement process. The profile is expressed in terms of the fluid fields describing the plasma. Assuming the spectrometer acquisition time to be much larger than the turbulent time scale, an ordering generally fulfilled in experiments, allows to develop a statistical formalism. We proceed by investigating the effects of density, fluid velocity and temperature fluctuations alone on the Doppler profile of a spectral line emitted by a charge exchange population of neutrals. Line wings are found to be affected by ion temperature or fluid velocity fluctuations, and can in some cases exhibit a power-law behavior. This study gives some insights in the appearance of non-Boltzmann statistics, such as Levy statistics, when dealing with averaged experimental data.