Effects of edge plasma turbulence on radial correlation length measurements with BES

Abstract

The recently developed technique of beam emission spectroscopy (BES) provides a tool to study long-wavelength density turbulence (coherence length ≫ ion gyroradius) in hot tokamak plasmas. To provide an accurate conversion of the measured light intensity fluctuations to a local ñ/n density fluctuation and to assess the influence of density fluctuations in the neutral beam induced by large edge turbulence, a multistate neutral beam excitation/transport code for realistic experimental geometries has been written. Results from this code show that the attenuation of the beam density induced by edge turbulence can give rise to significant levels of common-mode fluctuation power in signals from the plasma core and that the derivation of quantitative values of ñ/n from experimental measurements depends weakly on the radial extent of the density fluctuations.