SOL turbulence and the interpretation of BES and GPI experiments

Abstract In beam emission spectroscopy (BES) it is often assumed that fluctuations in the injected neutral beam density are negligible compared to fluctuations in the core plasma density of interest. However, here we present a model which suggests that, for a typical medium-sized tokamak in L-mode, beam density fluctuations – induced as the beam passes through the highly turbulent scrape-off layer (SOL) – can be of similar order to core plasma density fluctuations. In the closely related gas puff imaging (GPI) diagnostic, interpretation of the underlying SOL plasma depends on the local relationship between plasma density n and electron temperature Te as a function of time. We show that when n ∝ Te there is a natural tendency – not necessarily reflected in the underlying plasma – for the measured fluctuation magnitude to increase radially. Furthermore, for the often-measured HeI (587.6 nm) line, intensity fluctuations in hotter near-SOL regions may be so small that they go unseen.