Neutral particle diagnostics for ohmically and auxiliary heated tokamaks

Abstract

Protons and hydrogen atoms in a plasma undergo charge‐exchange collisions which give rise to an energetic charge‐exchange flux emerging from the plasma. The intensity and energy distribution of the charge‐exchange flux is measured separately for hydrogen or deuterium. Besides discussing the basic principles and the experimental techniques, this report concentrates on plasma characteristics accessible to neutral particle diagnostics. From the energy distribution of the charge exchange flux, the ion energy distribution can be inferred. In the case of a thermal plasma the measurement of the ion temperature profile allows the analysis of the ion energy transport, and with auxiliary heating the ion heating efficiency can be estimated. From the absolute intensity of the charge‐exchange flux the hydrogen atom density can be determined; strong toroidal variation, with sharp increases at limiter and gas input valve, is observed. The atom flux which leaves the plasma hits the wall and forms one branch of plasma wall interaction which can be investigated by measuring the backscattering of neutrals from the wall and by analyzing the wall sputtering due to charge‐exchange atoms. Neutral particle diagnostics carried out in the divertor chamber of a divertor tokamak reveal strong plasma wall interaction at the neutralizer plate.