Low energy neutral outflux from the PLT tokamak

Abstract

A time-of-flight system has been developed to measure the energy spectrum of neutral deuterium atoms emitted from the PLT tokamak plasma in the energy range 20–1000 eV. D 0 neutrals are mechanically chopped by a rotating slotted stainless steel disc. The gated neutrals impinge on a Cu-Be disc and eject secondary electrons which are collected and amplified by an electron multiplier; hence quantitative D 0 fluxes can be determined. The time evolution of the D 0 outflux emission spectra, dΓ/dEdΩ, has been measured for low density, ohmically heated PLT discharges. Our data, which covers the energy range 28–1000 eV, represent the first measurements of tokamak neutral outflux below 100 eV. For D 0 energies above 200 eV, we find that dΓ/dE dΩ vs. Particle energy monotonically decreases, typically in an exponential fashion. Early in a discharge dΓ/dEdΩ is weighted towards low energy. The measured plasma ion edge temperature increases with time, and concurrent with this rise the total particle flux falls by a factor of 25. Throughout these ohmically heated discharges the charge-exchange power loss away from the limiter is a small fraction, less than 2%, of the ohmic heating power; the charge-exchange particle loss away from the limiter is relatively unimportant in particle balance; and the bulk of the particle emission is typically at energies below 200 eV.