Fast gas injection as a diagnostic technique for particle confinement time measurements

Abstract

The determination of the effective particle confinement time (τp*), i.e., the particle confinement time normalized to recycling coefficient, is difficult when its value is long compared to the discharge duration in magnetically confined plasmas. Recent experiments on the current drive experiment upgrade (CDX-U) spherical torus have successfully achieved a significant reduction in recycling with large-area liquid lithium plasma-facing surfaces. The low recycling walls result in an increase in particle pumping and make it possible to measure τp* in short duration plasmas. Measurements of τp* are made using a supersonic gas injector which is closely coupled to plasma. A fast gas pulse is emitted from the supersonic gas injector, after which the density decay is measured using a microwave interferometer. The design of the supersonic gas injector and its configuration on CDX-U will be presented. The results of this technique will be shown as applied to the study of the effects of a liquid lithium toroidal limiter and evaporative lithium coatings on overall plasma density and τp*. © 2006 American Institute of Physics.