Fast Gas Injection as a Diagnostic Technique for Particle Confinement Time Measurements
Authors: T. Gray, R. Kaita, R. Majeski, J. Spaleta, and J. Timberlake
Date of PPPL Report: June 2006
Submitted to: Review of Scientific Instruments
The determination of the effective particle confinement time (τ*p), 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 τ*p in short duration plasmas. Measurements of τ*p 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 τ*p.