FAST PARTIAL AND TOTAL PRESSURE MEASUREMENTS DURING TOKAMAK DISCHARGES

A system consisting of magnetically shielded ion gauges and a differentially pumped quadrupole mass spectrometer is described. It provides time-resolved measurements of the neutral gas fluxes during discharges in the Tokamak de Varennes. A response time of 15 ms for hydrogen and ≈ 50 ms for impurity gases was obtained by a judicious choice of distance (to avoid magnetic disturbances), conductance, and pumping speed. An interface between the spectrometer and the data acquisition system of the tokamak allows fast (ms) selection of the mass and real-time (≈ 10 ms) adjustment of the gain to optimize the signal-to-background ratio. The usefulness of this diagnostic is illustrated by its use in three experiments: propagation of a plasma density perturbation following pulsed (5 ms) gas puffing, exchange of the discharge gas from H2to He, and chemical impurity (CO, CO2) production during discharges at the density limit and qa≈2. The correlation of low impurity fluxes with absence of disruptions in these last shots is shown. © 1990, American Vacuum Society. All rights reserved.