Ion temperature measurements on TMX-U using a gridless electrostatic analyzer

Abstract

A small retarding-potential analyzer was used on the Tandem Mirror Experiment-Upgrade (TMX-U) to investigate the radial profiles of ion temperature, density, and plasma potential during ion cyclotron resonance heating (ICRH). The analyzer uses the magnetic field of TMX-U, rather than biased grids, to prevent electrons from reaching the biased ion collector. The probe, the Radial Energy Analyzer (REA), has been inserted into the central-cell plasma at temperatures of 200 eV and densities of 3×1012 cm−3 without damage to the probe or major degradation of the plasma. This analyzer measured an increase in ion temperature from 20 to 150 eV with 60 kW of ICRH. The REA measurements agree with other diagnostics on TMX-U. A one-dimensional code was written to investigate the effect of space charge on the flow of ions inside the probe tip. The effect was found to be minimal, in agreement with experimental data.