Plasma Potential Measurements in the Tandem Mirror Experiment-Upgrade Using a Time-of-Flight Neutral Particle Analyzer

摘要

We have developed a technique to measure the magnitude of electrostatic potentials using neutral particle analysis. We have constructed a time-of-flight neutral particle analyzer to measure the velocity distribution of the neutral particles emitted from the midplane of the East end cell of TMX-U. The analyzer is collimated to accept particles with a 22.5 deg pitch angle, within the magnetic loss cone of the end cell. This location and orientation allows study of the potentially confined ions in the end cell. In particular, ions with sufficient energy to escape over the confining potential are lost in a single pass. The density of these ions can be substantially lower than the density of ions confined by the potential. The boundary between the confined and unconfined ions appears as a break in the measured distribution. From the energy of the break, one can determine the magnitude of the potential confining the ions. The depth of the thermal barrier and the existence and magnitude of a potential peak between the thermal barrier and the central cell can also be inferred from similar breaks in the distribution function. Information on the magnitude and time dependence of these potentials is important in understanding the formation of potentials in tandem mirrors and the role potentials play in plasma confinement. The potential measurements from the Time-of-Flight analyzer are found to be consistent with a thermal-barrier type axial potential profile. During plugging conditions, the ion confining potential is at least a few hundred volts, sufficiently large to reduce the ion end losses to immeasurable levels. During the loss-of-plugging, the ion confining potential and thermal-barrier depth both decrease. The measured axial confinement time is found to agree with the theoretical predictions for potential confinement. 63 refs., 110 figs., 4 tabs. (ERA citation 13:013711)