Stability studies of a hollow plasma in the double cusp experiment

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Abstract
Axisymmetric cusp end cells have application for the stabilization and plugging of a tandem‐mirror‐type reactor. Experiments have been performed on the RFC‐XX double cusp in Nagoya, Japan to measure the stability of the hollow plasma in the adiabatically confined region of the device. Experiments were performed with both a uniform field central section and with a mirror central section connecting the two cusps. The plasma is produced by an rf discharge of gas puffed into the vessel. Plasma fluctuations were measured by Langmuir probes and magnetic probes. Stable regions of operation with a hollow plasma in the cusp are found for both magnetic configurations. Various types of instabilities are observed: electrostatic drift type waves with m≥1, lower‐frequency waves with a magnetic perturbation associated with the density perturbation, an m=0 ion‐acoustic mode, and a large amplitude relaxation oscillation.