Electrode Biasing Experiment in KMAX Tandem Mirror

An electrode biasing system has been installed on the KMAX (Keda Mirror with AXisymmetricity) tandem mirror machine to control the rotation speed. It consists of a metal disk-type electrode and a concentric ring-shaped electrode. On each of them are 12 embedded single probes distributed uniformly in the azimuthal direction plus a single probe on the center. An adjustable power supply provides the biasing voltage from −1 kV to 1 kV, and a silicon controlled rectifier with rising time ~5 μs and maximum current up to 3000 A is used to switch on the circuit. While most of applied voltages are inevitably lost on the sheath as confirmed by the experiments, the plasma potentials have been found to change substantially.