Radial ion diffusion induced by cyclotron resonance heating at the thermal barrier region in the Phaedrus-B tandem mirror

Experiments with and without ion cyclotron resonance (1.275 MHz, fundamental ion cyclotron frequency of hydrogen plasma) at the thermal-barrier region have been carried out by varying the thermal-barrier magnetic field strength. Radial density profiles, potential profiles, and currents have been measured, as well as the fueling at the thermal-barrier region. The density at the plasma edge increased and the potential at the plasma edge decreased as the B field was decreased to bring the resonance into the thermal-barrier cell in both a balanced and imbalanced gas-puffing case. The data indicated that a possible mechanism is that ions can escape from the confinement region along the open resonance surface (the same as the mod-B surfaces) because the turning point of the trapped ions has a tendency to center on mod-B surfaces. An increasing potential observed in the present experiments in the edge region of the plasma with increasing _B field may be related to the higher degree of fluctuation of radial current to the collector.