Self-mode transition, oscillation and inverse hysteresis in ECR discharges

Positive and negative feedback of microwave (MW) ECR (electron cyclotron resonance) discharges in cross magnetic field were manipulated by a three-stub coaxial tuner. Unstable ECR discharges in the positive feedback region, such as the self-mode transition between the ordinary (O)-wave mode and extraordinary (X) one, inverse hysteresis, and oscillation, were investigated using a time-resolved MW power meter, high speed camera, and thermocouple. In the positive feedback region, the self-O → X wave mode discharge transition can be driven under the “hot” wall condition, while the counterpart, namely the self-X → O wave mode transition, is observable for the “cold” wall; the inverse hysteresis and oscillation take place at low and moderate gas flow rates, respectively. The mechanism underlying the self-mode transitions and instabilities is the discharge shift due to the gas heating effect. For MW ECR discharges to become stable, as indicated in previous experiments, they should be mismatched in the negative feedback region.