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Can the electrostatic ion-cyclotron instability by driven by a two-dimensional sheath?

An experiment in a single-ended Q-machine was conducted in which a voltage was applied between two concentric floating electrodes which terminated the plasma column. When the inner electrode was more than 200 V positive with respect to the outer one the electrostatic ion-cyclotron instability was observed. In the usual case the electron current is drawn between the grounded (plasma generating) hot plate and a positively biased electrode, with only a small voltage needed to trigger the instability. However, the current threshold is the same for both cases, indicating that it is a critical electron drift speed, not a sheath effect, which is necessary for the instability.