On the mechanism of the electrostatic ion cyclotron instability

Experimental investigations in a new arrangement in a single-ended collisionless Q-machine shed new light on the mechanism of the current-driven electrostatic ion cyclotron instability. The instability was excited by a positively-biased button surrounded by a coplanar usually floating ring electrode. By inserting an appropriately biased additional grid with central hole the plasma density between the ring and this grid could be controlled while the current channel in front of the button was not perturbed. Also in this arrangement the potential relaxation instability could be excited. By varying the bias and the distance of the grid-hole electrodes from the ring-button a smooth transition from a regime dominated by the potential relaxation instability to one dominated by ion cyclotron instability was obtained. In addition, the role of the reflected ions for the mechanism of the electrostatic ion cyclotron instability could be seen in a new light.