Electrostatic ion-cyclotron instability and potential relaxation instability excited by a ring-button electrode

Instabilities are studied which are excited by drawing an electron current to a button, surrounded by a concentric, axially moveable, floating ring in a single-ended Q-machine. Only the electrostatic ion-cyclotron instability is observed as long as the ring is behind the button or not farther than about two ion pitches in front of it. A strong enhancement of the ion-cyclotron instability occurs if the distance between button and ring is exactly one ion pitch. For larger distances the potential relaxation instability appears between the ring and the button. (author)