Modulation of the current-driven ion-cyclotron instability by the potential relaxation instability

An experimental investigation in a single-ended current-carrying Q machine is presented in which the radius of the current channel is varied. For increasing radius R (where R is normalized to the ion Larmor radius) the electrostatic ion-cyclotron instability is excited mainly at first. For R≳7, however, the potential relaxation instability mainly appears. In the transition region both instabilities are observed but the amplitude of the cyclotron instability is strongly modulated with the frequency of the relaxation instability. This is because of the periodic current limitation by the latter which nearly inhibits the electron drift necessary for the excitation of the former.