Feedback stabilization of plasma instabilities using a modulated ion beam

A new technique for feedback suppression using a modulated ion beam as a remote suppressor has been developed for the first time. A Langmuir probe was used as the sensor. The feedback loop was completed with a phase shifter and amplifiers. This technique was demonstrated by stabilizing the collisionless, curvature-driven trapped particle instability down to a baseline noise level with an appropriate feedback gain and phase. An important feature is that the injected ion beam is nonperturbing to the plasma, constituting only 1% of the plasma density. A simple theory to describe the experimental results is presented and its predictions show generally good agreement with the experiment. To indicate that this concept could also be extended to other plasma instabilities, the rotationally driven E×B mode was also stabilized. Since the basic mechanism of suppression is the cancellation of charge separation inherent in an instability, a modulated electron beam performed equally well. Such remote, nonperturbing particle beam suppressors have great potential for remote suppression of plasma instabilities in a variety of plasma devices.