EFFECTS OF A RADIAL ELECTRIC-FIELD ON LOW-FREQUENCY INSTABILITIES IN A MAGNETIZED PLASMA

In a single-ended Q-machine the effect of a radial electric field on c urrent-driven law frequency instabilities is investigated experimental ly. The field is produced by biasing a cylindrical wall surrounding th e plasma column. When the wall potential is positive, the low frequenc y potential relaxation instability is effectively suppressed, and the current is not limited by anomalous resistivity. In that regime a new type of instability is found with a higher frequency that increases pr oportionally to the wall potential. When the wall potential is negativ e, the potential relaxation instability is enhanced, and a strong modu lation and periodic limitation of the current occurs. The results show that the potential relaxation instability is well controlled by the w all potential, an effect which might be relevant for achieving a stabl e high current in a plasma. Our investigations also shed more light on the mechanism of the one-dimensional potential relaxation instability .