In a single-ended Q-machine the effect of a radial electric field on current-driven low frequency instabilities is investigated experimentally. The field is produced by biasing a cylindrical wall surrounding the plasma column. When the wall potential is positive, the low frequency 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 proportionally to the wall potential. When the wall potential is negative, the potential relaxation instability is enhanced, and a strong modulation and periodic limitation of the current occurs. The results show that the potential relaxation instability is well controlled by the wall potential, an effect which might be relevant for achieving a stable high current in a plasma. The investigations also shed more light on the mechanism of the one-dimensional potential relaxation instability.
Low-frequency instability in weakly ionized current-carrying magnetized plasmas. I. Experiments on instability behaviour