Collisional Drift Instability in a Variable Radial Electric Field

The collisional drift instability, which is observed in a weakly ionized argon plasma, is described by a cylindrical two-fluid model which allows for arbitrary density profiles and a sheared E*B rotation of the plasma. The waves are destabilized by a current drawn from a DC biased grid (in the path of the plasma) to ground. Variation of the grid-to-ground current is shown to alter the radial potential profile and thus the E*B rotation of the plasma. A threshold current destabilizes the plasma. Above this threshold, mode frequency is found to increase with current. Predictions of the model demonstrate both rigid body and sheared E*B rotation are destabilizing and that both provide the well known Doppler shift to the real frequency. When three versions of the model are compared, the best agreement with observed real frequency occurs when the radial electric field is included rigorously in the theory. The documented increase in the radial potential gradient with grid-to-ground current is shown to be responsible for the corresponding increase frequency. Through a transition from a stable to an unstable regime of grid operation the observed increase in radial potential gradient is strongly suggested as the destabilizing mechanism at work. The radial wave shape is predicted closely by two versions of the model in which the radial electric field is or is not included, and both provide excellent agreement with experiment.