Collisional drift instability in hollow and coaxial plasmas

The drift instability, which is known to localize in regions of maximum density gradient, is observed in a cylindrical discharge plasma which may be made hollow or coaxial in shape. In the hollow plasma, two distinct radial density gradients occur, forming the outer and inner edges of the plasma. These plasma edges are the sites of two distinct modes of the instability which may differ in mode number but not frequency. In the coaxial plasma, three plasma edges occur, and three distinct modes are observed. The plasma is structured and the waves are excited by positive d.c. voltages applied to grids in the path of the plasma. In predictions of radial wave shape, a two fluid model which allows for arbitrary radial density profiles provides excellent agreement with experiment. Real frequency predictions of the model are Doppler shifted by E*B column rotations which are treated as uniform in the regions of mode propagation. In most cases, good agreement with observed frequency is obtained.