Enhancement and Suppression of Velocity-Shear-Driven Drift Instability due to Negative Ions

The collisionless electron drift wave instability in a plasma involving sheared magnetic-field-aligned positive-ion flow and negative ion species has been experimentally investigated. Surveying wide ranges of the shear strength and of the negative ion exchange fraction, which was first made possible by our new apparatus mounted in a Q machine, reveals detailed characteristics of the instability. The kinetic dispersion relation suggests that the wave observed for positive shear is the current-driven shear-modified drift wave. However, for negative shear the wave exhibits peculiar behavior which cannot be directly interpreted by the linear local theory. We also found that negative ions tend to stabilize the instability.