Observation of velocity-shear flute instability in the region of bad curvature

Abstract

A low-frequency flute instability in a collisionless plasma flowing along a curved magnetic field has been observed to localize in regions of bad curvature (∇B2∇≳0). The experiment was performed in a single-ended Q machine. A low density plasma was injected along the lines of force which were bent into the arc of a circle. As the flow velocity of the plasma was increased, the flute type oscillations were found to be stabilized in the region of good curvature and to be destabilized in the region of bad curvature. The experimental results are well explained by simplified calculations. For slow flow velocity the instability is mainly driven by a transverse Kelvin–Helmholtz effect due to the velocity shear of the E×B rotation, so that it is almost independent of the magnetic field curvature. For large velocities, however, the curvature effect stems from a Rayleigh–Taylor ’’gravitational’’ effect due to the centrifugal force, which makes the oscillation localize in the region of bad curvature.