Transverse Velocity Shear Instabilities within a Magnetically Confined Plasma

Abstract

Strong shear in the plasma rotation is produced in the interior of a magnetically confined Q ‐machine plasma column by means of an externally controlled nonuniform radial electric field. Coherent low‐frequency oscillations are observed localized in the velocity shear layer, which contains up to seven ion gyroradii. The observed oscillations are shown to result from the transverse Kelvin‐Helmholtz instability by the good agreement with finite‐Larmor‐radius fluid theory for the mode frequencies, radial dependences of the potential and density fluctuations, and the mode structure at large magnetic field and small rotation. At large electric fields the low‐frequency Kelvin‐Helmholtz modes are completely suppressed. Simultaneously, there appear oscillations with ω>̃ω c , whose properties are described by the fluid equations when extended to arbitrary rotation and wave frequencies.