Parallel velocity shear instabilities in an inhomogeneous plasma with a sheared magnetic field

Abstract

Injection of fast neutrals with a component of the beam velocity along the magnetic field B0 results in a plasma having a sheared mean velocity along B0. If this parallel velocity shear U′ and the density inhomogeneity scale length Ln are large enough so that U ′|Ln |/(τ + 1) 1/2υi > 1 then a parallel Kelvin-Helmholtz instability is excited, where vi is the ion thermal speed and τ is the electron temperature/ion temperature. Magnetic shear stabilization requires a shear length Ls comparable to or less than |Ln |, namely, τL s /3 (τ + 1) < υi /U ′ < |Ln |/(τ + 1) 1/2. For U ′|Ln |/(τ + 1) 1/2υi < 1, however, this parallel Kelvin-Helmholtz instability is stabilized and only drift wave instabilities can be excited. For τ ≈ 1 and (M / m) 1/2 ≈ 43, the first of these drift wave instabilities is unstable for U ′/Ωi > 172| (Ln / Ls)3/2 | provided |Ln /Ls| < ai /30|Ln |, while the second is stable for U ′ /Ωi < 136ai /|Ls | provided |Ls /Ln | > 600 and U ′ /Ωi ≪ 1, where ai and Ωi are the ion gyration radius and frequency.