Instabilities of a high-pressure plasma contained by a dense blanket of neutral gas

The paper examines instabilities in an inhomogeneous plasma whose pressure, p, is balanced by the pressure of a dense blanket of neutral gas and is large compared with the pressure of the magnetic field, B 2 /8π, ß ≡ 8πp/B 2 ≫1. It was shown earlier tha t with ∂ ln B/∂ ln n ≥ 0 (n being the plasma density) instabilities may arise in such plasma with a growth rate of the order of the gradient frequency, γ ≃ω n , and a longitudinal wave number k z ≃ ω n /v T i , v T i being the thermal velocity of the ions. Where ∂ ln B/∂ ln n < 0 such dangerous instabilities do not arise; hence this represents the most favourable conditions of containment. However, the present paper shows that even in these conditions the plasma is not free of instabilities. It is established that with ∂ ln B/∂ ln n < 0 perturbations with a large k z value can arise, e.g. k z ≳ ω n /v T i . However, the growth κ rate of such perturbations is small by comparison with the frequency γ ≃ (k ⊥ ρi) 2 √B ω ni (where k ⊥ is the transverse wave number and ρ i the Larmor ion radius).