Internal plasma pressure peaking in low-shear open and closed magnetic confinement systems

Abstract

The plasma convective (flute-interchange) stability for low magnetic shear systems, with a low collisionality and a low beta, is considered in terms of the necessary and sufficient collisionless kinetic criterion. The magnetic confinement systems under consideration are axially symmetric mirrors equipped with outer divertors and inner field reversing rings (cusps, internal rings, high-beta cells) and closed multimirror traps. A simple approach is proposed for plasma stabilization resulting in a substantial steepening of the critical pressure profile. The essence is the combination of the plasma stabilization by a strong and an alternating-sign field-line curvature. Axially symmetric tandem mirror systems composed of a mirror device having an outer divertor and an inner field reversing ring are shown to have an internal radially peaked stable pressure profile. Bumpy tori are also shown to have an internally peaked stable pressure profile.

Stable anisotropic plasma confinement in magnetic configurations with convex–concave field lines

“…The key result of [14] in comparison with previous works is that even for a partial compensation of the drift-induced space-charge separation due to the particle longitudinal motion, the plasma stability is improved: the allowable pressure profile is much steeper (figure 2).…”

“…This explains the shift of the peaking position for an anisotropic plasma [14,42], namely its position shifting towards the min dl for plasmas with prevailing longitudinal velocity and towards min λ max for plasmas with prevailing transverse velocity 3 . It is clear that the main contribution to the vanishing of ∫ G(λ, ψ)∂J /∂ψ dλ arises from the passing particles (low λ) in the first case and from the trapped particles (large λ) in the second case.…”

“…It is found that the combination of the both stabilization methods, when plasma is confined at convex-concave field lines, provides a strong stabilizing action against convective (flute-interchange) plasma instability [14]. In particular, for the tandem trap mirror-cusp a strong internal peaking of the stable plasma pressure profile is observed (see figure 1).…”

Plasma confinement by magnetic field with convex-concave field lines

“…For the particle distribution function F (ε, λ, ψ) = f (ε) G (λ, ψ) p m (ψ) (where ψ is the flux function, λ is the pitch angle, ε is the energy, and ∂f /∂ε should be <0) the kinetic criterion [13,[33][34][35][36][37] gives the stability condition (see [38,39] and [14,15]):…”

“…It has been found that a combination of the convex and the concave part of a field line provides a strong stabilizing action against convective (flute-interchange) plasma instability [14,15]. This results in internal peaking of the stable plasma pressure profile that is calculated from the collisionless kinetic stability criterion for any magnetic confinement system with a combination of mirrors and cusps.…”