Second stability region for gyrokinetics and the L-H transition

TL;DR

This paper demonstrates the existence of the second stability region in gyrokinetics within a simple tokamak model and shows how plasma transitions to H-mode involve entering this region due to current and pressure effects, reducing turbulence.

Contribution

It extends the concept of the second stability region from MHD to gyrokinetics and links it to the L-H transition through magnetic shear modifications and turbulence reduction.

Findings

Second stability region exists for gyrokinetics in simple tokamak geometry.

Transition to H-mode involves entering the second stability region due to bootstrap current and pressure effects.

Significant reduction in electrostatic turbulence when moving from L- to H-mode profiles.

Abstract

Using a simple circular tokamak geometry, we show the well-known `second stability region' of MHD-ballooning modes exists for linear gyrokinetics too -- whether electrostatic or electromagnetic -- and we suggest that the plasma enters this region in H-mode as a consequence of the bootstrap current and Shafranov shift altering the magnetic field, which may occur if the normalised pressure gradient is $\alpha_{\rm MHD} \simgt 1$ and collisionality is low. By performing simulations in more realistic magnetic geometries, we demonstrate a large reduction in collisionless, electrostatic turbulent transport when going from density and temperature profiles typical of L- and H-mode, respectively. This reduction is shown to be a consequence of both the bootstrap current lowering the global magnetic shear, and the pressure gradient altering the local magnetic shear, pushing the plasma towards the second-stability region. A path connecting the L- and H-mode equilibria is constructed, along which the energy and particle fluxes exhibit non-monotonic behaviour as a function of the pressure gradient.