New glance at resistive ballooning modes at the edge of tokamak plasmas

Abstract

To understand the L–H transition, one has to identify the modes to be stabilized at the edge of L-mode plasmas, roughly from ρ = 0.7 to the last closed flux surface. To address this issue, realistic edge tokamak parameters inspired by three different L-modes from DIII-D and Tore Supra have been investigated with a gyrokinetic code GENE (Jenko et al 2000 Phys. Plasmas 7 1904). Former fluid theories for such parameters predict resistive ballooning modes (RBMs) to be unstable (Rogers et al 1998 Phys. Rev. Lett. 81 4396). In this paper, linear gyrokinetic simulations demonstrate that, under realistic L-mode conditions, RBMs are linearly unstable at every edge, i.e. ρ ⩾ 0.93. These modes predominantly drift in the electron diamagnetic direction at low wave numbers and are destabilized by higher collisionality. They are further destabilized by higher normalized temperature gradient and higher q. The magnetic shear and the density gradient length have a weaker impact.