New unstable branch of drift resistive ballooning modes in tokamaks

Abstract

Drift resistive ballooning modes (DRBM) are studied for present-day tokamak edge plasmas. A ballooning equation has been derived for DRBM, corresponding to an ‘‘optimal ordering,’’ i.e., such a choice of the length and time normalizing units that make the three terms in the vorticity equation to be of the same order. A simple criterion to distinguish strong and weak ballooning regimes for DRBM has been obtained. The DRBM in the strong ballooning regime have been studied and it has been found that they are robustly unstable with an ideal growth rate γ∝cs/(RLn)1/2. The weak ballooning regime also has been studied. It has been found that there exist two different unstable branches in this case. The first one has been identified as a conventional DRBM studied earlier. It has been shown, however, that this solution is stabilized when magnetic shear ŝ=aq′/q is order of 1. The other branch has a stronger ballooning structure compared to the conventional DRBM branch and is localized on the outside of the torus. It is less unstable for a weak shear, but is the only one that is robustly unstable for ŝ∼1. A numerical simulation has shown that this branch matches the strong ballooning solution.