Edge plasma relaxations due to diamagnetic stabilization

Abstract

A new mechanism for pressure profile relaxations in an edge tokamak plasma is derived from simulations within the two-fluid three-dimensional turbulence code EMEDGE3D. The relaxation is due to diamagnetic coupling in the resistive ballooning/drift wave dynamics: Unstable modes experience explosive growth at high pressure gradients after a phase in which they are stabilized by the diamagnetic coupling leading to the onset of a transport barrier. The sheared E × B flow does not play any significant role. After relaxation, the transport barrier forms again and it sets the conditions for a new relaxation event, resulting in an oscillatory behavior. We find that energy flux into the scrape-off layer decreases with the increasing oscillation frequency and that the oscillations are tamed by increasing plasma temperature. This behavior is reminiscent of the so-called type-III edge localized modes. A one-dimensional model reproducing the relaxations is also derived.