Edge plasma relaxations due to diamagnetic stabilization

TL;DR

This paper introduces a new mechanism for edge plasma relaxation in tokamaks driven by diamagnetic coupling, leading to oscillatory behavior similar to type III ELMs, supported by simulations and a 1D model.

Contribution

It presents a novel diamagnetic coupling mechanism causing edge plasma relaxations, supported by 3D turbulence simulations and a simplified 1D model.

Findings

Relaxations are driven by diamagnetic coupling in resistive ballooning/drift wave dynamics.

Oscillations decrease in energy flux with higher frequency and are damped by increased plasma temperature.

The behavior resembles type III Edge Localized Modes (ELMs).

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 $E\times B$ flow does not play any significant role. After relaxation the transport barrier forms again and it sets the conditions for a novel relaxation, resulting in an oscillatory behavior. We find that energy flux into the scrape of layer decreases with increasing oscillation frequency and that the oscillations are tamed by increasing plasma temperature. This behavior is reminiscent of so-called type III Edge Localized Modes. A one-dimensional model reproducing the relaxations is also derived.