Journal of Plasma Physics

Review Article

Recent theoretical progress in understanding coherent structures in edge and SOL turbulence

S. I. KRASHENINNIKOVa1, D. A. D'IPPOLITOa2 and J. R. MYRAa2

a1 University of California, San Diego, CA, USA

a2 Lodestar Research Corporation, 2400 Central Avenue, Boulder, CO, USA (skrash@mechanics.ucsd.edu)

Abstract

In this paper we review some theoretical aspects of the dynamics of the mesoscale filaments extending along the magnetic field lines in the edge plasma, which are often called ‘blobs’. We start with a brief historical survey of experimental data and the main ideas on edge and SOL plasma transport, which finally evolved into the modern paradigm of convective very-intermittent cross-field edge plasma transport. We show that both extensive analytic treatments and numerical simulations demonstrate that plasma blobs with enhanced pressure can be convected coherently towards the wall. The mechanism of convection is related to an effective gravity force (e.g. owing to magnetic curvature effects), which causes plasma polarization and a corresponding E× B convection. The impacts of different effects (e.g. X-point magnetic geometry, plasma collisionality, plasma beta, etc.) on blob dynamics are considered. Theory and simulation predict, both for current tokamaks and for ITER, blob propagation speeds and cross-field sizes to be of the order of a few hundred meters per second and a centimeter, respectively, which are in reasonable agreement with available experimental data. Moreover, the concept of blobs as a fundamental entity of convective transport in the scrape-off layer provides explanations for observed outwards convective transport, intermittency and non-Gaussian statistics in edge plasmas, and enhanced wall recycling in both toroidal and linear machines.

(Received August 17 2007)

(Revised October 26 2007)

(Online publication January 02 2008)