Review of MARFE phenomena in tokamaks

Abstract

This paper presents a review of MARFEs and detached plasmas. A MARFE is a toroidally symmetric tokamak phenomenon occuring at minor radii near the plasma boundary, defined by a limiter or magnetic separatrix. The MARFE poloidal width is approximately 30° in extent at the smaller major radius edge of the plasma. It is characterized by greatly increased radiation, high ion densities and density fluctuations, and relatively low electron temperature. These changes in the edge plasma are consistent with the MARFE being the manifestation of a thermal instability. MARFEs tend to occur in most tokamaks at similar values of ρ = n e κJ , a fraction of the density disruption limit. A related phenomenon, the detached plasma, is different in that the boundary layer radiation emission is poloidally symmetric. The radiated power, in this case, is approximately equal to the input power, thereby reducing power flows to the limiter to negligible values. A review of experimental data and theoretical treatments of MARFEs and detached plasmas are provided herein. The relationship between boundary layer parameter scalings, the MARFE threshold and the density limit is explored.