Explanation for MARFE formation and subsequent evolution into a detached symmetric plasma edge

An explanation is proposed for the experimentally observed tokamak phenomenon of a symmetric radiating edge evolving with increasing density into a multifaceted asymmetric radiation from the edge (MARFE) condition and then cooling into a detached plasma with a symmetric edge. The thermal stability impurity density limits increase with the poloidal mode number, m, of the edge plasma distribution. When the edge impurity density exceeds the m=0 mode density limit, the edge plasma evolves into a mixture of m≳0 modes which becomes more highly asymmetric as the density increases further and exceeds the density limits of successively higher modes, forcing the plasma into the remaining stable higher-m modes—the stable MARFE. If the radiative cooling of the edge is sufficient to reduce the temperature to Te∼10 eV, the various thermal modes are stabilized by the strong positive temperature dependence of the ionization cooling in the presence of neutral atoms, the impurity density limits are thereby increased, and the plasma evolves back into a stable m=0 mode. On the other hand, if the plasma becomes highly enough asymmetric before sufficient cooling takes place, nonlinear effects drive a disruptive collapse of the radial temperature distribution.