Ionization and edge-density gradient effects on MARFE instabilities in tokamak plasmas

Abstract

Ionization and edge-density gradient effects on the long-wavelength radiative condensation (MARFE) instability in tokamak edge plasmas are considered. A self-consistent coupled equilibrium is derived from the continuity and temperature equations including the effects of radiation and ionization. In the limit relevant to edge plasmas, a modification of the temperature equilibrium derived earlier [Phys. Fluids 30, 2429 (1987)] is obtained due to the rapid edge-density variations. The critical radiation (density) limit is enhanced and results in significantly larger growth rates for the pure radiative mode than previously reported. Ionization processes affect the MARFE through a source term in the continuity equation, and produce an effect which qualitatively depends on the edge temperature T_L. For T_L less than a critical temperature, ionization phenomena weaken the MARFE instability. This behavior reverses at higher temperatures.