Ionization-recombination instability and multifaceted asymmetric radiationfrom the edge in magnetic fusion devices

Recent experimental studies of MARFEs (Multifaceted Asymmetric Radiation Fr om the Edge) in the Alcator C-Mod [I. H. Hutchinson , Phys. Plasmas 1, 1511 (1994)] and TEXTOR-94 (upgraded Torus Experiment for Technology Oriented R esearch) [A. M. Messiaen , Phys. Plasmas 4, 1690 (1997)] tokamaks reveal lo w plasma temperature T similar to 0.7 eV, strong hydrogen radiation, and pl asma recombination in the MARFE region. These results contradict a well-acc epted model, based on low-Z impurity radiation, and can be better explained by a nonlinear stage of an ionization-recombination instability, discussed recently in the literature. In this paper, a qualitative analysis of the i nstability is performed, and its growth rate gamma and the range of wavelen gths of unstable perturbations are estimated. It is found that the instabil ity exists for plasma temperatures near the wall T less than or similar to 10 eV; and for lower temperatures, T similar to 1 eV, volumetric recombinat ion becomes important and can increase gamma considerably compared to the c ase without the volumetric recombination. The results of the qualitative an alysis are confirmed by a numerical solution of the linearized fluid equati ons with recombination and ionization describing MARFEs. (C) 2000 American Institute of Physics. [S1070-664X(00)02903-7].