A ONE-DIMENSIONAL MODEL FOR RADIATIVE THERMAL-EQUILIBRIUM AND STABILITY OF THE TOKAMAK EDGE PLASMA

The multifaceted asymmetric radiation from the edge (MARFE) instabilit y observed in the peripheral radiative layer in the high-density tokam aks is considered for a spatially smooth profile of radiative-loss fun ction in equilibrium. The instability which is driven by the impurity radiation and opposed by the finite thermal conductivity, was consider ed previously by Drake [J. F. Drake, Phys. Fluids 30, 2429 (1987)] as a surface wave on a sharp radiative layer (step-function-like) at the edge. A novel exponential model is presented for the radiative loss fu nction and it is shown that the two crucial features of MARFE: (i) sta bility to poloidally symmetric perturbations and (ii) a threshold valu e of radiative loss for the growth of asymmetric perturbations, explai ned earlier by Drake's theory, are contained in this model of radiativ e equilibrium, also. Since, from the earlier theory, these features ap pear to be equilibrium specific and related to the form of radiative-l oss function, the present work broadens the class of radiation profile s which can explain the MARFE, by recovering all the old results by an entirely different mathematical treatment.