A one-dimensional model for radiative thermal equilibrium and stability of the tokamak edge plasma

Abstract

The multifaceted asymmetric radiation from the edge (MARFE) instability observed in the peripheral radiative layer in the high-density tokamaks is considered for a spatially smooth profile of radiative-loss function in equilibrium. The instability which is driven by the impurity radiation and opposed by the finite thermal conductivity, was considered 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 function and it is shown that the two crucial features of MARFE: (i) stability to poloidally symmetric perturbations and (ii) a threshold value of radiative loss for the growth of asymmetric perturbations, explained earlier by Drake’s theory, are contained in this model of radiative equilibrium, also. Since, from the earlier theory, these features appear to be equilibrium specific and related to the form of radiative-loss function, the present work broadens the class of radiation profiles which can explain the MARFE, by recovering all the old results by an entirely different mathematical treatment.