Radiation-driven turbulence at the plasma edge in toroidal devices

The radiation drives for plasma edge parameters are reexamined. By including the electron density gradient, the radiation drive through density fluctuations can be strongly destabilizing even in a parameter regime away from force balance. This results in the dominance of the magnitude of the radiation power (γn drive) over its derivative (γT drive) for plasma edge parameters of Ohmic tokamak discharges. This leads to instability over a range of poloidal mode numbers much broader than in the case of the γT drive and to temperature fluctuations of opposite phase to the density fluctuations. Nonlinear numerical calculations show that this opposite phase relationship persists nonlinearly. As a consequence, the fluctuations induce a heat flux that is observed to be inward, while the particle and energy fluxes that they generate are calculated to be outward.