The role of current-driven drift mode on various scaling laws

The anomalous thermal heat conduction and particle diffusion caused by current driven drift modes (CD-mode) can affect the energy balance of an ohmically heated plasma appreciably. Transport coefficients scaling as 1/n are derived using simple assumptions for the fluctuation level. Energy balance equations with the parameters of the W VII-A stellarator with ohmic heating are evaluated in order to explain the temperature scaling Tsub(e) = Tsub(e)(anti n), anti n = density. At high density the anomalous transport coefficient increases with density but the increase is not large enough to explain the rapid deterioration of confinement at high density as found in W VII-A. In the low density regime the CD-mode may explain the observed scaling. The ions are assumed to behave neoclassically through the analysis. (orig.)