Generation of non-axisymmetric scrape-off layer perturbations for controlling tokamak edge plasma profiles and stability^a)

A critical requirement for tokamak fusion reactors is the control of the divertor heat load, both the time-averaged value and the impulsive fluxes that accompany edge-localized modes. We propose driving toroidally varying currents through the scrape-off layer (SOL) plasma both to broaden the SOL by inducing radial convection and to control the edge pressure gradient by inducing resonant magnetic perturbations. The generation of additional convective transport via steady-state convective cells or increased turbulence drive requires that the electric potential perturbations exceed a threshold in amplitude that depends on wavelength. The generation of a coherent magnetic perturbation is optimized by choosing the appropriate width and phasing of the biasing region at the target plate in order to optimize the profile of the SOL current. Longer wavelength modes produce a larger effect because they are not sheared as strongly by the magnetic X-point. Generation of the necessary currents is challenging due to the possibly substantial power requirements and the possible need for internal insulators. We analyze passive current-drive mechanisms that rely on puffing and pumping of neutral gas in a toroidally asymmetric fashion using the UEDGE code to model the ITER divertor.