Driving toroidally asymmetric current through the tokamak scrape-off layer. I. Potential for edge localized mode suppression

Abstract

A potential technique for suppressing edge localized modes is theoretically analyzed. Recent experiments have shown that externally generated resonant magnetic perturbations (RMPs) can stabilize edge localized modes (ELMs) by modifying the density profile [T. E. Evans et al., Nat. Phys. 2, 419 (2006); Y. Liang et al., Phys. Rev. Lett. 98, 265004 (2007)]. Driving toroidally asymmetric current internally through the scrape-off layer (SOL) plasma itself can also generate RMPs that are close to the required threshold for ELM control. Ion saturation current densities can be achieved by producing potential differences on the order of the electron temperature. Although the threshold is uncertain in future devices, if driven coherently through the SOL, the upper limit for the resulting perturbation field would exceed the present experimental threshold. This analysis provides the tools required for estimating the magnitude of the coherent SOL current and RMP generated via toroidally asymmetric biasing of the target. Flux expansion increases the perturbation near the X-point, while phase interference due to the shearing of field lines near the X-point reduces the amplitude of the effective SOL perturbation and makes the result sensitive to both toroidal mode number n and the phasing at the target plate. If the current density driven at the target plate decays radially, the amplitude over the useful coherence width of the current profile will be reduced. The RMP can still exceed the present threshold at low n if the radial location and width of the biasing region are optimally chosen.