Magnetic Perturbation Spectrum Produced by Non-Axisymmetric Scrape-Off Layer Current and the X-Point Shadow

摘要

The applied magnetic perturbation spectrum produced by non-axisymmetric scrape-off layer current is calculated for the symmetric two-wire model of a diverted plasma at large aspect-ratio. The spectral peak lies close to the magnetic field line pitch-resonance condition near the separatrix, but moves toward larger poloidal harmonics on flux surfaces in the core. Dissipative and kinetic effects limit the ability of the asymmetric parallel current to reach the midplane when the biasing region is sufficiently close to the strike point. Thus, the X-point casts a thin “shadow” across the separatrix where it is not possible to drive current. The size of the region of reduced current drive efficiency is estimated based on fluid and kinetic limits (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)