Universal asymptotics of poloidal spectra of magnetic perturbations of saddle coils in tokamaks

Abstract

Universal and generic features of poloidal spectra of external magnetic perturbations created by a set of saddle coils in tokamak plasmas are studied in a vacuum approximation. It is found that the poloidal mode spectra with high-accuracy are described by a linear combination of three universal asymptotical formulas which depend only on the safety factor of the equilibrium plasma and the geometry of perturbation coils. The validity of these formulas is confirmed by numerical calculations of the mode spectra in the DIII-D plasma [G. L. Jackson, Europhys. Conf. Abstr. 27A, P-4.47 (2007)] and a spherical tokamak, National Spherical Tokamak Experiment [D. A. Gates, Nucl. Fusion 49, 104016 (2009)], plasmas perturbed by external control coils. © 2011 American Institute of Physics.