New Understanding of Tokamak Plasma Response to 3D Magnetic Fields

The performance of both present and future tokamaks such as ITER can be greatly degraded or enhanced by small 3D magnetic perturbations. An important new tool for understanding 3D magnetic field effects in tokamaks is the Ideal Perturbed Equilibrium Code (IPEC), which computes 3D perturbed tokamak equilibria including plasma response effects such as poloidal harmonic coupling, shielding, and amplification. IPEC predictions have been verified by recent error field correction results on NSTX and DIII-D, where the strong plasma response effects are essential to explain observed Locked Mode (LM) behavior. Also, a generalized Neoclassical Toroidal Viscosity (NTV) theory in tokamaks indicates that the variation of field strength predicted by IPEC, including effects of the deformed flux surfaces, is consistent with the experimentally measured rotational damping rates. Based on these results, IPEC has been used to characterize Resonant Magnetic Perturbation (RMP) for Edge Localized Mode (ELM) suppression, and to optimize ITER RMP field. The initial results show that the optimization of three rows of coils (two off-midplane rows and one midplane row) can give a great benefit to RMP by minimizing core degradation toward a theoretical minimum level.