New aspects of error fields from high field side in tokamaks*

Recent error field (EF) studies in NSTX-U and COMPASS are revealing new aspects of magnetic perturbations from the high field side (HFS) in tokamaks, challenging conventional EF correction strategies. First, attempts to correct EFs produced by a misaligned TF coil in NSTX-U were not fully successful, often driving a disruptive m/n=1/1 mode and almost always generating 2/1 locking in the edge. The optimal correction often varied dynamically depending on scenarios despite the statically generated nature of EFs. The controllable HFS EFs in COMPASS are correctable against 2/1 locking but not fully against other degrading effects such as a disruption during L-H transition, even with top and bottom (T/B) coils in addition to low field side (LFS) coils. Response simulations using IPEC, MARS, and M3D-C1 all indicate the complex m/n mode coupling and scenario dependences of HFS EF, as opposed to the single-mode characteristics of LFS EFC. NSTX-U recovery is accordingly preparing various EFC strategies with multi-level physics requirements including locking, NTV, and heat flux mitigation. COMPASS is testing various EFC scenarios for ITER using its unique coil sets, as will be discussed in detail.