Tearing Mode Locking Avoidance by Applying Rotating External 3D Field

Recent cylindrical non-linear reduced MHD simulation [S. Inoue (PPCF 2018)] has demonstrated a possibility of “shielding-out” an error field by applying a rotating 3D field, weakening significantly a single-helicity 2/1 locked mode (LM). DIII-D experiments support this proposal. The applied n=1 fields include several poloidal components, even for m/n=2/1 LM avoidance and produce a shielding-like response at q≥3 and weak response inside of q<3. This suggests the 1/1 and 2/1 resonant components are shielded-out. 3D feedback for LM avoidance produces an edge tearing resonant magnetic perturbation (RMP) in such a way that the edge RMP appearance coincides in time with disappearance of the interior RMP, again suggesting a shielding effect. The relative dependence of the tearing and shielded response on the frequency of the applied field and other physical parameters will be discussed in a qualitative manner with the M3DC1 code, using both one- and two-fluid models.