Error field impact on mode locking and divertor heat flux in NSTX-U

Abstract

Results from the 2016 NSTX-U campaign and the subsequent recovery effort have led to new insights regarding error fields in the NSTX-U experiment in particular, and in spherical tokamak configurations in general. During the experimental campaign, many L-mode discharges were found to be locked from the q = 2 surface outward, indicating the presence of error fields. Extensive metrology and plasma response modeling with IPEC and M3D-C1 indicate that misalignment of the toroidal field coil (TF) center rod, while small, produces the largest resonant error field among the sources considered. The plasma response to the TF error field is shown to depend significantly on the presence of a q = 1 surface, in qualitative agreement with experimental observations. It is found that certain characteristics of the TF error field present new challenges for error field correction. Specifically, the error field spectrum differs significantly from that of coils on the low-field side—such as the NSTX-U resonant magnetic perturbation (RMP) coils—and does not resonate strongly with the dominant kink mode, thus potentially requiring a multi-mode correction. Furthermore, to mitigate heat fluxes using poloidal flux expansion, the pitch angle at the divertor plates must be small (~1°), and must be sustained by inner poloidal field (PF) coils for NSTX-U s divertor geometry. It is shown that uncorrected error fields from these inner PF coils may result in potentially significant local perturbation to the pitch angle without contributing significantly to the core resonant error field components responsible for mode locking. Tolerances for coil alignments in the NSTX-U restart are derived based on both heat flux considerations and core resonant fields independently.