2D Spatial Structures of Fishbone instabilities inferred from Beam Emission Spectroscopy in the Mega Amp Spherical Tokamak Upgrade

Structures of fast particle-driven internal kink modes (“fishbones”, FBs) are obtained using beam emission spectroscopy (BES) in the Mega Amp Spherical Tokamak Upgrade (MAST-U). FBs are sometimes observed to cause redistribution and loss of beam-injected ions in tokamaks. These processes reduce the effectiveness of beam heating and current drive, and the losses pose a threat to plasma-facing components. The contribution of FBs to plasma fluctuations is isolated using cross-correlation analysis of measurements from Mirnov coils and a BES diagnostic. Preliminary analysis of BES data shows that the radial structures of FBs are peaked at or near the magnetic axis as they were in MAST. The upgraded BES with a larger 2D poloidal view enabled measurements of poloidal wavenumbers (k_??) and the poloidal spatial scales of the FBs. These measurements can be used for validation of nonlinear theories of FBs in the future. The work also shows that 2D BES could be an alternative to 2D electron cyclotron emission imaging (ECEI) for measuring 2D spatial structures of core instabilities in tokamaks with low magnetic fields that prohibit the implementation of ECEI diagnostics.