Initial FIDA results on MAST-U with on and off-axis beam deposition

A tangentially-viewing fast ion D-alpha spectrometer has been re-installed on MAST-U in order to diagnose the spatial and velocity distribution of the high-energy portion of the fast ion distribution [1]. Comparison of the signal to calculations based on the TRANSP/FIDASIM codes can be used to assess fast ion transport models. The influence of fast ion and bulk plasma-driven MHD on the FIDA signals can also be investigated with high time resolution (~.3ms). With two on-axis beams operating in MAST, there was strong mode activity and consequential fast ion transport [2]. However, because one of the beams is shifted off-axis in MAST-U, fast ion gradients are reduced in the core, suppressing the drive for core instabilities in the region of low magnetic shear. This is expected to result in improved fast ion transport and enhanced current drive. To assess this, FIDA data will be compared with classical modelling in cases with both on-axis single beam heating as well as combined on/off-axis beams. Consistency with other fast ion diagnostics will be investigated, in particular with a fission chamber, but also including an NPA, FILD and neutron camera. The operational windows for good FIDA signal to background ratio will also be assessed.