Assessment of magnetic smearing effects for the MAST-U beam emission spectroscopy system

The MAST-U beam emission spectroscopy (BES) diagnostic measures D${}_{\alpha }$ emission from the on-axis neutral deuterium heating beam. From the measured light intensity variations, the local density fluctuations can be characterized, which is a valuable method to study both core and edge plasma turbulence, as well as various MHD phenomena. The predecessor 2D turbulence imaging BES diagnostic system was installed on MAST in 2010, with the first mirror location optimized so that measurements had the best achievable poloidal and radial resolution at a specific observation location along the beam. This system was operational in the M8 and M9 campaigns and was successfully used in several physics programs. In the MAST Upgrade, the BES system was relocated due to technical constraints and installed in a dedicated port. The assessment of the new configuration’s spatial resolution was required so that the measurements could be interpreted correctly and consistently. The spatial resolution assessment was performed for multiple observation locations covering the minor radius of the device. Our study concentrated on the dominant smearing caused by the misalignment between the lines of sight and the magnetic field lines within the beam geometry. For comparison, the same analysis was also performed on pre-upgrade scenarios. The relocation of the observation system had a slightly negative effect on the poloidal resolution due to an increase in the misalignment between the lines of sight and the magnetic field lines at the beam location. However, the values in general remain in the region of the old setup, posing no obstacle to the continued utilization of the system for density fluctuation measurements.