Calculation of spatial response of 2D beam emission spectroscopy diagnostic on MAST

Abstract

The beam emission spectroscopy (BES) turbulence diagnostic on MAST is to be upgraded in June 2010 from a one-dimensional trial system to a two-dimensional imaging system (8 radial X 4 poloidal channels) based on a newly developed avalanche photodiode array camera. The spatial resolution of the new system is calculated in terms of the point spread function to account for the effects of field-line curvature, observation geometry, the finite lifetime of the excited state of the beam atoms, and beam attenuation and divergence. It is found that the radial spatial resolution is similar to 2-3 cm and the poloidal spatial resolution similar to 1-5 cm depending on the radial viewing location. The absolute number of detected photons is also calculated, hence the photon noise level can be determined. [doi:10.1063/1.3479037]