Calculation of spatial response of 2D BES diagnostic on MAST

The Beam Emission Spectroscopy (BES) turbulence diagnostic on MAST is to be upgraded in June 2010 from a 1D trial system to a 2D imaging system (8 radial \times 4 poloidal channels) based on a newly developed APD array camera. The spatial resolution of the new system is calculated in terms of the point spread function (PSF) 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 ~ 2-3 cm and the poloidal spatial resolution ~ 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