Calculation of spatial response of 2D BES diagnostic on MAST

TL;DR

This paper calculates the spatial response of a 2D Beam Emission Spectroscopy diagnostic on MAST, considering various physical effects, to determine its resolution and photon noise levels for improved turbulence measurement.

Contribution

It provides a detailed calculation of the spatial resolution and photon detection characteristics of the upgraded 2D BES system on MAST, incorporating effects like field-line curvature and beam attenuation.

Findings

Radial spatial resolution ~ 2-3 cm

Poloidal spatial resolution ~ 1-5 cm

Photon noise levels can be estimated from detected photon counts

Abstract

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