Status of the Alcator C-MOD scanning two-dimensional Thomson scattering diagnostic

The detailed study of noncircular tokamak discharges will require measurements of local Te and ne over a two-dimensional region. A novel scanning Thomson scattering system is under construction for use on the Alcator C-Mod device. A multipulse (50 Hz pulse rate) Nd:YAG laser (1 J, 15 nS) will be used to scan the plasma major radius within a 14 cm range during a 160 mS period with a spatial step size of 2 cm. The laser beam sweep, which is limited by the size of the diagnostic ports, will cover ∼50% of the plasma discharge cross section. The scattered laser light spectra at various positions along the vertical laser beam path will be analyzed by a set of up to 11 (initial operation is planned for six) Wadsworth spectrometers. A prototype spectrometer with associated light collection optics has been tested and the total Thomson system optical transmission has been measured to be ∼12–15%. System details and performance measurements will be presented.