Density measurements in the edge, divertor and X-point regions of Alcator C-Mod from Balmer series emission

Abstract

The electron density in the edge, divertor and X-point regions of Alcator C-Mod [Proceedings of the IEEE 13th Symposium on Fusion Engineering (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] has been measured using the Stark broadening of high-n (n=8 through 11) transitions of the Balmer series of deuterium. These measurements have been made during typical single null, diverted operation and during gas-puffing through capillaries located within the first wall and divertor. Electron densities up to 1.6×1021 m−3 have been measured in the X-point region and slightly lower densities (5−7×1020 m−3) have been measured in the divertor and edge regions. These results are factors of 2 to 5 larger than the density measurements from the CO2 laser interferometer system in the main chamber (3×1020 m−3) and comparable with probe measurements in the divertor. The ratios of the widths of adjacent lines within the Balmer series have been compared with ratios calculated for the Stark broadening of these lines. These transitions can be used to measure electron densities from 3×1019 to 2×1021 m−3 with a modest resolution spectrograph [(Δλinst=1 Å full width at half maximum)].