Spectroscopic Diagnostics using a Visible Transmission Grating Spectrometer at the Alcator C-Mod Tokamak*

A high throughput, f/\#$\sim$3.5, transmission grating spectrometer for visible light (350-670 nm), is being used to make localized measurements of bulk flow velocities and temperatures of impurity ions and deuterium atoms, in the edge and scrape off layer of C-Mod. The flows and temperatures of the main atoms are investigated by detecting D$_{\beta}$ emission from charge exchange recombined deuterons which is enhanced by providing a local atom source from a gas puff. The CCD detector used with the spectrometer allows a simultaneous measurement from 4 different spatial points with a time resolution $\geq$10 ms/frame. Comparison between our results and the edge flows measured by a Mach probe and those from other spectroscopic diagnostics will be given. A higher time resolution ($\sim$100 $\mu$s/frame) is possible by using a special readout mode of the CCD. This allows time resolved electron density measurements from Stark broadened D$_{\beta}$ during disruptions.