Multichannel Doppler transmission grating spectrometer at the Alcator C-Mod tokamak

Upgrades to an existing transmission grating spectrometer for visible light used for Doppler velocity and temperature measurements at the Alcator C-Mod tokamak are given. These include a new Princeton Instruments Photonmax 512B charge-coupled device and a four channel fiber optic input. These together allow improvements to the signal level, time resolution, and the number of spatial channels. The fiber optic input allows four simultaneous spatial channels each of which offers a larger percentage of input light flux when compared to the standard fiber slit combination. The "on chip" amplification combined with versatile pixel binning further increases the signal to noise ratio allowing a continuous acquisition of spectra every 8 ms. The error bars for extracted velocity and temperature values are potentially smaller owing to the smaller pixel size and increased light flux which allow a more detailed line shape and simplifies line fitting. Examples of time and space resolved spectra are shown and further improvements are discussed.