Design and Operation of a Frequency Doubled Nd:YAG Thomson Scattering System with Transmission Grating ICCD Spectrometer

A novel Thomson scattering system has been deployed on the Pegasus Toroidal Experiment. It provides a relatively low-cost, simplified design. Scattering is achieved using a 7 ns, 2 J frequency doubled Nd:YAG laser operating at 532 nm. The laser focuses to $\sim $3 mm diameter within the plasma via a 7 m beam-line. The beam-line contains cameras as beam finders and remotely adjustable mirrors for shot-to-shot alignment. A custom multi-element lens collects scattered photons from 15 cm $<$ R$_{maj}<$ 85 cm with 1.2 cm radial resolution. Eight fiber optic bundles provide 8 spatial points for sampling the laser or background light. Each set of 8 channels is measured in a single spectrometer that utilizes a high efficiency ($\sim $80{\%}) volume phase holographic grating and a high quantum efficiency ($>$ 40{\%}) image intensified CCD (ICCD) camera. Three spectrometers provide a total of 24 channels. Two interchangeable gratings exist to cover low (T$_{e}$ = 10--100 eV) and high (T$_{e}$ = 0.10--1 keV) electron temperature regimes on Pegasus. The spectrometer is optimized for n$_{e}$ from mid-10$^{18}$ to mid-10$^{19}$ m$^{-3}$. The signal-to-noise expected is $\sim $0.5 of an equivalent system using Nd:YAG at 1064 nm and avalanche photodiode detectors.