Progress on Thomson scattering in the Pegasus Toroidal Experiment

Abstract

A novel Thomson scattering system has been implemented onthe Pegasus Toroidal Experiment where typical densities of 1019 m−3 and electron temperatures of 10 to 500 eV are expected. The systemleverages technological advances in high-energy pulsed lasers, volume phaseholographic (VPH) diffraction gratings, and gated image intensified (ICCD)cameras to provide a relatively low-maintenance, economical, robustdiagnostic system. Scattering is induced by a frequency-doubled, Q-switchedNd:YAG laser (2 J at 532 nm, 7 ns FWHM pulse) directed to the plasma over a7.7 m long beam path, and focused to < 3 mm throughout the collectionregion. Inter-shot beam alignment is adjustable with less than a 0.01 mmspatial resolution in the collection region. A custom lens system collectsscattered photons at radii 15 cm to 85 cm from the machine's center, at ∼ F/6 with 14 mm radial resolution. The initial configuration providesscattering measurements at 12 spatial locations and 12 simultaneousbackground measurements at adjacent locations. If plasma backgroundsubtraction proves to be insignificant, these background channels will beused as viewing channels. Each spectrometer supports 8 spatial channels andcan provide 8 or more spectral bins each. The spectrometers usehigh-efficiency VPH transmission gratings (eff. > 80%) and fast-gatedICCDs (gate > 2 ns, Gen III intensifier) with high-throughput (F/1.8),achromatic lensing. A stray light mitigation facility has been implemented,consisting of a multi-aperture optical baffle system and a simple beam dump.Successful stray light reduction has enabled detection of scattered signal,and Rayleigh scattering has been used to provide a relative calibration.Initial temperature measurements have been made and data analysis algorithmsare under development.