Large area avalanche photodiode detector array upgrade for a ruby-laser Thomson scattering system

A low-cost upgrade has been implemented on the Madison Symmetric Torus (MST) ruby-laser Thomson scattering (TS) system to increase spectral coverage and substantially improve the signal-to-noise ratio (SNR). The spectral resolution has been increased from five channels on the long-wavelength side to an 11-channel system, which covers both sides of the laser line. Coverage on both sides of the spectrum allows for more accurate detection of subtle changes in the distribution function, particularly relativistic spectral shifts of high-temperature plasmas during auxiliary current drive. The previous microchannel plate (MCP) detector was replaced with an array of modular large area avalanche photodiode detectors, which have approximately 16 times the quantum efficiency of the MCP detector. Scattered light collection has also been upgraded, allowing the radial viewing location of the TS system to be easily changed between plasma discharges. Improved SNR and upgraded light collection hardware in the Thomson scattering system have facilitated first-time measurements of the evolution of the electron temperature profile in the MST under a variety of discharge conditions, leading to increased understanding of the underlying dynamics of reversed-field pinch plasmas.