Characterizing the Thomson Scattering System on the Pegasus-III Experiment

The Pegasus-III experiment uses a multi-point Thomson scattering system to observe the electron temperature and density evolution of non-solenoidal startup plasmas. As part of the upgrade to Pegasus-III, detailed characterization and calibration of the Thomson system was performed to improve the measurement quality and ease-of-use of the diagnostic. Specifically, a series of photometric calibrations were used to quantify the gain and noise enhancement as a function of voltage for the intensifiers used on the CCD cameras. Additionally, improvements were made to the spectrometer's fast shutter design to reduce the shutters' open time from 10 ms to near 2 ms, depending on the components used. One near-term use for the system is to examine the temperature evolution of the coaxial helicity injection (CHI) plasmas. CHI is a form of non-solenoid startup using current driven on open field lines connecting two co-axial, electrically isolated divertor plates to create the plasma. CHI modeling indicates that the electron temperature plays a key role in plasma current evolution and closed flux current generated. Thus, Thomson measurements are critical to understanding CHI dynamics and comparing to these models.