Impurity Identification and Transport Modeling of Pegasus-III LHI Discharges

Pegasus-III is a spherical tokamak dedicated to studying solenoid-free tokamak plasma initiation and current drive. It’s important to understand the impurity dynamics during this initial ramp-up phase that could limit the performance of the plasma. The Pegasus-III SPRED spectrometer has been upgraded with a higher resolution grating with (2100 g/mm) achieving a spectral resolution of ~0.1 nm for metallic line identification in the ~10–32 nm wavelength range. This enhancement helps identify impurities arising from potential harmful plasma-material interactions (PMI) during local helicity injection (LHI). N, O and Ti are consistently observed in LHI discharges; however, cathode spots and PMI with the injector limiter result in additional spectral lines identified as Mo, likely coming from injector shield rings, and Fe from the limiter. Aurora, an impurity transport code, is being used to model impurity transport and is constrained by the SPRED-identified species and an estimation of radiated power made with a multi-channel AXUV photodiode-based diagnostic. To improve the fidelity of impurity transport modelling, photometrically calibrated filterscopes designed to monitor specific impurity lines are being commissioned.