The Behavior of Plasma Impurities and Radiation Losses During LHI and their Effect on Helicity Dissipation

The new Pegasus-III experiment provides a dedicated US facility to investigate solenoid-free tokamak initiation such as local helicity injection (LHI). LHI utilizes high-power electrodes localized in the plasma edge to inject magnetic helicity and drive a plasma current. The efficiency of LHI directly depends on the amount of plasma impurities because they enhance helicity dissipation via increased resistivity and radiative cooling. Thus, it is necessary to identify and limit the mechanism by which impurities are sourced into the plasma. A SPRED spectrometer records time-evolving spectra in the vacuum ultraviolet (VUV) range and gives information about impurity species. The observed contaminants likely result from wall interactions that release the titanium used for gettering and residual trapped gases like N and O. A tangential 32-channel AXUV photodiode array provides measurements for estimating the radiation losses with good spatial and temporal resolution. Radiated power is approximated to be ~ 200 kW, comparable to the available input power I_pV_LHI from helicity injection, suggesting a radiation-dominated regime. These measurements are used to constrain impurity transport modeling and quantify the effects of helicity dissipation.