Study of Impurities in the \textsc{Pegasus} Toroidal~Experiment

In the \textsc{Pegasus} experiment, three distinct operating regimes are currently under study: 1) ohmic heating; 2) helicity injection from local arc guns in the plasma edge; 3) and helicity-driven startup followed by ohmic drive. The levels and roles of impurities are significant for each regime. They are of particular importance in helicity-driven plasmas due to the presence of local plasma gun injectors in the scrape-off region. Plasma impurities are monitored by: a 32 channel AXUV bolometer diode array to determine $P_{RAD} $; a visible bremsstrahlung spectrometer to determine $\left\langle {Z_{eff} } \right\rangle $; and a SPRED VUV spectrometer to identify the impurity species. Ohmic plasmas show very low radiated power, with VUV spectra dominated by O emission. Helicity driven plasmas show strong N and O emissions, which presumably arise from interaction with the guns' BN shields. Determination of absolute impurity concentration is in progress. Qualitatively, it is noted that plasmas with handoff from helicity to ohmic drive do not show radiative collapse, despite $P_{RAD} $ levels comparable to that of a purely helicity-driven plasma.