Characterization of Neutral Fueling During LHI in the Pegasus-III Experiment

Pegasus-III relies on non-solenoidal startup via helicity injection. Local helicity injection (LHI) creates a localized arc plasma that is biased against the wall of the vessel, allowing electron beam extraction. The bias voltage of several hundred volts required to achieve successful LHI operation can lead to cathode spots on injector electrode surfaces. The existence of these spots can be minimalized by slowly conditioning the electrode surfaces to the voltages required. At fueling rates required to sustain desired plasma, suspected charge exchange neutrals may escape the injectors and enter the edge plasma. Uncontrolled fueling is incompatible with an integrated core/edge operation scenario and can have variable effects on confinement properties. When scaling LHI, more gas flow will be needed, making understanding neutral leakage necessary. Experiments will be conducted to characterize neutral leakage from LHI injectors, including changing the arc fuel gas from deuterium to argon. Studies will be done using a fast ion gauge and filterscopes to measure neutral pressure and density, respectively. The density and temperature of the arc will be studied by measuring stark broadening of spectral lines.