Relationship Between Cathode Spot Formation and Injector Properties During Local Helicity Injection on the Pegasus-III Experiment

Local Helicity Injection (LHI) is a plasma startup method employed on Pegasus-III that utilizes localized arc plasma sources that are electrically biased to extract an electron beam into the main vessel parallel to the vacuum magnetic field. Metallic injector electrode surfaces can become the source of electron current rather than the arc plasma, leading to cathode spots that can damage the electrodes, decrease injector lifetime, and reduce LHI performance. Occurrence of these spots can be minimized by conditioning the electrode surfaces. Scanning electron microscopy (SEM) and optical profilometry are used to track the evolution of injector surface properties as they are conditioned. SEM images of used injector components show surface damage typical of materials with a history of cathode spots and non-conductive oxide particles, confirmed by energy-dispersive X-ray spectroscopy (EDS). The density of craters suggests a non-uniform spatial distribution of cathode spot-induced material degradation. Further work involves imaging the cross-section of previously installed samples and comparing to that of pristine samples. Optical profilometry will obtain a profile of surface roughness with changing voltage.