Vacuum Barriers for XHV Operation of DC Electron Guns

The operational lifetime of photocathodes is a critical metric if their application in continuous operation accelerators is viable. Their lifetime is often limited by the vacuum level within the electron gun, due to surface poisoning or through the process of ion back- bombardment. This process is generated by the ionization of residual gas by the electron beam produced in the gun, and these ions are then accelerated by the gun towards the photocathode causing surface degradation. Any improvement of the vacuum level within the gun reduces the residual gas and thus the rate of degradation. Vacuum barriers applied to the vacuum chamber walls could be a method of trapping gas from the chamber walls and flanges, which is the primary source of gas in the XHV (Extreme High Vacuum) range at which photocathode guns operate. The TiN family of coatings has the potential to be effective vacuum barriers and are widely used in industry for their surface hardness and wear properties. There are a number of other benefits that could be advantageous to high voltage DC photo guns. These coatings can be tailored (for example AlTiN) for a black, non-reflective finish, which acts as a light absorber, collecting any scattered stray light from the laser illumination of the photo cathode. This stray light can lead to unwanted halo formation from the cathode. This family of coatings is also extremely hard, which can lead to improved high voltage breakdown properties. In this Phase I project the vacuum outgassing properties will be the primary focus. Coatings will be applied to large surface area test chambers and compared with identical uncoated chambers. Outgassing rates can be measured by a spinning rotor gauge monitoring and compared with other coatings to isolate potential beneficial coatings. Xelera Research LLC, with broad experience in accelerator injector design and construction, particle-free techniques and ultra-high vacuum technology, has the scientific and engineering skills to tackle this challenging R&D project.