9b Bright Electron Source for X-ray Free Electron Lasers

The Basic Energy Sciences Report on Facility Upgrades has identified The Linear Coherent Light Source- II “upgrade” at Stanford Linear Accelerator as “absolutely central to contribute to world-leading science”. This Light Source upgrade will provide X-ray energies up to 20 keV necessary to study fundamental atomic structure dynamics, electronic and nuclear coupling in biological and chemical processes. But to realize this upgrade there is a need to improve the reliability and the performance of the photocathode where the electrons are photoemitted in the accelerator. The currently-grown photocathodes at the Light Source are inadequate to meet the desired upgrade requirements. RMD will address the Light Source upgrade needs for the Stanford Linear Accelerator by reliable manufacturing of cathodes via sputtering technique and packaging them under vacuum for shipment to Stanford. Because the sputter-grown cathodes have a better performance and more robust compared to the conventional cathodes, they will facilitate the necessary upgrade. These cathodes will be protected by graphene top-layers that will extend the cathode lifetime in the accelerator. Also, sputtering allows mass production and thus stockpiling of cathodes, which the Linear Accelerator could purchase and introduce the same into their photo guns in a reliable fashion, when necessary. During Phase I, RMD was able to demonstrate the growth of bialkali antimonide photocathodes by sputtering technique, where multiple cathodes were demonstrated with similar performance specifications. The sputtered cathodes have exceptional surface smoothness and high-efficiency in green wavelength which will facilitate the desired beam brightness. A compatible vacuum packaging for the cathode was also independently demonstrated, which was unsealed at the Cornell beamline successfully in order to produce an electron beam. During the Phase II effort, the various unit operations demonstrated in Phase I will be integrated together to usher the manufacturing process for advanced photocathodes designed for Stanford Linear Accelerator energy upgrade. This effort will entail construction of a Photocathode Manufacturing Tool and optimizing the processes to produce high-efficiency and long-lifetime photocathodes. In a parallel effort, RMD will incorporate monolayer graphene to protect the cathode surface to improve the overall performance. RMD will develop technologies to package the cathode and unpack the same in order to deliver the same into a photo-gun at the Linear Accelerator. The availability of packaged cathodes will streamline the supply for accelerators at various national labs, small industries and Universities. The proposed technology will enable manufacturing of photocathodes for accelerators in the United States. The technology will also enable the realization of cost-competitive new detectors such as the Large Area Picosecond Photo Detectors. Availability of such detectors will impact medical investigations for cancer detection, border security screening, scattering neutron detectors for spallation sources to perform basic sciences, and deep underground neutrino experiments.