"Black Gun" Technologies for DC Photoinjectors

DC photoinjectors relate to several current and future accelerators, such as the JLab CEBAF and the Electron-Ion Collider. However, stray light in the gun causes emission from off-nominal photoemissive regions, initiating electron trajectories which intercept downstream surfaces. This causes electron-stimulated desorption of atoms, which ionize and back- bombard the cathode, reducing its charge lifetime. (Such issues are separate from well-known "dark current" effects: field emission currents independent of illumination.) This work is important for high average current, high rep rate designs where shorter charge lifetimes can be quickly exceeded. Phase I will develop the technologies necessary to create a true "black gun" DC photoinjector. Our goal is to reduce the scattered drive laser light in a DC photoinjector, and by extension the unwanted photoemission, by an order of magnitude. This in turn will reduce stray beam, ion back-bombardment, x-ray generation, and radioactivation throughout, and enable significantly higher average current operation. In Phase I a highly incremental program will be performed. First we will model and fabricate optical elements of a Black Gun - Butler baffles, Brewster windows, and other low-scattering elements. Second, we will develop several candidate surface treatments for a Black Gun: anodization, femtosecond laser blackening, and/or resonant nanostructure- enhanced absorption. Vacuum compatibility and optical constants will be tested. A gun mock-up will be constructed to demonstrate, as we expect based on literature, that stray light can be reduced by one to two orders of magnitude.. The combination of methods minimizing all but the essential drive laser light is what makes this proposal the first true "black gun." At the end of Phase I, we will quantitatively conclude which "black gun" technologies have the most promise for incorporation into photoinjector testing in Phase II of the project. The potential market for high QE, ion-damage-tolerant GaAs photocathodes is significant. Spin-polarized photocathodes are essential at proposed electron-ion collider facilities such as JLEIC and eRHIC, and advancements would also affect existing facilities such as JLab's CEBAF. In addition, high QE and long-lived photocathodes are of broader interest including SRF facilities such as SLAC's LCLS-II, Fermilab's IARC, and RF linac based FELs for industrial applications. Spin-polarized transmission electron microscopy will also benefit from advanced spin-polarized photocathodes.