Fabrication and Measurement of Regenerable Low Workfunction Photocathodes

Abstract

Laser-switched photoemitters are a source of electrons for high current applications such as free electron lasers. Laser-modulated photoemission permits rapid switching of the electron beam, far surpassing what can be achieved using electric-field gated emission. Photoinjector systems consist of a drive laser producing short bunches of photons and an efficient photocathode, which converts photon bunches into electron beam pulses. Development of both technologies is required, but the scope of this project is restricted to improvement of the photocathode. Most high-efficiency photocathodes employ cesium-based surface coatings to reduce work function and enable efficient electron emission in the visible range. Lifetime is severely limited by the loss of this delicate coating, which degrades rapidly in practical vacuum environments. More robust photocathodes exist, but have much lower efficiency, and place unrealistic demands on drive laser power and stability. This research proposes a novel dispenser concept that dramatically extends the lifetime of high efficiency cesium-based cathodes by continuously or periodically restoring the cesium surface monolayer during an in situ rejuvenation process. Sintered tungsten provides an interface between a cesium reservoir and the photoemitting surface. During temperature-controlled rejuvenation, cesium diffuses through and across the sintered tungsten to create and sustain a low-work function photocathode. The prototype dispenser cathode was fabricated and tested for two modes of operation: continuous and periodic near-room temperature rejuvenation. The data are compared with a photoemission model of partially covered surfaces under design for integration with existing beam simulations. Overall performance suggests that this cesium-delivery mechanism can significantly enhance the efficiency and operational lifetime of a wide variety of present and future cesium-based photocathodes. Also reported are surface characterization, ion beam cleaning, and fabrication techniques used to optimize performance of the dispenser photocathode.