Modeling Electron Emission and Surface Effects of High Quantum Efficiency Photocathodes

Statement of the problem or situation that is being addressed The successful operation of the Department of Energy (DOE) X-ray light sources, free elec- tron laser (FEL), and linear accelerator facilities depends on developing advanced photoinjectors. These require photocathodes capable to deliver high quantum e ciency (QE), high-brightness, low emittance, high-average current electron beams. Measurements have demonstrated the potential to generate electron beams with the high repetition rate and QE required for FEL and next gener- ation light source applications, however, high- delity simulation capabilities are needed to enable detailed understanding of the physics of electron emission from rough surfaces that a ect QE dy- namics, response time, and dark current properties of photocathodes. We propose to develop new software to address these issues in order to provide simulation tools for the design of photocathodes that meet or exceed the desired operational parameters for next generation DOE facilities. General statement of how this problem or situation is being addressed Although charge transport in GaAs photocathodes can now be simulated with the three di- mensional (3D) computational physics kernel Vorpal, this code currently lacks algorithms for the high- delity modeling of time-dependent electron generation, the representation of rough cathode- vacuum interfaces, and electron emission from rough surfaces in an applied eld. We will develop and implement algorithms for accurate modeling of electron emission from rough photocathode surfaces with negative electron a nity, surface physics phenomena related to electric eld enhance- ment and varying electron a nity within Vorpal to enable realistic 3D modeling of time-dependent electron emission and dark current e ects. What is to be done in Phase I? We will investigate and prototype proof-of-concept algorithms for time-dependent electron gen- eration due to absorption of laser pulses, representation of rough photocathode surfaces, calculation of electric elds on photocathode-vacuum interfaces with di erent pro les, and electron emission that takes into account eld enhancement, space-charge, and varying electron a nity e ects on rough surfaces. Commercial applications and other bene ts The proposed new modeling capabilities will aid researchers in developing electron sources that meet or exceed the desired operational parameters for future DOE facilities. This project will produce commercial quality software, including a state-of-the-art graphical user interface, and will increase Vorpal's capability to generate further commercial revenue as it nds use in the industrial design of reliable and durable high quantum e ciency photocathodes. In addition, Field Emitter Array manufacturers can create increasingly accurate models with the developed tools. Key words high quantum e ciency photocathode, electron emission, semiconductor, modeling, simulations Summary for Members of Congress Novel electron sources combining low emittance with high quantum e ciency, current, and brightness are required for next generation free electron lasers and X-ray light source facilities as new energy-e cient technologies are explored by the Department of Energy. We will develop high- delity software capable of designing the required high-quantum e ciency photocathodes.