Software for Modeling and Design of Robust GaAs Photocathodes

The successful operation of the Department of Energy X-ray light sources, free electron laser, and linear accelerator facilities depends on providing reliable photocathodes for generation of low emittance, high-brightness electron beams using conventional lasers. Experiments on GaAs-based photocathodes have demonstrated the potential to generate high-brightness, unpolarized and polarized, electron beams with low emittance. However, high-fidelity simulation capabilities are needed to design and engineer robust GaAs photocathodes that meet or exceed the desired operational parameters for research facilities run by the Department of Energy. Three dimensional, particle-in-cell codes can accurately simulate the evolution of electron beams in accelerator cavities but lack algorithms for modeling the physics of electron emission from photocathodes. We will develop and implement software code for modeling electron emission from GaAs photo cathodes within the VORPAL Computational Framework to enable simulations of the complete electron generation process including creation of electron-hole pairs due to photon absorption, transport of charge carriers, and emission from negative electron affinity surfaces. We investigated and implemented proof-of-concept algorithms for photon absorption distributions and a three band model for electron transport in GaAs. Results from VORPAL transport simulations with the prototyped algorithms are in agreement with existing experimental and numerical data. These results demonstrate the importance of the new capabilities for high-fidelity modeling of GaAs photocathodes. We will implement accurate modeling of electron emission, surface band banding potentials, effects of surface roughness, localized states, and varying electron affinity to enable detailed investigation of electron emission from GaAs via simulations. We have designed experiments to collect data for validation of the implemented models. We will run extensive simulations to compare with experimental data and investigate time response, emittance, and energy distribution properties of electrons emitted from GaAs. Commercial Applications and Other Benefits: The proposed code for high-fidelity simulations of GaAs photocathodes will directly benefit scientists designing electron sources for industrial and modern research facility applications. The new simulation capabilities will also allow the existing code VORPAL (already commercially distributed) to increase its potential for generating further commercial revenue when used in the industry to design photocathodes and semiconductor devices.