Modeling Electron Emission and Surface Effects of High Quantum Efficiency Photocathodes

Successful operation of the Department of Energy (DOE) X-ray light sources, free electron laser (FEL), Ultrafast Electron Diffraction (UED), and linear accelerator facilities requires photocathodes capable of delivering high quantum efficiency (QE), high-brightness, and ultra-low emittance electron beams. Cryogenic temperature experiments have demonstrated the potential to lower the intrinsic emittance of high QE alkali-antimonide photocathodes operated at emission threshold. However, high laser intensity is required leading to heating of charged carriers and the lattice effectively increasing the emittance. Accurate simulations are needed to understand how heating effects and rough surfaces affects QE, intrinsic emittance, and response time. We propose to develop a new computational tool for high-fidelity simulations of photocathodes designed for high QE and ultra-low emittance applications. Although charge transport in specific high QE semiconductor materials can now be simulated with several codes, they do not provide capabilities for simulation of transient carrier transport and heating effects due to intense laser photoexcitation. We will implement algorithms for accurate modeling of transient dynamics and heating effects to enable high-fidelity simulations of alkali-antimonide photocathodes with general rough surfaces. We have successfully implemented algorithms for time-dependent electron generation due to absorption of laser pulses, representation of rough surfaces, calculation of electric fields on rough material interfaces, and electron emission from rough photocathode surfaces including field enhancement effects. We validated results from our simulations on quantum yield and emittance against experimental data. We showed that realistic models using detailed materials properties were needed to obtain agreement with the experimental data. We will implement realistic models for photoexcitation, transient charge transport, charged carrier and lattice heating effects, electron emission, surface roughness (both physical and chemical), and space-charge effects for alkali-antimonides photocathodes. We will validate the implemented code against experimental data from our collaborators and existing measurements. Commercial applications and other benefits: The proposed new modeling capabilities will aid researchers in developing high QE and ultra-low emittance electron sources that meet or exceed the desired operational parameters for future DOE facilities. This project will produce a unique computational software tool with a state-of-the-art graphical user interface. We expect it will generate commercial revenue as it finds use in the semiconductor device modeling industry.