A Moments-Model of Photoemission
and Its Application to Characterization and Beam Optics Simulations

Photocathodes are a critical component of linear beam devices and particle accelerators for directed energy, next generation x-ray FEL’s and light sources. Models of photoemission that can be adapted for characterization but also for inclusion in beam optics codes for designing injectors in a manner that is not computationally burdensome are therefore of interest, in addition to their utility in addressing questions of fundamental interest related to tunneling/transport behavior, photocathode yield, emittance, emission promptness and beam characteristics dependent upon material and environmental parameters. Moreover, it is expedient to have such models operate in tandem with those describing thermal-field emission. Complications that attend real sources are then progressively added, such as surface roughness, field enhancement, thermal-field effects, emission delay, resonant tunneling, and optical properties. In the present work, we shall: (i) show how the canonical thermal, field, and photoemission equations (Richardson, Fowler-Nordheim and Fowler-Dubridge, respectively) follow from the Moments Model framework (akin to the Three Step model of Spicer), (ii) show how findings from Density Functional Theory (DFT), exact tunneling/transmission evaluations, materials physics, and surface coating models can be incorporated in the development of a ”next generation” Moments model with respect to coatings, barriers, optical constants, and dark current, (iii) connect the aforementioned physics with features of concern to beam op- tics simulations, in particular, emittance and launch velocity, delayed emission, surface roughness, shielding, and methods to characterize work function and field enhancement factors associated with surface structure.