• K. L. Jensen, J.L. Shaw, J.E. Yater
  NRL, Washington, DC
• D.W. Feldman, E.J. Montgomery, P.G. O'Shea, P.Z. Pan
  UMD, College Park, Maryland
• N.A. Moody
  LANL, Los Alamos, New Mexico
• J.J. Petillo
  SAIC, Burlington, Massachusetts

Funding: We gratefully acknowledge funding provided by the Joint Technology Office and the Office of Naval Research

Laser switched photocathodes are now the electron source of choice for short wavelength Free Electron Lasers. The photocathode requirements are profound: ideally, capabilities such as high peak and average current, high quantum efficiency (QE) in the visible, long lifetime in an rf injector and the ability to be repaired in situ are desired. We are pursuing cathodes with self-rejuvenating surfaces based on cesium dispenser cathode technology*,**, in which the physics of recesiation, evaporation, diffusion, and evolution of the surface coating and the QE are the metrics of performance. Here, we present predictive theoretical models of surface evolution and QE in a manner appropriate for inclusion in beam simulation codes, wherein emission non-uniformity and dark current affect emittance, beam halo, and dynamic evolution of bunched electron beams***. The emission models focus on bulk transport issues (including scattering processes) and surface conditions (including diffusion in the presence of random, non-uniform sub-monolayer coverage), and relate these factors to recent experimental characterizations of the surface evolution.

*Jensen, et al., JAP{10}2, 074902 ; Moody, et al., APL90, 114108.
**E. Montgomery, et al., (this conference)
***Petillo, et al., Proc IEEE PAC (2007); Jensen, et al., PRST-AB 11, 081001.

• E.J. Montgomery, D.W. Feldman, P.G. O'Shea, P.Z. Pan
  UMD, College Park, Maryland
• K. L. Jensen
  NRL, Washington, DC
• N.A. Moody
  LANL, Los Alamos, New Mexico

Funding: This project is funded by the Joint Technology Office and the Office of Naval Research.

High performance FELs require photocathodes with quantum efficiencies of several percent at green wavelengths, kHr lifetime, kA/cm² peak and A/cm² average current, and ps response. Such cathodes are challenged to maintain requisite high quantum efficiency while in harsh accelerator vacuum conditions. Delicate surface coatings are often cesium-based, and therefore are reactive with contaminant gases. The dispenser photocathode architecture resupplies the cesium coating from a subsurface reservoir through a porous substrate, thereby extending lifetime*. Recesiation has been shown to rejuvenate Cs:Ag cathodes from O2, CO2, and N2O contamination**, and theory of dispenser photocathodes is advancing***. We here investigate the fabrication, contamination, and external recesiation of alkali antimonides with high quantum efficiency, in support of the dispenser photocathode design.

*Moody et al., APL90, 114108.
**Montgomery et al., Proc. AACW 2008 (submitted for publication).
***K. Jensen et al., (this conference).