• N.A. Moody, D.W. Feldman, P.G. O'Shea
  IREAP, College Park, Maryland
• A. Balter
  ,
• K. Jensen
  NRL, Washington, DC

Funding: We gratefully acknowledge our funding agencies, Joint Technology Office (JTO) and the Office of Naval Research (ONR).

Photoinjector performance is a limiting factor in the continued development of high powered FELs. Presently available photocathodes have limited efficiency and short lifetime in an RF-gun environment, due to contamination or evaporation of a photosensitive surface layer. An ideal photocathode should have high efficiency at visible wavelengths, long lifetime in practical vacuum environments, and prompt emission. High efficiency cathodes typically have limited lifetime, and the needs of the photocathode are generally at odds with those of the drive laser. A potential solution is the low work function dispenser cathode, where short lifetimes are overcome by periodic in situ regeneration that restores the photosensitive surface layer, analogous to methods used in the power tube industry. This work reports on the fabrication techniques and performance of cesiated metal photocathodes and cesiated dispenser cathodes, with a focus on understanding and improving quantum efficiency and lifetime, analyzing issues of emission uniformity, and optimizing the activation procedure needed to rejuvenate the cathode. The efficiency versus coverage behavior of cesiated metals is discussed and closely matches that predicted by recent theory.

• K. Jensen
  NRL, Washington, DC
• A. Balter
  ,
• D.W. Feldman, N.A. Moody, P.G. O'Shea
  IREAP, College Park, Maryland

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

Photocathodes for FELs and accelerators will benefit from rugged and self-rejuvenating photocathodes with high QE at the longest possible wavelength. The needs of a high power FEL are not met at present by existing photocathode-drive laser combinations: requirements generally necessitate barrier-lowering coatings which are degraded by operation. We seek to develop a controlled porosity dispenser cathode, and shall report on our coordinated experimental and theoretical studies. Our models account for field, thermal, and surface effects of cesium monolayers on photoemission, and compare well with concurrent experiments examining the QE, patchiness, and evolution of the coatings. Field enhancement, thermal variation of specific heat and electron relaxation rates and their relation to high laser intensity and/or short pulse-to-pulse separation, variations in work function effects due to coating non-uniformity, and the dependence on the wavelength of the incident light are included. The status of methods by which the theory can be extended to semiconductor photocathodes and efforts to provide emission models to beam simulation codes is also treated.