• L. Monaco, P.M. Michelato, C. Pagani, D. Sertore
  INFN/LASA, Segrate (MI)

The production of Cs2Te photoemissive films used as laser driven electron sources in the high brightness photoinjectors at FLASH and PITZ, is a well established activity at INFN Milano since the '90s. Our total production is of more than 100 photocathodes, with an average QE of 8% (@ 254 nm) for fresh films and an operative lifetime that increased up to some months at FLASH. In the last two years, we have improved the standard diagnostic used during the cathode growth to better understand the material properties of the films. This activity is motivated by the need to improve the photocathode properties, mainly the energy distribution of the photoemitted electrons that influences the thermal emittance. The multiwavelengths diagnostic, i.e. the on-line measurements of the photocurrent and reflectivity from the film during its growth in the 239 nm ~ 436 nm range, has been deeply applied on several cathodes and the potentiality of this technique are discussed in this paper.

• S. Lederer, S. Schreiber
  DESY, Hamburg
• P.M. Michelato, L. Monaco, D. Sertore
  INFN/LASA, Segrate (MI)

Caesium telluride photocathodes are used as laser driven electron sources at the Free-Electron-Laser Hamburg, FLASH, and will be used at the European XFEL. One concern of the operation of photocathodes in these user facilities is the degradation of the quantum efficiency during operation. After improving vacuum conditions and removing contaminants, the cathode life time increased from a couple of weeks to several months. In this contribution we report on long time operation of Cs2Te cathodes in terms of QE measurements and investigations on the homogeneity of the electron emission. Another concern of electron guns operated with long RF-pulses (0.8 ms at FLASH) is the generation of dark current either from the cathode or from the gun body. During the last years a constant high amount of dark current, emitted from the gun body itself, was observed at FLASH. Caused by that during the shut-down 2009/2010 the RF-gun at FLASH, operated more than five years, was replaced. The improved dark current situation with the new RF-gun is presented in terms of dark current measurements under different operational conditions.