The effects of oxygen-induced phase segregation on the interfacial electronic structure and quantum efficiency of Cs3Sb photocathodes.

High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

A cryogenically cooled high voltage DC photoemission electron source.

Linear electron accelerators and their applications such as ultrafast electron diffraction require compact high-brightness electron sources with high voltage and electric field at the photocathode to maximize the electron density and minimize space-charge induced emittance growth. Achieving high brightness from a compact source is a challenging task because it involves an often-conflicting interplay between various requirements imposed by photoemission, acceleration, and beam dynamics. Here we present a new design for a compact high voltage DC electron gun with a novel cryogenic photocathode system and report on its construction and commissioning process. This photoemission gun can operate at ∼200 kV at both room temperature and cryogenic temperature with a corresponding electric field of 10 MV/m, necessary for achieving high quality electron beams without requiring the complexity of guns, e.g., based on RF superconductivity. It hosts a compact photocathode plug compatible with that used in several other laboratories opening the possibility of generating and characterizing electron beam from photocathodes developed at other institutions.

Review and demonstration of ultra-low-emittance photocathode measurements.

This paper reports the development of a simple and reliable apparatus for measuring ultra-low emittance, or equivalently the mean transverse energy from cryogenically cooled photocathodes. The existing methods to measure ultra-low emittance from photocathodes are reviewed. Inspired by the available techniques, we have implemented two complementary methods, the waist scan and voltage scan, in one system giving consistent results. Additionally, this system is capable of measuring the emittance at electric fields comparable to those obtained in DC photoinjectors.

Maximum achievable beam brightness from photoinjectors.

Electron injectors delivering relativistic electron beams with very high brightness are essential for a number of current and proposed electron accelerator applications. These high brightness beams are generally produced from photoemission cathodes. We formulate a limit on the electron beam brightness from such cathodes set by the transverse thermal energy of the electrons leaving the photocathode and the accelerating field at the cathode. Two specific examples--direct measurement of the transverse phase space of a space charge dominated beam from a high-voltage photoemission electron gun and a numerical optimization of the same at a higher gun voltage--illustrate the importance of this limit.