Evolution of the transverse and longitudinal energy distributions of electrons emitted from a GaAsP photocathode as a function of its degradation state

L B Jones,T C Q Noakes,D V Gorshkov,B L Militsyn,H E Scheibler,A S Terekhov

doi:10.1063/1.4984603

Abstract

We present measurements of the transverse and longitudinal energy spread of photoelectrons emitted from a GaAsP photocathode as a function of its degradation state. The cathode was initially activated to a state of negative electron affinity in our photocathode preparation facility, achieving a quantum efficiency of 3% at a wavelength of 532 nm. It was then transferred under XHV conditions to our transverse energy spread spectrometer, where energy spread measurements were made while the photocathode was progressively degraded through a controlled exposure to oxygen. Data have been collected under photocathode illumination at 532 nm, and the changing photoelectron energy distribution associated with the changes in the level of electron affinity due to quantum efficiency degradation through an exposure to 0.25 L of oxygen has been demonstrated. Our experiments have shown that GaAsP boasts a significantly higher resilience to degradation under exposure to oxygen than a GaAs photocathode, though it does exhibit a higher level of mean transverse energy. Coupled with the favourable published data on GaAsP photoemission response times, we conclude that GaAsP is a viable candidate material as a particle accelerator electron source.

Highlights

The evolution of particle accelerator technology has created a well-documented requirement for high-brightness electron sources coupled to linear and energy recovery accelerators to effectively drive 4th generation free-electron laser (FEL) light sources.1,2Electron beam brightness in an accelerator is fundamentally limited by injector brightness, and this is itself limited by the source beam emittance or the intrinsic emittance of the photocathode source
Our experiments have shown that GaAsP boasts a significantly higher resilience to degradation under exposure to oxygen than a GaAs photocathode, though it does exhibit a higher level of mean transverse energy
We found that the left edge of Longitudinal energy distribution curve (LEDC) lies at the same voltage and has the same shape, implying that both photocathodes had the same workfunction and confirming that we had achieved a good level of negative electron affinity (NEA) in our activated GaAsP photocathode

Summary

Introduction

The evolution of particle accelerator technology has created a well-documented requirement for high-brightness electron sources coupled to linear and energy recovery accelerators to effectively drive 4th generation free-electron laser (FEL) light sources.1,2Electron beam brightness in an accelerator is fundamentally limited by injector brightness, and this is itself limited by the source beam emittance or the intrinsic emittance of the photocathode source. The operational lifetime of photocathodes used as particle accelerator electron sources is of significant importance as any periodic work required to maintain the photocathode in an operational state constitutes an interruption to accelerator operations. This fact is relevant to III-V semiconductor photocathodes activated to a negative electron affinity (NEA) state, such as GaAs7–13 and GaAsP, which are candidate electron sources for high-current energy recovery linacs, high-repetition rate FELs, and high-energy physics applications using strained-layer superlattices to generate highly spin-polarised electron beams.. Significant work has been published documenting the effects of such contaminants on GaAs, and this shows that an exposure of just 0.025 L O2 is sufficient to reduce QE by one order of magnitude, necessitating the heat cleaning of the photocathode and reactivating to the NEA state

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