Charge lifetime measurements at high average current using aK2CsSbphotocathode inside a dc high voltage photogun

R R Mammei,J L Mccarter,S Covert,T Rao,J Grames,D Machie,J Feingold,J Smedley,J Clark,J Hansknecht,M Poelker,J Walsh,M Ruiz-Osés,P A Adderley,R Suleiman

doi:10.1103/physrevstab.16.033401

Abstract

Two ${\mathrm{K}}_{2}\mathrm{CsSb}$ photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{Torr}$. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditions the charge lifetime was poor due to extensive ion bombardment under severely degraded vacuum conditions, and as a result of localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope to determine surface morphology.

Highlights

There are two popular photocathode choices for generating high average current electron beams at accelerators using relatively inexpensive rf-pulsed green-laser light: GaAs and K2CsSb
The quantum efficiency (QE) remained constant for days before extracting beam, suggesting a very long dark lifetime under these vacuum conditions
The photocathode was biased at 100 kV and charge lifetime measurements were performed at different radial locations at 1 mA beam current

Summary

Introduction

There are two popular photocathode choices for generating high average current electron beams at accelerators using relatively inexpensive rf-pulsed green-laser light: GaAs and K2CsSb. The GaAs photocathode can exhibit very high quantum efficiency (QE) and it can produce a beam with small thermal emittance [1], but it is widely recognized to be a very fragile photocathode requiring strict adherence to procedures that maintain cleanliness of the photocathode surface on an atomic scale. The K2CsSb photocathode has a slightly larger thermal emittance [2] compared to GaAs but it is considered to be a prompt emitter because of its positive-electron affinity (PEA) nature, producing shorter bunches than GaAs. The biggest advantage is the photocathode’s ability to survive under markedly harsher vacuum conditions compared to GaAs [3]

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