A test system for optimizing quantum efficiency and dark lifetime of GaAs photocathodes

Abstract

Gallium-arsenide-based photocathodes are used for a variety of applications, including electron-beam production for particle accelerators. An ultra-high vacuum teststand has been developed for cleaning and preparation of photocathodes. First studies using the apparatus were carried out investigating quantum efficiency and vacuum (dark) lifetimes of GaAs photcathodes that were prepared using different procedures involving Cs, Li, and O 2 . While the two-stage activation procedure with Cs and O 2 yielded the highest quantum efficiency of up to (25.9 ± 1.3) % at wavelength 405 nm, the vacuum lifetime of cathodes activated with Cs, Li, and O 2 using a two-stage process exceeded those obtained in single-stage process by more than a factor of three.

Lifetime Measurements of GaAs photocathodes at the Upgraded Injector Test Facility at Jefferson Lab*

“…This should be considered when measuring η in the activation chamber and could be the cause of the apparently lower η for Li-enhanced activation that has been reported in ref. [12]. Also, the difference between η 0 for activation 1 and activation 2 is significantly smaller compared to the same difference between corresponding η max .…”

“…Quality and composure of this layer determines the two main photo-cathode parameters: quantum efficiency η and lifetime τ, where η characterizes the electron emission efficiency, i.e. how many electrons are emitted per incident photon, and is given by emitted photo-current I p , incident laser power P L and wavelength λ as [12]…”

“…It has been reported that introducing the alkali metal lithium (Li) into the activation process with Cs and NF 3 (GaAs:(Cs, Li, NF 3 )) can enhance the surface layer quality, hence increasing both η and τ d compared to the "standard" GaAs:(Cs, NF 3 ) activation [16,17]. Recent studies at Photo-CATCH on GaAs:(Cs, Li, O 2 ) yielded increased values for τ d , but did not report an increase in η compared to the GaAs:(Cs, O 2 ) process [12]. Recent collaborative efforts of our groups at JLab and TU Darmstadt have focused on investigations of τ c for both GaAs:(Cs, Li, NF 3 ) and GaAs:(Cs, Li, O 2 ), since the performance of such cathodes have not yet been evaluated during gun operation.…”

“…Comparably, albeit at a smaller scale, a dedicated test stand for DC photo-gun research and development, Photo-CATCH, with a 60 kV photo-gun and 60 keV beamline has been constructed at the Institut für Kernphysik of the Technische Universität Darmstadt. It provides an environment for research and development to optimize the existing DC photo-gun, which is located at the Spin-Polarized Injector SPIn of the in-house electron accelerator S-DALINAC [11][12][13]. A collaboration between the two institutes has been established in order to effectively capitalize on our joint area of research.…”

Cryogenic GaAs cathode development for improved lifetime

“…3), which will be used to measure the achievable temperature and cooling duration in near future. A test stand (Photo-CATCH) with a cleaning and activation chamber, as well as a short beamline with diagnostic instruments is available in Darmstadt [8] for further testing and setup of the final source.…”