Temperature-dependent quantum efficiency degradation of K-Cs-Sb bialkali antimonide photocathodes grown by a triple-element codeposition method

K-Cs-Sb bialkali antimonide photocathodes grown by a triple-element codeposition method have been found to have excellent quantum efficiency (QE) and outstanding near-atomic surface smoothness and have been employed in the VHF gun in the Advanced Photoinjector Experiment (APEX), however, their robustness in terms of their lifetime at elevated photocathode temperature has not yet been investigated. In this paper, the relationship between the lifetime of the K-Cs-Sb photocathode and the photocathode temperature has been investigated. The origin of the significant QE degradation at photocathode temperatures over $70°\mathrm{C}$ has been identified as the loss of cesium atoms from the K-Cs-Sb photocathode, based on the in situ x-ray analysis on the photocathode film during the decay process. The findings from this work will not only further the understanding of the behavior of K-Cs-Sb photocathodes at elevated temperature and help develop more temperature-robust cathodes, but also will become an important guide to the design and operation of the future high-field rf guns employing the use of such photocathodes.

Figure 1

Cross section of the APEX VHF gun with its main components in evidence. The inset shows the detail of the cathode plug area.

Figure 2

Left: Detail of the fine mesh used for the FEA analysis. Right: FEA analysis results.

Figure 3

(a) Decay curves of a series of K-Cs-Sb photocathodes at different temperature. The photocurrent was monitored by illuminating the cathode surface with a 532-nm laser. (b) $1/\mathrm{e}$ lifetime of photocathodes as a function of different decay temperature. Due to the very large decay constants below $71°\mathrm{C}$ we indicate the $1/\mathrm{e}$ lifetimes below these temperatures as the lower bounds for the lifetime. All other error bars of the estimated lifetime values that are calculated based on the 95% confident bounds of the fitted decay constants and amplitudes are within 10%, therefore smaller than the points. The red line is an exponential fit of $1/\mathrm{e}$ lifetime data of $77°\mathrm{C}$, $93°\mathrm{C}$ and $110°\mathrm{C}$.

Figure 4

(a) X-ray fluorescence results of the photocathode before and after the decay, the stoichiometry of the photocathode before and after the decay are shown in the inserted table. (b) Atomic ratio of K to Sb and Cs to Sb during the decay.

Figure 5

X-ray reflectivity experimental data (circles) and simulated curves (red lines) of the K-Cs-Sb photocathode at different stages during the beam line experiment. The reflectivity intensity that was measured from the Si substrate, before-decay photocathode and after-decay photocathode, is demonstrated with an offset for clarity.

Figure 6

Spectral response comparisons of the K-Cs-Sb photocathodes decayed at $110°\mathrm{C}$ for 1 hour (a) and 12 hours (b).

Figure 7

Ratio of the QE before decay to the QE after decay and the ratio of the QE before the decay to after recesiation on photocathodes decayed at $110°\mathrm{C}$ for 1 hour (a) and 12 hours (b).