Fabrication and measurement of efficient, robust cesiated dispenser photocathodes

Abstract

Photocathodes for high power free electron lasers face significant engineering and physics challenges in the quest for efficient, robust, long-lived, prompt laserswitched operation. The most efficient semiconductor photocathodes, notably those responsive to visible wavelengths, suffer from poor lifetime due to surface layer degradation, contamination, and desorption. Using a novel dispenser photocathode design, rejuvenation of cesiated surface layers in situ is investigated for semiconductor coatings building on previous results for cesiated metals. Cesium from a sub-surface reservoir diffuses to the surface through a microscopically porous, sintered tungsten matrix to repair the degraded surface layer. The goal of this research is to engineer and demonstrate efficient, robust, long-lived regenerable photocathodes in support of predictive photocathode modeling efforts and suitable for photoinjection applications.

Development and Characterization of Multi-Alkali Antimonide Photocathodes for High-Brightness RF Photoinjectors

“…In general, due to having a lower melting point (i.e., 28.44 • C) and high vapor pressure [29], a part of the Cs atoms that were deposited on the sample (where the temperature is around 90-110 • C) should have been prone to evaporate. Such a loss of Cs atoms due to evaporation over the substrate temperature of 50 • C was previously reported for K 2 CsSb and Cs 3 Sb photocathodes [30,31]. As a consequence of such evaporation, a temperature-sensitive surface layer may have been formed near the subsurface region of the films during the deposition, which caused a balance between the adsorption (inward cesium being distributed and diffused) and desorption (evaporated cesium from the surface) of cesium atoms near the surface.…”

Synchrotron x-ray study of a low roughness and high efficiency K2CsSb photocathode during film growth

“…Since Cs has a low melting point of 28 °C [33], the Cs atoms deposited on the sample at 125 °C have a much faster evaporation rate than those deposited on the water cooled QCM. The quick loss of cesium atoms in K 2 CsSb and Cs 3 Sb photocathodes due to evaporation over 50 °C was also reported previously [34]. As in a dynamic vacuum system, extra Cs atoms which evaporated from the photocathode surface were pumped out through the vacuum pump system.…”

Final Technical Report

“…UMD also investigated cathodes fabricated with various materials, including cesium, potassium, and sodium, as both alkali metal coatings and multialkali antimonides, deposited on tungsten and silver [4,5,6,7]. Re-cesiation of alkali-based cathodes was effective in experiments at UMD for restoring QE and extending lifetime.…”