Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

• Published in: Physical review. Accelerators and beams Vol. 19; no. 10; p. 103401
• Main Authors: Xie, Huamu, Ben-Zvi, Ilan, Rao, Triveni, Xin, Tianmu, Wang, Erdong
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 01-10-2016
• Subjects: Brightness; Cathodes; Cooling rate; Cryogenic temperature; Electrode materials; Electron guns; Electron sources; Emittance; Energy gap; Free electron lasers; Hadrons; High acceleration; Ionization; Laser cooling; Liquid nitrogen; Mathematical models; Monte Carlo simulation; PARTICLE ACCELERATORS; Photocathodes; Photoelectric emission; Photon absorption; Reduction; Room temperature; Simulation; Spectral sensitivity; Temperature; Temperature dependence; Valence band
• ISSN: 2469-9888; 2469-9888
• DOI: 10.1103/PhysRevAccelBeams.19.103401

High-average-current, high-brightness electron sources have important applications, such as in high-repetition-rate free-electron lasers, or in the electron cooling of hadrons. Bialkali photocathodes are promising high-quantum-efficiency (QE) cathode materials, while superconducting rf (SRF) electron guns offer continuous-mode operation at high acceleration, as is needed for high-brightness electron sources. Thus, we must have a comprehensive understanding of the performance of bialkali photocathode at cryogenic temperatures when they are to be used in SRF guns. To remove the heat produced by the radio-frequency field in these guns, the cathode should be cooled to cryogenic temperatures. We recorded an 80% reduction of the QE upon cooling the K2CsSb cathode from room temperature down to the temperature of liquid nitrogen in Brookhaven National Laboratory (BNL)’s 704 MHz SRF gun. We conducted several experiments to identify the underlying mechanism in this reduction. The change in the spectral response of the bialkali photocathode, when cooled from room temperature (300 K) to 166 K, suggests that a change in the ionization energy (defined as the energy gap from the top of the valence band to vacuum level) is the main reason for this reduction. We developed an analytical model of the process, based on Spicer’s three-step model. The change in ionization energy, with falling temperature, gives a simplified description of the QE’s temperature dependence. We also developed a 2D Monte Carlo code to simulate photoemission that accounts for the wavelength-dependent photon absorption in the first step, the scattering and diffusion in the second step, and the momentum conservation in the emission step. From this simulation, we established a correlation between ionization energy and reduction in the QE. The simulation yielded results comparable to those from the analytical model. The simulation offers us additional capabilities such as calculation of the intrinsic emittance, the temporal response, and the thickness dependence of the QE for the K2CsSb photocathode.