Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation
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.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Nuclear Physics (NP)
- Contributing Organization:
- Stony Brook Univ., NY (United States); Peking Univ., Beijing (China)
- Grant/Contract Number:
- SC0012704
- OSTI ID:
- 1329340
- Alternate ID(s):
- OSTI ID: 1346618
- Report Number(s):
- BNL-113333-2016-JA; PRABFM; 103401
- Journal Information:
- Physical Review Accelerators and Beams, Journal Name: Physical Review Accelerators and Beams Vol. 19 Journal Issue: 10; ISSN 2469-9888
- Publisher:
- American Physical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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