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

Abstract

High-average-current, high-brightness electron sources have important applications, such as in highrepetition-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 K 2 CsSb 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 K 2 CsSb photocathode.

Towards the operation of Cs-K-Sb photocathodes in superconducting rf photoinjectors

“…Models suggest that at the envisaged gradients for SRF photoinjectors (15)(16)(17)(18)(19)(20) the surface roughness of the photocathode must be reduced to values of R q ≤ 10 nm to maintain a roughness induced emittance contribution that is below the intrinsic emittance [9,10]. From recent works of Schubert et al and Ding et al it is apparent that the final roughness of alkali antimonide photocathodes depends strongly on the growth procedure and the crystallization processes involved [11,12].…”

“…III D). This is a critical area of research as there continues to be lively discussion within the accelerator physics photocathode community [17,18] on the topic of operation under cryogenic temperatures. Experiences related to the storage and transfer of these extremely sensitive photocathodes are also presented in Secs.…”

Photoemission and degradation of semiconductor photocathode

“…For semiconductors, the intrinsic emittance can be lower (0.2-0.3 mm mrad=mm rms) when the incident photon energy is near or below the threshold [11][12][13], while QE still supports low average photoelectron current operation. Some previous work has studied the photoemission mechanisms of semiconductors [16][17][18][19], but deviations still exist between theories and experiments. Moreover, the semiconductor is less robust against poisoning by the residual gases and has a relatively short lifetime in practice, and a dynamical model to investigate the cathode performance during the degradation process is still missing.…”

Temperature dependence of alkali-antimonide photocathodes: Evaluation at cryogenic temperatures

“…Any reduction in quantum efficiency (QE) observed when a photocathode is cooled must be a result of adsorbed gas contamination on the photocathode surface and/or the shifting band gap that results from mechanical stress on the photocathode crystal structure. Brookhaven National Laboratory researchers reported a drop in QE of about 20% when a K 2 CsSb photocathode was cooled to 166 K inside their 704 MHz SRF gun [10], while Cornell researchers reported a drop of ∼80% in QE at 680 nm when a Cs 3 Sb was cooled to 90 K inside a 20 kV dc gun [9].…”

“…When samples were cooled to 196 and 77 K, the QE values decreased due to the expected modification of the semiconductor band gap [13]. Photocathode QE also decreased when samples were cooled as a result of surface contamination caused by gas adsorption [10], but this ill effect could be minimized by reducing the water partial pressure inside the vacuum apparatus. In contrast to the referenced work mentioned above, the photocathode QE at 532 nm decreased by only ∼50% when the photocathode was cooled from room temperature to 77 K. By measuring the photocathode QE spectral response over a wide range of temperatures, the band gap dependence on the temperature was empirically determined and extrapolated to 4 K, which is the nominal liquid helium operating temperature of L-band SRF photoguns [14].…”