Photoemission and degradation of semiconductor photocathode

Abstract

In state of the art photoemission electron sources, the transverse emittance of the electron beam is approaching its intrinsic value from the cathode. To reduce the intrinsic emittance, it is straightforward to tune the photon energy of the drive laser toward the photocathode emission threshold. This paper aims at constructing an improved model which takes into account an intermediate energy level to better explain the near threshold photoemission for semiconductors. A dynamic model is also proposed to evaluate the cathode degradation process due to surface oxidation. The models agree well with published results for different materials under various conditions, including electric field, temperature and vacuum condition. The dynamic model predicts that near threshold emission of an oxidized Cs 3 Sb cathode outperforms UV photoemission of typical metal cathodes in terms of both quantum efficiency and intrinsic emittance. With the oxidization layer, the robustness of semiconductor cathodes to gun vacuum condition may be comparable to a metal cathode. This, however, needs further testing.

Overview of the Semiconductor Photocathode Research in China

“…This is helpful in high brightness electron sources which low intrinsic emittance and high QE are needed at the same time. 1.8 and 1.9 eV, respectively [66].…”

“…The researchers at PKU [55], SARI [53], and THU [66] have simulated the emission process of the alkali antimony photocathodes.…”

“…Furthermore, the intrinsic emittance of the photocathode remains substantially constant, which means it is possible to obtain a photocathode with high QE and low emittance at the same time. The cryogenic performance of the Cs3Sb cathode and its degradation mechanism [66], caused by residual gas in the chamber, is also simulated at THU. The acceptor level of the Cs3Sb photocathode is considered in the Monte Carlo model.…”

“…In addition, the simulated results suggest that the intrinsic emittance of the near-threshold emission (driven by a 690 nm laser) of the Cs3Sb photocathode is better than UV laserdrived metal cathode when the QE remains above an acceptable minimum level. This is The cryogenic performance of the Cs 3 Sb cathode and its degradation mechanism [66], caused by residual gas in the chamber, is also simulated at THU. The acceptor level of the Cs 3 Sb photocathode is considered in the Monte Carlo model.…”

Single shot cathode transverse momentum imaging in high brightness photoinjectors

“…In state of the art photoinjectors, thermal emittance dominates the final injector emittance, so low thermal emittance cathode developments and characterizations are extremely important for further emittance reduction [3][4][5]. In the real gun environment, cathode properties change over time due to residual gases in the gun vacuum, particle back bombardments, RF heating and radiations [6][7][8][9][10]. Both quantum efficiency (QE) and thermal emittance should be routinely measured to help injector tuning and optimizations.…”