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Because the work function (WF) of the thermionic cathode of an electron gun is the primary factor for determining emission current density, the precise monitoring of the WF in actual operational conditions under high temperatures is essential for tracking the status of the cathode surface. We adjusted a theoretical model of quantum efficiency under high temperatures as proposed by R. H. Fowler to include the final energy state in the photoexcitation process. This modification is necessary for estimating the WF precisely with the measurement results obtained by photoelectron yield spectroscopy using a tunable pulsed laser in the wavelength range from 410 to 709 nm. As the first step of the investigation, we conducted WF measurements of a single-crystal CeB6 cathode, which is employed for the SACLA x-ray free-electron laser. The WF of the unused fresh CeB6 was 2.44 ± 0.02 eV at a high temperature of 836 °C. Our future plans for WF measurements are also discussed below. [ABSTRACT FROM AUTHOR]

The CeB6(001) single crystal used as a cathode in a low‐emittance electron gun and operated at the free‐electron laser facility SACLA was investigated using cathode lens electron microscopy combined with X‐ray spectroscopy at SPring‐8 synchrotron radiation facility. Multilateral analysis using thermionic emission electron microscopy, low‐energy electron microscopy, ultraviolet and X‐ray photoemission electron microscopy and hard X‐ray photoemission spectroscopy revealed that the thermionic electrons are emitted strongly and evenly from the CeB6 surface after pre‐activation treatment (annealing at 1500°C for >1 h) and that the thermionic emission intensity as well as elemental composition vary between the central area and the edge of the old CeB6 surface. [ABSTRACT FROM AUTHOR]