Direct observation of thermionic emission pattern of hemispherical single-crystal LaB6

Thermionic emission pattern from a hemispherical LaB6 cathode of 〈110〉 orientation was made using an experimental setup similar to Martin’s experiment. The emission pattern from a clean surface has clearly indicated that the bright region consists of (100) spot surrounded by four (210) spots. This suggests that the bright spots (except for the center spot in the cross-over images observed for 〈100〉, 〈110〉, and 〈111〉 LaB6 cathodes correspond to (210) spots as deduced by Oshima et al. Emission patterns at different vacuum conditions are also observed to find that a specific emission pattern appears with good reproducibility corresponding to the change of vacuum condition, leading to existence of enhanced thermionic emission.