Comparison of discharge and surface characteristics of single-crystal and polycrystalline lanthanum hexaboride emitters in a hollow cathode

The applicability of single-crystal lanthanum hexaboride emitters in hollow cathodes was investigated through comparative operational tests and surface analyses with polycrystalline emitters. The discharge voltage of the single-crystal emitter was slightly higher than that of the polycrystalline emitter, but the difference was not significant, while the electron temperature and electron density were nearly identical. This behavior is presumably due to the fact that the low-work function crystal planes were not effectively utilized in the single-crystal emitter. The single-crystal emitter exhibited superior resistance to contamination, preserving surface integrity even in high-temperature regions, while the polycrystalline emitter accumulated more carbon and oxygen contamination. However, although the sample number was limited to two, rapid increases in discharge current caused cracks due to thermal shock in both of the two single-crystal emitters tested. In one case, a complete brittle fracture was observed, as evidenced by hackle patterns, rib marks, and fatigue striations. In contrast, no cracking or fracture was observed in the polycrystalline emitter, likely because grain boundaries mitigated the thermal stress. These results highlight a critical trade-off: single-crystal emitters offer contamination resistance, but require careful thermal design to prevent fracture, whereas polycrystalline emitters provide structural robustness at the expense of contamination susceptibility. This study provides guidance for selecting lanthanum hexaboride crystal structures in hollow cathode applications.