Thermionic cathodes are central to plasma and vacuum devices, where emission efficiency, sheath coupling, and thermal robustness strongly influence performance and lifetime. Hybrid cathodes that integrate carbon nanotubes (CNTs) with low-work-function oxides have demonstrated enhanced thermionic emission in vacuum, but their behavior in plasma environments remains largely unexplored. Here, a Ba–Sr–O-coated CNT thermionic cathode is investigated in a low-pressure Ar–Hg glow discharge plasma and directly compared with a conventional Ba–Sr–Ca (BSC) oxide cathode fabricated on identical triple-coiled tungsten filaments. In vacuum, the Ba–Sr–O–CNT cathode delivers 3–4 times higher emission current than the BSC cathode at comparable temperature and electric field, despite exhibiting a higher work function (1.6 vs 1.3 eV), indicating strong CNT-induced Schottky field enhancement. In plasma operation, cathode fall voltages are measured using a capacitive band probe, and cathode surface temperatures are simultaneously determined by optical pyrometry over a range of discharge and auxiliary heating currents. Under all tested conditions, the Ba–Sr–O–CNT cathode exhibits substantially lower peak cathode fall (9.8–14.6 V), and hotspot temperatures reduced by 70–80 °C relative to the conventional cathode at the same discharge current. These cathode fall values place the CNT cathode below the ∼16 V threshold commonly associated with excess ion sputtering in Ar–Hg plasmas, whereas the conventional cathode operates near or above this limit. The results demonstrate that CNT-enabled emission enhancement persists in plasma, reducing sheath potential and thermal loading and suggesting improved resistance to sputtering-induced degradation.
Jin et al. (Tue,) studied this question.