Low temperature and cold emission of scandate cathodes

The unprecedented emission capability of top-layer scandate cathodes prepared by laser ablation deposition (LAD) was already reported yielding 460 A cm−2 at 1030 °C true temperature, which is a factor of about 20 higher than for the best impregnated (I) cathodes. Therefore their emission behaviour has been investigated down to room temperature, yielding 0.5 A cm−2 at 500 °C and 0.001 A cm−2 at 300 °C. The measurements underline, that the work function (WF) remains about constant over a wide temperature range. The effective work function with the theoretical thermionic constant of 120.4 A (cm K)−2 amounted to 1.42 eV, a Richardson plot determination resulted in a Richardson WF of 1.16 eV and a Richardson constant of 8.5 A (cmK)−2. Field emission was observed at room temperature and at about 300 °C for LAD-top-layer scandate cathodes (‘Sc’/Re–I) and was compared with other Ba dispenser cathodes. The threshold field strengths were 3.2 V μm−1 for ‘Sc’/Re–I, 12.4 V μm−1 for Re–I and 15.7 V μm−1 for W–‘I. The emission stability at low operating temperatures is dependent on residual gas pressure and composition.