Abstract
The unprecedented emission capability of top-layer scandate cathodes prepared by laser ablation deposition (LAD) was already reported [1,2] 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.
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