Low temperature properties of Ba-dispenser cathodes

The low temperature emission properties of Ba-dispenser cathodes with different metal and metal+oxide top layers were determined as a function of anode voltage and operating time. A “Sc”/Re–I cathode showed room temperature emission, which was nearly four magnitudes larger than for a W–I cathode. At 300 °C, thermionic and field emission were both found superimposed. A Fowler–Nordheim fit derived a remarkable low field emission threshold of 3–4 V/μm. Nevertheless, emission degradation with time was observed due to oxidation of the Ba–O dipoles. At temperatures below 600 °C, oxygen adsorption time constants could be lesser than time constants for desorption of oxygen and resupply the Ba due to oxygen induced desorption, which led to a continuous degradation of emission current. Using oxygen desorption energies and energies required for the Ba resupply (determined from high temperature measurements), we could estimate boundary condition for stable long-term emission. Above 600 °C, required low background pre...