Investigation of Nanosized-Scandia-Doped Dispenser Cathodes With Machined Surfaces

Abstract

Since the cathodes with machined surfaces are required for the practical applications in some high-power vacuum electron devices, the characteristics of nanosized-scandia-doped dispenser (SDD) cathodes with machined surfaces have been investigated, especially in dc applications. In order to reopen the surface pores, which may be smeared by machining procedure, the cathode surface was washed by Ethylene Diamine Tetraacetic Acid (EDTA) solution and prolonged time was taken for the cathode activation. The emission test results show that the SDD cathodes with machined surfaces exhibit better emission performances after treatment. The space-charge-limited dc emission current density of over 20 A/cm2 at 900 °Cb has been measured with temperature compensation for electron cooling after full activation. An on-going life test with a dc load of up to 40 A/cm2 at 970 °Cb has been completed $\sim 5000$ h of operation to date. Since the emission capability can be fully restored after exposing to air and reactivating for a short time, a preheating procedure is favorable for SDD cathode with machined surface in the practical applications. Surface analysis indicates that a recrystallization of tungsten grains occurs during the prolonged heating process, leading to further reopening of the surface pores. As a result, cathode surface is covered uniformly by active elements of Ba and Sc (as well as O) as on an unmachined surface, thereby maintaining the high emission property of the SDD cathode.