Development of high current-density cathodes with scandia-doped tungsten powders

The development of high current-density cathodes employing scandia-doped tungsten powders is reviewed in this paper. A matrix with a submicrometer microstructure characterized by uniformly distributed nanometer particles of scandia is believed to play a dominant role in the improved emission capability of these cathodes. Space-charge-limited current densities of over 30 A/cm(2) at 850 degrees C-b have been repeatedly obtained for many runs of cathodes fabricated from the different batches of scandia-doped tungsten powders. A lifetime of over 10000 h at 950-degrees C-b 2-A/cm(2) dc loading in a test diode has been achieved. Periodic high current-density pulse testing was also carried out during the test. The performance for both the dc and pulsed current densities remained stable. When tested at Stanford Linear Accelerator Center in a cathode life test vehicle with a Pierce gun configuration, the cathode operated for 500 h at 1170 degrees C-b, with a pulsed loading of 100 A/cm(2) and with less than 5% degradation in current density. The outstanding performance of these cathodes is attributed to a surface multilayer of Ba-Sc-O of about 100-nm thickness that uniformly covers the W grains with nanometer-size particles distributed on the growth steps. The layer is formed after proper activation by diffusion of free or ionic Sc together with Ba and 0 from the interior of the cathode to its surface. This highly mobile, free, or ionic Sc is liberated from constituents produced during impregnation and activation by reactions between the matrix materials and impregnants.