Surface oxide charge accumulation induced current suppression in LaB6 thermionic cathodes at high temperature

Abstract

Thermionic cathodes are widely used in applications ranging from electron microscopes and x-ray tubes to microwave power tubes. However, their short lifetime remains a significant limitation that urgently needs to be addressed. In this report, a lanthanum hexaboride (LaB6) thermionic cathode with severe surface poisoning was measured and an abnormal phenomenon was observed: as the temperature increased, the emission current first increased and then decreased, exhibiting current suppression at high temperatures. This behavior contradicts Richardson's law of thermionic emission. After a large emission current self-flashing process, the current suppression phenomenon disappeared. It is proposed that, at high temperatures, the accumulation of electronic charge within the oxide layer on the LaB6 cathode surface generates a reversed electric field, which in turn suppresses the emission current. This proposal has been verified by measuring the electron emission characteristics of a tungsten thermionic cathode equipped with a metal suppression shell under different degrees of charge accumulation. Furthermore, the quantitative electronic charge accumulation was resolved through temperature and electric field simulations combined with theoretical calculations. These findings contribute to understanding the failure mechanism of LaB6 thermionic cathodes and open up a new route for extending their lifetime.