Large-area lanthanum hexaboride electron emitter

Abstract

A large-area cathode assembly which is capable of continuous, high-current electron emission is described. The cathode utilizes an indirectly heated lanthanum hexaboride (LaB6) disk as the thermionic electron emitter. The LaB6 cathode emits over 600 A of electrons at an average of 20 A/cm2 continuously with no observable lifetime limits to date after about 400 h of operation in a plasma discharge. Proper clasping of the LaB6 disk is required to avoid impurity production from chemical reactions with the holder and to provide adequate support if the disk fractures during rapid thermal cycling. Modification of the LaB6 surface composition due to preferential sputtering of boron by hydrogen and argon ions in the plasma discharge has been observed. The surface appearance is consistent with the formation of LaB4 as a result of boron depletion. The electron emission capability of the cathode is not significantly altered by the surface change. This surface modification by preferential sputtering is not observed in hollow cathodes where the ion energy from the cathode sheath voltage is typically less than 50 V. The electron emission by the cathode has not been affected by exposure to both air and water during operation. Utilizing thick disks of this intermediate temperature cathode material results in reliable, high-current, long-lifetime electron emitter assemblies.