Single crystal lanthanum hexaboride electron beam emitter having high

Abstract

I have discovered that an electron beam emitted from an LaB.sub.6 single crystal cathode has higher brightness when a significant portion of the actual emitting surface of the LaB.sub.6 crystal comprises flat surfaces oblique to the electron beam axis and when these flat surfaces expose relatively low work function crystal planes. I have defined as a relatively low work function crystal plane those crystal planes having a lower work function than the average work function for sintered LaB.sub.6. My preferred geometry for a single crystal LaB.sub.6 electron emitting tip is a pyramid oriented such that the apex points in the electron beam emission direction and preferably also points in a direction perpendicular to a relatively low work function crystal plane. The pyramidal tip may have three, four, or more flat sides, all of which contribute electrons to the beam, from at least an area in the vicinity of the apex. The apex of the pyramid may be rounded or flat. Relatively low work function crystal planes include the (100), (110), (111), (210), (321), and (311) crystal planes, but additional relatively low work function crystal planes which have not yet been tested probably exist. In general, the brightness of any single crystal LaB.sub.6 cathode may be improved by facetting the emitting tip with flat surfaces so as to expose relatively low work function crystal planes. However, highest brightnesses occur when the emitted electron beam is perpendicular to and the flat surfaces expose the lowest work function crystal planes.