What ever material we may choose for an emitter, in a dc machine, it need only emit about 10-30 ma of electron current. As an emitter in a Hirsch-Meeks fusor would be a much larger surface area structure than in a simple vacuum tube, most anything that has a half-way decent emission will do well.
If you are going for high DC power or high pulse currents of long duty cycle, that is another matter.
Richard Hull
Created on Friday, December 22, 2000 12:11 AM EDT by Richard Hull