Patent ID: 7902529

Claim:
An apparatus comprising an electron source for selectively providing a first stream of electrons that travels in a direction along an imaginary line to a location remote from the electron source, and for selectively providing a second stream of electrons that travels in the direction along the line to the location, the electron source including: a first electron emitter for selectively emitting electrons for the first stream; and a second electron emitter positioned in a close fixed relationship to the first electron emitter for selectively emitting electrons for the second stream, wherein the imaginary line is normal to and extends through the central axes of the emission sections of the first electron emitter and the second electron emitter while both emitters are positioned in the direction of the imaginary line; and wherein a distance between the central axes of the emission sections of the first electron emitter and the second electron emitter is less then half the length of either the first electron emitter or the second electron emitter, such that heat transferred from the first electron emitter in an active state to the second electron emitter in an inactive state reduces condensation on the second electron emitter.