Ultrahigh-brightness microbeams: considerations for their generation and relevance to FEL

Field-emission tips have always been the brightest electron source, but the focused beam current from them has been very low. A substantial improvement in beam intensity and brightness is expected with the use of a microfabricated gated-emitter which produces a parallel beam near the emitter. A high-gradient accelerating field suppresses large displacement of electrons from the beam axis and thus reduces the emittance growth due to aberration. The emission is typically 100 /spl mu/A per tip, while higher currents are generated by tightly-packed arrays of the gated-emitters. The attainable normalized brightness is estimated to exceed 10/sup 13/ A/m/sup 2/rad/sup 2/, and such beams have unique uses for extending the laser wavelength to the X-ray region at moderate beam energies.<>