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 μA per tip, while higher currents are generated by tightly-packed arrays of the gated-emitters. The attainable normalized brightness is estimated to exceed 10¹³ A/m²rad², and such beams have unique uses for extending the laser wavelength to the X-ray region at moderate beam energies