Vanderbilt University, Nashville
High-quantum-efficiency photocathodes used for free-electron lasers tend to be fragile and demand complex drive lasers. Field-emitter arrays eliminate both these problems, but introduce other problems along with interesting new physics. Diamond field-emitter arrays are rugged and forgiving of poor vacuum. They are easily conditioned to give uniform emission, current density on the order of 100 A/cm2 before phase compression, and emittance smaller than 10 μm-radians. In gated versions the emission can be phased to the rf drive and the emittance can be reduced by the focusing effect of the gate. Experimental evidence from diamond pyramids and carbon nanotubes suggests that field emission is enhanced by resonant tunneling through molecules adsorbed on the surface. The emission from individual molecules appears to reach the fundamental limits imposed by the Heisenberg uncertainty principle and by the Pauli exclusion principle.
