HIGH BRIGHTNESS ELECTRON-BEAM SOURCES FOR FEL APPLICATIONS

A new generation of field emitter array (FEA) cathode materials is under development at SAIC, in collaboration with NRL and GTE Laboratories. The emitter structures under consideration consist of large area (approximately 1 cm2) arrays of large numbers (approximately 10(6)) of microscopic field emitting tips. The structures can be fabricated so as to choose an emitter tip microstructure that is a solid cone, a hollow cylinder, or a variety of other shapes. These microstructures evidence very high local field enhancement factors, controllable from a factor of approximately 200 to > 2000. This large local field enhancement allows quantum field emission of significant current from the large area array while the applied macroscopic electric field is still quite low (approximately 20 kV/cm). Single-tip, noninteracting particle, multigrid simulations indicate that beam brightnesses B(n) = I/pi-2-epsilon(n)2 > 10(10) A/cm2 rad2 may be possible. Beams with such high brightnesses allow for a greatly expanded field of FEL applications, including high gain and harmonic operation in the FIR wavelength regime. Experiments have so far demonstrated DC average current densities > 1 A/cm2, uniform emission, and improved characteristics when run for long periods of time ( > 100 h, DC). Our present efforts are concentrated on optimizing the available cathode current density, measuring the actual beam brightness, and including self-field and 3-D effects in the numerical simulations.