High-brightness photoelectric field-emission cathodes for free-electron laser applications

Field emission from very fine needles in an electric field is characterized by a very small energy spread, less than 1 eV, and emittance of the order of 10(-8)pi m rad, normalized. In microsecond pulses, total currents in excess of 6 A have been observed, corresponding to normalized brightness of the order of 10(15) A/m(2) str, which is four orders of magnitude higher than conventional sources. Recently, photoemission has been observed from needles operating in fields too small for significant field emission. Total currents as high as 10 A have been observed in nanosecond pulses. The quantum efficiency is of the order of unity. Using e-beams of such high brightness, single-pass FELs can operate at very long and very short wavelengths, with very low total e-beam current. In the FIR, it is possible to construct Cerenkov FELs with enough gain (e(20)) to reach saturation in a single pass using only 10 mA total electron-beam current. In the UV, it is possible to build single-pass Compton-backscatter FELs operating at total current less than 300 mA.