Pulsed photoelectric field emission from needle cathodes

Experiments have been carried out to measure the current emitted by tungsten needles with 1-μm tip radius operated up to 50 kV. This corresponds to electric fields in the order of 109 to 1010 V/m. The needles were illuminated with 10-ns laser pulses at 532, 355 and 266 nm. The laser intensity was varied from 1010 to 1012 W/m2, limited by damage to the needle tip. The observed quantum efficiency depends on the wavelength and the electric field, approaching unity at the highest electric fields when illuminated at 266 nm. Peak currents up to 100 mA were observed in nanosecond pulses, corresponding to an estimated brightness of 1016 A/m2 sr. Since the current is controlled by the laser intensity, with only a weak voltage dependence, these cathodes can be used for infrared and ultraviolet tabletop free-electron lasers and other applications that demand short electron-beam pulses with high brightness.