Electron beams formed by photoelectric field emission

Previous measurements of electron emission by cw laser-irradiated tungsten needles have been extended to pulsed laser irradiation using a 7-ns long, doubled Nd : YAG laser at intensities up to 1011 W/m2. The results show that the mechanism for emission changes from the tunnelling mechanism observed in the cw case, characterized by exponential voltage dependence, to an ablative mechanism characterized by very weak voltage dependence. The observed emission has a threshold around 1011 W/m2, just below the damage threshold for the needles. Using needles with a 1-μm tip radius at voltages up to 50 kV, current densities as high as 1011 A/m2 were observed.