Studies of electron beams propagation in space-charge regime

We report the experimental characteristics of electron beam propagation under space-charge regime. The electron beams were generated by a Nb polycrystalline photocathode illuminated by two different excimer lasers, a XeCl (308 nm) and a KrCl (222 nm). The laser photon energies were very close to the Nd work function. The cathode surface was mechanically worked in order to study the photoemission from a smooth and a rough surface. At low accelerating voltage the electron beam was dominated by the space-charge effect and its resulting pulse never clipped as predicted by the Child-Langmuir law. Instead, it presented as fast a rise time as the laser one, an intermediate zone, and a tail longer than the laser pulse one. On the other hand, under saturation regime the output current wave form was similar to the laser one. The quantum efficiency was higher for the rough cathode. It corresponded to 3.2x10(-5) and 6.7x10(-7) for the KrCl and XeCl irradiation, respectively. The maximum current value was an electron bunch containing 980 mA (8.9 nC), by means of 1.7 mJ KrCl laser energy and 10 kV of accelerating voltage. (C) 2002 American Institute of Physics.