Anomalous Energy Spreads in Electron Beams

Energy distributions of 2- to 10-eV electron beams were measured in a deflection analyzer. The performance of the analyzer was monitored by use of a second one in tandem. In agreement with earlier measurements in high-energy beams the energy distribution varied with current. At low beam currents the energy distribution was Maxwellian at a temperature corresponding to the cathode temperature. As the current increased toward the beam space-charge limit, the energy width increased. The excess energy width could be fitted to a curve linear in current density and varying inversely with beam energy to the three-halves power. No shift in most probable energy was found. The results are not in quantitative agreement with the higher-energy low space-charge measurements.