Energy Distribution of Electrons from Thermionic Cathodes with a Wehnelt Cylinder

Abstract

The energy distribution in 26 keV electron beams was measured by a retarding potential analyser. Beams of small angular divergence were used to eliminate the apparent increase of energy width. The beam current was changed by varying the negative bias voltage V_g at fixed cathode temperatures. For a tungsten hairpin cathode at 2620 K (Maxwellian width=0.62 eV), for example, the energy width ΔE_i was 0.33 eV just above the cut-off voltage. It increased with the decrease of |V_g|, reached a maximum (ΔE_i=2.3 eV) and then decreased. When the image of the cathode tip was focused on the entrance aperture of the analyser, ΔE_i decreased to 1.1 eV. These results can be interpreted by the filtering effect of the bias potential and the change of the effective size of electron source which is caused by the change of the bias voltage.