Energy distribution of electrons emitted from a thermionic-field emission cathode operating under space-charge limitation

A tungsten field-emission tip, with Ga surface diffusion, is a novel thermionic-field emission cathode capable of emitting milliampere-level currents from the same area as that of a tungsten field-emission tip. In this study, to evaluate the cathode temperature, the energy distribution was experimentally measured using the potential-retarding method. The results confirmed that, up to 100 μA, the energy distribution increased slightly toward only high energy. In the range from 100 to 800 μA, as the emission current increased, the distribution of higher-energy electrons significantly increased. This result indicated that the temperature increased upon increasing the current. Furthermore, at currents >1.5 mA, the energy distribution broadened to approximately 5.2 eV in both the deceleration and acceleration directions. This broadened distribution suggested that strong Coulomb repulsion occurred under a space-charge-limited current.