Noise and energy broadening in field emission systems

To improve the performance of the field emission electron gun requires that greater total currents are drawn from the field emission cathode. This involves various practical difficulties associated with the vacuum environment, and in addition, the Boersch effect energy broadening may become important at high currents and for certain applications, for example in pulsed transmission microscopy (Smith and Cleaver, 1976). As part of a study of field emission guns under high current conditions we have measured the energy spreads in a pulsed triode gun system. Positive going pulses on the control electrode, or Wehnelt, were used to draw peak currents of up to 2 mA for 10 μS durations, the pulses being of 4 kV amplitude. A (111) orientated single crystal tungsten cathode was used, of 150 mm tip radius, and the energy of the emitted electrons measured in an analyser due to Brack (1962). This consists of a three electrode einzel lens with a superimposed axial magnetic field, and is designed to have a high coefficient of chromatic aberration.