ABSTRACT
For a practical application, basically only the electrons emitted by the (central part of the) end facet of a Schottky source are of interest, as these end up in the final beam that illuminates the specimen. But before zooming in we first take a broader look and consider the total emission from a Schottky source. The total emission from a Schottky source is relevant, for example, the gun design, because it could cause unwanted charging or heating of parts, which could damage the system. Although the content of this chapter has not been an area of thorough investigation, it is presented up front to sketch the general picture of a working Schottky emitter. How much current a Schottky source emits can be calculated with the theory of the previous chapter if the field, temperature, and work function across the emitter surface are known. The work function is basically a material property, but the field and temperature are applied by biasing the emitter and by resistive heating. Below we will address the relation between the operating parameters for a Schottky emitter (bias, heating current) and the input parameters for emission theory (field and temperature at the emitting surface). The temperature is relatively constant across the emitter cone (McGinn, 2005), but as will be shown, the field and work function strongly vary. As a consequence, the current density distribution is a strong function of the position on the emitter, and the total emission is to be found from a surface integral.
