Experimental Laboratory Exercises
The emission current is typically between 10 pA and 500 pA depending on the acceleration voltage and the spot size. The current can be measured by focusing the beam into a Faraday cup. This is a metal container that is grounded through a current meter. The container has a small hole into which the electron beam is focused. Secondary electrons get trapped inside the cup.
The beam current depends on the acceleration voltage, the spot size, the filament, and on the age of the filament. Measurements of the beam current as a function of spot size were performed for a particular filament for three different acceleration voltages. All of the data could be fit by a single function,
I = 8×10-6 V exp(0.15 SS).
Here V is the acceleration voltage in kV and SS is the spot size.