Try changing the three parameters which control the electron
1. Filament current, A
The filament current controls the temperature of the filament
and hence the number of electrons emitted or 'beam current'. Generally we want a large number of electrons
emitted from a small region of the filament. This is done
by saturating the filament - increasing the filament current
until the beam current no longer rises.
The bias potential controls the size of the region of filament
which emits electrons and hence it affects both the source
size and the beam current. If the bias is too high no region
of the filament will emit and the beam is said to be pinched
The main reason to alter bias is to change the brightness
of the beam.
the controls on the right to start the simulation … and try
to set the gun at saturation.