Try changing the three parameters which control the electron
gun:
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.
2. Bias
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
off.
The main reason to alter bias is to change the brightness
of the beam.
Use
the controls on the right to start the simulation … and try
to set the gun at saturation.
What
are the units of brightness? How is the brightness of an
electron beam changed as it passes through a condenser lens
with a magnification of 0.05? 
