A thermionic electron gun is operated as follows:

• Select the accelerating potential ( kV ).
• Increase the current passing through the filament until the knee of the emission curve is reached (saturation), giving the best compromise between the beam current emitted (as high as possible) and the filament lifetime (as long as possible).
• Adjust the bias to give the desired combination of source size and beam current.

The simulation is on the next page - click on 'Continue'.

Why is it not a good idea to use a filament current higher than that which will just saturate the filament?"