Electron Beam Ion Sources such as the Dresden EBIT and Dresden EBIS-A use cathodes of

high emissivity to form electron beams to ultimately produce highly charged ions.

These cathodes have a limited lifetime and need to be replaced after several thousand hours

of source operation.

In this case, a spare electron gun with a readily installed new cathode can be used for

instantaneous replacement to have the ion source back in operation as quick as possible.

Also, a spare gun can be used to have a different cathode diameter in stock.

With larger cathodes higher electron currents and, thus, more ion output can be achieved.

However, the electron beam compression by the magnetic field can be slightly less efficient

reducing the ionization factor and the highest achievable charge states.

If an EBIT or EBIS is supposed to be used for such opposite fields of applications

(high currents versus highest charge states), having two different cathode diameters available is

an advantage.