An electron gun includes the following: a primary thermionic electron source, a secondary thermionic electron source and a focusing electrode disposed within a first housing that includes one or more reference members adjustably attached to a housing support connected to a first platform; an anode and one or more focusing coils disposed within a second housing comprising one or more insulating members adjustably connected to the first platform; and one or more deflection coils disposed within a third housing connected to the second housing and located opposite said first housing.