Electron Beam Scanner and Tungsten Target

The electron beam gun is a triode system operated with a small-size tungsten bolt cathode that is indirectly heated by electron bombardment. The Wehnelt electrode and the anode are shaped as a Rogowski transducer with a bell-shaped through-hole anode. This configuration has been chosen since it greatly reduces the dependence of focal spot size on electron beam current. The electron beam gun is operated at about 10−6 mbar gas pressure which is provided by a vacuum pump system, consisting of a scroll pre-pump and a turbo molecular main pump operated in series. The gas pressure in the beam column and the scanner head is somewhat higher in the range of 10−5 mbar.