A MAGNETIC ION-SEPARATOR PROTECTING THE CATHODE OF A HIGH-POWER ELECTRON-GUN

The operation time of the cathode of high-power electron guns under un favourable vacuum conditions is limited by ion bombardment. Theoretica l estimates of the dependence of the sputtering rate on the origin of the ions indicate that ions generated in the whole acceleration gap co ntribute to the destruction. A permanent magnetic deflector incorporat ed in the Wehnelt electrode is used to form a transverse magnetic fiel d close to the cathode which decreases strongly towards the anode. Due to the different Lorentz force acting on particles of equal kinetic e nergy but different mass, the electrons are strongly deflected already close to the cathode while most ions, following the electric field, h it the cathode far from the emitting area. The resulting deflection of the electron beam is compensated by a mechanical gun shift and anothe r magnetic deflecting system beneath the anode. Thus, an effective pro tection device for the cathode of a 35 keV electron gun for intaglio p rinting engravure was developed which extends the operational time at 45 mA beam current in a vacuum of 8 . 10(-3) Pa from 1/2 h to 10 h. Us ing an inert gas to avoid reactive removal some further improvement is possible. The ion separator would even become dispensable if Helium g as could be applied.