Improvements Relating To The Focusing Of Electron Beams

1,237,636. Electron beam focusing apparatus. WELDING INSTITUTE. 24 March, 1970 [31 March, 1969], No. 16767/69. Heading H1D. A magnetic lens for focusing an electron welding beam of a diameter greater than 10 mm. comprises a coil 13, Fig. 1, which is unshrouded over an internal length S which is at least equal to the diameter D so that the coil produces a radial field to deflect the electrons towards a focus and an increase in the current through the coil produces a decrease in diameter of the beam emerging from the lens. The coil may be unshrouded or may be shrouded by a mild steel tube 13 as shown in Fig. 1, and with a 100 mA 30 kV beam the focal length may be varied between 19 and 300 mm. (Fig. 2, not shown). Preferably, the conditions are such that 4(r L )<SP>3</SP>(1+K)<(S+D)<SP>2</SP><9f<SP>2</SP> min, where K is the magnification, f min is the minimum focal length, and r L is the radius of the beam at the centre of the lens. Spherical aberration may be reduced by using a hollow beam, and Fig. 4 illustrates a suitable magnetron injection gun comprising a cathode 17, focusing electrode 18, and anode 16 surrounded by a coil 20; the main lens is shown at 21.