Patent GB-1183061-A (GB)
Title: Electron Focus Projection and Scanning System
IPC: H01J29/46
Filed: 1967-03-03
Abstract: 1,183,061. Electron beam focusing and deflecting systems. GENERAL ELECTRIC CO. 3 March, 1967 [12 May, 1966], No. 10083/67. Heading H1D. A "focus projection and scanning" or "FPS" electron optical system for simultaneously focusing an electron beam and scanning it in two co-ordinate directions, e.g. in a vidicon, utilizes a longitudinal magnetic field combined with a rotatable bi-axial electric field, the coil or permanent magnet 25, Fig. 1, and the electrostatic yoke 26, preferably of the type employing pairs of interleaved sinusoidal electrodes, being of the same length. A pre-focus Einzel lens 16 forms a real image of the object aperture 12 at 20 within the FPS system or "cavity" 2. According to the invention the "dwell phase" of the electrons within the FPS cavity is adjusted to a "strong focusing" condition (see below). In an alternative embodiment (Fig. 4) the electron beam is divergent as it enters the FPS cavity. The beam may be accelerated or decelerated in the drift space 5 between the FPS cavity and the target 3 by means of a spiral electrode and a mesh electrode may be provided to terminate the field of the spiral electrode (Fig. 3). Post-FPS cavity collimation may be achieved by extending the magnetic coil or by means of a spiral lens (Figs. 6 and 7). In a further system (Fig. 8) a beam cross-over may be formed in the entrance aperture to the FPS cavity or the aperture itself may form the real object of the system. In this arrangement there are no fieldfree spaces at either end of the FPS cavity. The characteristics of the FPS system are analysed to indicate the relationships between dwell phase # where # = #T (T is the dwell time in the FPS cavity) and such factors as image rotation, compression and resolution. Optimum anastigmatic operation is obtained when # = 110-140 degrees, but for improved resolution and according to the invention # > 180 degrees (strong focusing condition) and at # = 360 degrees landing errors are reduced to a minimum. In the strong focusing condition a minimum FPS cavity length is necessary to avoid a high deflection voltage and hence in the embodiment of Fig. 12, a conical electrostatic yoke 136 is used. In addition the drift space 131, non-immersed in the magnetic field, is provided to reduce the dwell phase # and Einzel prefocusing lens 135, 133, 139 is used. A greatly increased drift space 131, or a spiral electrode may be used in this region where the FPS system is used in electron beam welding apparatus (Figs. 14A, 14B). The system may also be used in image orthicon tubes, monochrome or colour television tubes where a plurality of beams may be provided, electron beam machining or welding apparatus or in X-ray tubes. In the electrostatic scanning yoke with interleaved sinusoidal electrodes, the pitch of the sinusoids may be graded in the axial direction and especially at the exit end of the yoke in dependence on the proximity of the beam with the walls of the yoke to avoid fringe effects (Figs. 10A, 16B). The polarity of the longitudinal magnetic field is chosen in relation to the direction of rotation of the sinusoids. Where the FPS system is operated in a mode (dwell phase # = 270-360 degrees) in which image rotation is a minimum, a non-rotating magnetic lens is used, the field being generated by a three-sectioned coil. The sections have equal numbers of ampere turns, but the middle section is energized oppositely to the outer sections (Fig. 17).