Electron Beam Deflection in the Focusing Magnetic Field of a Camera Tube

Abstract

Magnetic focusing of an electron beam in camera tubes has the advantage of making it easy to attain high resolution—because the beam can be focused onto a minute spot with quite a small beam. In such a magnetic focusing field, however, electron beam deflection causes inevitable helix-like twisting of the beam accompanied by serious degradation of the tube characteristics. This chapter discusses the twisting motion of a beam and to improve those tube characteristics such as spot and raster distortions, as well as beam landing error, which may arise from a twisting motion. To eliminate the twisting motion after deflection, “double deflection” is carried out. It is argued that calculation and experimental tests of the effects of double deflection are in good agreement. The technique ensures perfect beam landing on the target. In normal camera tubes, shading caused by oblique beam landing may be removed by the adjusting action of the target surface equilibrium potential and the collimating action of the electron lens at the mesh-electrode. However, the twisting motion of the beam cannot be completely counteracted by an electrostatic collimation lens. The double deflection method, on the other hand, can eliminate the twisting motion completely and lead to perfect beam landing.