Spiral focusing for cathode-ray tubes

Conventional cathode-ray tubes with e-s focus employ short-lens devices following an object-drift space. This combination promises small spot-size. This is, however, seldom realized because of severe aberrations in short lenses. Superior results recently have been obtained with long-lens structures, extending to, and including, the object. Such long unipotential lenses have been realized by coating high-resistance spirals inside the tube neck. Theory indicates that the magnification of this arrangement is much smaller than might be expected. Typically, (m) is only 23 per cent up from a thin-lens gun of the same overall length. In overall performance, however, the long lens proves superior because of its greater freedom from spherical aberration. This is brought out by beam-pulsing tests. Here, the spiral lens is found to compare favorably with a magnetic-focus coil and to outperform a conventional e-s lens, both in spot resolution and in ability to handle large beam currents. The spiral-lens has found use in certain types of radar indicators. It is capable of handling beams of large current, emerging from small apertures under large angles of divergence. The spiral lens can be adjusted by one external resistor to focus in the zero-voltage, or "stable-focus" mode. Its power dissipation can be held low since there is no interception of beam current. Photographs of gun assemblies with spiral lenses will be shown, as well as typical characteristics achieved with tubes of this kind.