Field perturbing means for preventing beam scalloping in reversed field focusing system

摘要

1,049,875. Travelling wave tubes. WESTERN ELECTRIC CO. Inc. June 25, 1963 [July 3, 1962], No.25142/63. Heading H1D. A travelling wave tube has a magnetically shielded cathode and an electron gun for producing a converging electron beam, the resulting electron beam being focused by a magnetic field which has a strength higher than that required for Brillouin focusing whereby the beam is caused to commence to scallop, the focusing field being distorted to produce an imbalance of the forces governing the flow of the beam so as to produce a cancellation of the scalloping effect of the focusing field. The focusing field is five or six times larger than that required for Brillouin flow at the point of entry of the beam into the focusing field, and the distortion of the focusing field is at a point from the place of entry of the beam into the focusing field e.g. between “ and ¢ of a scallop wavelength such that the beam is compressed to the Brillouin diameter corresponding to the focusing field. The field distortion is a decrease in the strength of the magnetic field over a distance of up to ¢ a scallop wavelength. The focusing field is preferably produced by a period permanent magnet system, and the magnet field distortion is produced at every field reversal to eliminate scalloping, the field being distorted both before and after each field reversal. The field distortion is produced by a ring of magnetic material.