GB806890A - Improvements in or relating to velocity modulation apparatus and electron guns therefor - Google Patents

Abstract

806,890. Velocity modulated tubes. WESTERN ELECTRIC CO. Inc. Nov. 18, 1955 [Nov. 29, 1954], No. 33079/55. Class 39(1). In a travelling wave tube or klystron convection current noise in the beam is reduced by a specially designed gun having a control electrode 23 for space charge limiting the electron flow, the control electrode 23 being spaced from the cathode by a distance short relative to the period of frequencies in the design band of the apparatus and convection current noise is still further reduced by a cavity resonator not excited by an external source and having the cathode and control electrode 23 forming opposite surfaces thereof. The control electrode 23 is located slightly closer to or further from the cathode than the potential minimum so that the control electrode 23 rather than the space charge minimum controls the current flow. The cathode-control electrode space is designed as a cavity resonator resonant at the mid-point of the design band. The resonator should be as close as possible to the cathode where the current and velocity fluctuations are uncorrelated so that the current fluctuations can be reduced without affecting the velocity fluctuations. To reduce noise velocity fluctuations a velocity jump gap 27 is provided between the first and second anodes but the velocity jump may be omitted in some cases at an A.C. velocity maximum. Control electrode 23, which carriers a fine wire grid, and beam forming electrode 24 may be integral. The helix 14 may commence at an optimum position to minimise noise ; for a klystron the buncher is similarly at a minimum noise position the buncher being at final anode potential. A longitudinal magnetic focusing field is provided which may include the gun region, or Brillouin flow may be used, e.g. 25A may be the magnetic shield for the gun. Attenuation may be provided by conventional lossy material or ferrite. Input and output coupling are provided by helices 15, 16, and the cathode and control grid may carry different D.C. potentials the control grid being the more negative while providing h/f continuity by means of insulating spacers 30 forming a choke or by a dielectric ring, Fig. 2 (not shown) in the wall of the cathode-control grid resonator. In Fig. 2 (not shown) the control electrode has no grid. Prior art velocity jump devices which are alleged to increase convection current noise are referred to. Specification 761,279 is referred to.