An advanced type of non-intercepting grid for convergent beam electron guns

Conventional intercepting grids are used in convergent beam electron guns to pulse the beam on and off using a relatively low voltage pulse compared to the cathode voltage, thus reducing modulator size and power requirements. This type of grid is fundamentally limited to low average power tubes in order to prevent thermionic grid emission or melting of the center portion of the grid. This paper discusses a type of non-intercepting grid designed for use on convergent beam electron guns where excessive grid temperature would prohibit use of an intercepting type grid. The non-intercepting nature of the grid is achieved by means of a double aligned pair of grids with the shadowing grid closest to the cathode electrically connected to the cathode. Using this type of grid, guns for high power traveling wave tubes have been built and tested with excellent results. In a particular case, a gun which typically intercepts 10% to 15% of the cathode current on the grid using a conventional grid had only 0.11% interception with the double grid. This gun, which is used on a C-band tube, has been successfully operated to 2.5% duty at 37 KV. In addition to the C-band tube, nonintercepting gridded guns are being incorporated into S-band and X-band tubes. Fabrication techniques for this type of structure will be discussed and experimental test data will be shown. Extensive measurements have been taken with a pinhole beam analyzer in a demountable beam tester to determine how the current density profile of the beam is changed by adjusting the relative position of the aligned grid. Empirical design curves have been obtained for optimizing the grid parameters.