Convergent electron gun with bonded nonintercepting control grid

The bonded grid is a metal-insulator-metal sandwich attached directly to impregnated tungsten cathodes. The metallic top surface of the grid structure is the control grid, and the metallic surface on the bottom provides a diffusion barrier for barium between cathode and insulator. A bonded grid for use in microwave triodes was developed and described in a paper at IEDM in 1976. The present paper describes the extension of this work to convergent guns for linear beam microwave tubes. Low grid emission, a strict requirement of linear beam tubes, was achieved after experiments with various control grid materials. A gun using pyrolytic carbon coated with a thin layer of boron nitride as control grid was fabricated and evaluated for electrical and electron optical performance in a beam analyzer. Emission from the control grid has been reduced by three orders of magnitude. Means of permanent attachment of the bonded grid package to cathode surface is presented. Fabrication technique and test results from the guns are described in detail.