Electron gun with bonded nonintercepting control grid

The bonded grid consists of a metal-insulator-metal sandwich attached directly to an impregnated tungsten cathode. 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 the cathode and the insulator. The major advantage of this design approach is that the close tolerance required in grid cathode spacings is easily obtained and maintained with the cathode in hot condition. The first bonded grid structure described in the paper was developed for a microwave triode having a grid-to-cathode spacing of 2 mils. The dimensions of the grid cells are 5 × 35 mils. The grid structure consists of a thin wafer of anisotropic boron nitride insulator metallized on both sides by RF sputtering. The fabrication techniques were later extended to construction of convergent guns for linear-beam tubes. The progress and the problem areas experienced in both phases of this program are discussed.