An ultrahigh precision electron gun-tube alignment technique for millimeter-wave applications

This paper describes a simple technique to measure and adjust electron gun-circuit concentricity for millimeter-wave applications. Optimization of the electron gun design and magnetic focusing circuit is not sufficient to insure low circuit interception. High power, higher frequency millimeter wave tubes, some with circuit diameters less than 0.010 inch, must be precisely aligned from the cathode to the collector. A solution adopted by the authors is to use an inexpensive, low-power helium-neon laser to measure the concentricity of the cathode to the microwave circuit and correct by mechanical adjustment before final assembly. A laser beam is shot through a hole in the cathode, down the rf circuit, and picked up by a photodetector in the collector region. The technique is simple and can improve the gun-tube alignment by an order of magnitude.