A theory for coupling gridded gun design with PPM focussing

It is pointed out that a major fraction of Pierce guns in use today are gridded to allow beam current control and on/off switching by grid voltage levels roughly two orders of magnitude less than the applied cathode-to-anode voltage. Such guns are used in various electron-beam devices, such as microwave and millimeter-wave tubes, linear accelerators, and ubitrons or free electron lasers. It has been customary practice in the design of a gridded gun, to develop first a suitable ungridded or diode gun. In the considered case, difficulties begin to arise after the design of the ungridded gun. In the present paper, a description is given of the theoretical and experimental methods which are useful in the design of gridded guns. Simplified relationships are developed from which the optimum periodic permanent magnetic (PPM) field level for a given gun can be determined in a priori fashion. Attention is given to the concept of tunnel emittance, focussing stability criteria for nonlaminar beams, and the coupling between the gun and PPM focussing problems.