Computer Simulations Of Relativistic Mlg's With Magnetically Compressed Beams For High Power Microwave Tubes

High power microwave tubes require well focussed high energy electron beams High frequencies cause the r-f interaction with the beam to concentrate in the outer edge of the beam making the center relatively unimportant The Magnetron Injection Gun (MIG) uses a truncated cone for a cathode and an axial magnetic field to concentrate the electron current in an annulus eliminating the current in the center of the beam High power microwave tubes used for r f plasma generation, and for accelerators require these high current relativistic beams A theoretical study of Relativistic MIG's has been carried out using PBGUNS', a digital program for the Poisson simulation of particle beams The simulations show that there are two types of guns that need to be considered those with cathodes that are short axially compared with their mean radius from the axis and those whose length are comparable to or longer than the mean radius Short cathodes are dominated by the axial end conditions which need to be close to the Pierce angle (67 5 degrees) but are little effected by the shape of the accelerating electrode The longer cathodes are critically dependent on both the end conditions and the accelerating electrode One or two transition regions are needed, one to transporl the beam to its final diameter and a second to turn it axially in such a way that it does not ripple excessively before it reaches the front of the cathode, but can be relieved by reducing the magnetic field so that the electrons rise well above the cathode Longer cathodes can also be stabilized by using larger cone angles Cone half angles of less than 15 degrees tend to be more unstable The longer the cathode and the higher the current the greater the difficulty in forming a transportable, compressed beam Several examples will be presented