The design of space-charge limited magnetron injection guns with control electrodes for gyroklystron applications

Temperature-limited (TL) magnetron injection guns (MIGs) suffer from nonuniform current density distributions due to thermal and work-function variations. A preliminary design of a space-charge-limited (SCL) MIG which could overcome this disadvantage was presented earlier. The design indicated that comparable beam quality as the TL MIG could be achieved, but with a slight cost of increased peak electric fields and cathode loading densities. One disadvantage of the SCL design is that the ability to control the beam current by varying the cathode temperature is lost. In this paper, we present two designs that use nonintercepting control electrodes to address this issue. Both designs have been developed with the basic goal of achieving a 500-kV, 500-A beam for the gyroklystron experiment in our laboratory. The first design has small electrodes that can be adjusted by +/-2% of the main beam voltage to adjust the current by +/-60%. The second design has larger control electrodes that can be biased sufficiently to cut off the beam flow even when the full beam voltage is applied. The design details and parametric studies are presented.