Abstract: A repetitive pulsed magnetic field serving as the guiding magnetic field for the repetitive high-power microwave source has been proposed, which makes the source compact with low loss and small heat. The design started from the expression of the current producing the pulsed magnetic field, and was based on the following principles: low eddy current loss, small capacitive energy storage and large quality factor. The optimization method of the storage capacitor and charging voltage of the field producing system was presented. Applying this method to the design of a Ka-band back-wave oscillator (BWO), the circuit parameters were determined, corresponding simulation and experiment were conducted, and the measured results accorded with the calculated requirements. The BWO worked steadily at a r