Design of an Electron Gun and a PPM System for G-Band TWTs

In this paper, a design for the electro-optics system of G-band travelling wave tubes (TWTs), including an electron gun and a permanent magnet focusing system, is presented. The electron gun utilizes the classical Pierce electron gun structure, which suppresses the spurious emission at the cathode edge by adding a casing to the outer layer of the cathode emitting surface. A cylindrical control electrode is used to reduce the processing error and sets a negative bias voltage at the focusing pole, compresses the emitted electron injection into shape, which reduces the radius of the electron beam injection waist and extends the firing range, so as to ensure that the electron beam reaches the focusing system. The working voltage of the designed electron gun is 19 kV, the current is greater than 45 mA, the radius of the waist is 0.078 mm, and the firing range is 14.5 mm. In order to make the electron beam stably transmitted in the electron channel with a radius of 0.12 mm, a periodic permanent magnet (PPM) focusing system was used to focus the electron beam. The peak magnetic field of the focusing system is 2.9 times that of the Brillouin magnetic field, which increases the electron note rigidity. The simulation results show that the maximum fluctuation radius of the transmitted electronic note is less than 0.1 mm. The electron gun and PPM focusing system were processed and assembled into a flow-tube sample tube to verify the effectiveness of the G-band TWT electron-optical system. The experiment showed that the sample tube emission current was 43.3 mA and the collector current was 40.8 mA, achieving an electron flux rate of 94.2%, and the system will be optimized in the next step.