Fig. 1. Maxwell velocity distribution with 5 sample areas

Fig. 2. Simulation model and the obtained electron beam track of the optimized electron gun

Fig. 3. Beam cross section at beam waist

Fig. 4. Current distribution under different radial distance at beam waist

Fig. 5. Simulation model of PPM focusing magnetic system

Fig. 6. Electron trajectories with axial magnetic field of PPM

Fig. 7. Radial current of beam cross section at different z-positions

Fig. 8. Schematic diagram of the TWT electron gun validation

Fig. 9. Schematic diagram of the 140 GHz FTWT