Fig. 1. Schematic diagram of electron beam propagation and cavity structure

Fig. 2. x_mn of different modes versus the R_out/R_in

Fig. 3. Dispersion curves under different modes at U_b=300 kV, B₀=0.5 T and ɑ=1.2

Fig. 4. Coupling factor under each mode versus R_b

Fig. 5. Normalized amplitude of hot cavity and efficiency versus distance of the axle of the cavity

Fig. 6. Distribution and FFT of the azimuthal electric field E_phi in 3D PIC simulation under the condition Q_cav more than 90

Fig. 7. The start oscillation currents I_start of TE₀₁ and TE₃₁ modes

Fig. 8. Distribution and the FFT of the azimuthal electric field E_phi in 3D PIC simulation under the condition Q_cav less than 60

Fig. 9. Variations of cavity length thresholds of TE₂₁ mode BWO versus the start current

Fig. 10. FFT of the azimuthal electric field E_phi at 20 ns to 30 ns and output power variation over time under the condition Q_cav less than 60

Fig. 11. Distribution and FFT of the azimuthal electric field E_phi under the optimal parameters of 3D PIC simulation

Fig. 12. Output power variation over time

Fig. 13. Output power variation with velocity spread of electron beams