Fig. 1. Schematic of a high efficiency klystron-like RBWO. (1 pre-modulation cavity; 2 modulation ridge; 3 slow wave structure; 4 extraction cavity; 5 electron beam collector; 6 resonant reflector; 7 electron beam; 8 guiding magnet; 9 cathode)

Fig. 2. Pre-modulation structure with two cavities.

Fig. 3. Effect of the two modulation cavity spacing L₁ on efficiency.

Fig. 4. Schematic of enhanced extraction cavity: (a) Rectangular extraction cavity; (b) perfect extraction cavity.

Fig. 5. Schematic of two extraction cavities: (a) Rectangular extraction cavity; (b) ellipse extraction cavity.

Fig. 6. Field distribution in rectangular extraction cavity in PIC simulation: (a) E_z orients – z direction; (b) E_z orients +z direction.

Fig. 7. Longitudinal electric field distribution in ellipse extraction cavity in PIC simulation: (a) E_z orients – z direction; (b) E_z orients +z direction.

Fig. 8. Magnetic field of superconductor magnet.

Fig. 9. Phasespace of electron beam under solenoid and superconductor magnet: (a) Case with solenoid magnet; (b) case with superconductor magnet.

Fig. 10. Variation of efficiency with L_ec under solenoid and superconductor magnet.

Fig. 11. Typical results in PIC simulation: (a) Microwave power; (b) frequency spectrum.

Fig. 12. Effect of diode voltage under different L_ak: (a) Variation of efficiency with diode voltage; (b) variation of frequency with diode voltage.

Fig. 13. Schematic of experiment system.

Fig. 14. Picture of the Klystron-like RBWO.

Fig. 15. Schematic of measurement system.

Fig. 16. Calibration result of envelope detector.

Fig. 17. Surface with different roughness: (a) Roughness Ra = 0.4; (b) roughness Ra = 0.05.

Fig. 18. Output waveform for different roughness: (a) Roughness Ra = 0.4; (b) Roughness Ra = 0.05. (channel 1, online microwave; channel 2 and 3, radiation field)

Fig. 19. Breakdown traces after 120 pulses for different roughness: (a) Roughness Ra = 0.4; (b) roughness Ra = 0.05.

Fig. 20. Lifetime of extraction cavity: (a) After 100 pulses; (b) after 130 pulses. (channel 1, online microwave; channel 2 and 3, radiation field)

Fig. 21. Waveform under rectangular and ellipse extraction cavity: (a) Rectangular extraction cavity; (b) ellipse extraction cavity. (channel 1, diode voltage; channel 2, diode current; channel 3, radiation field)

Fig. 22. Breakdown trace after 40 pulses: (a) Rectangular extraction cavity; (b) ellipse extraction cavity.

Fig. 23. Power density distribution of radiation field.

Fig. 24. Microwave waveform and frequency spectrum.