Fig. 1. Block diagram of RF system at the phase II of SSRFLLRF—Low level radio frequency, Feedback—Coupling voltage, Beam—Beam position monitor

Fig. 2. Model of electrical model of resonance cavity

Fig. 3. Simulation result of the voltage of the RF system

Fig. 4. Schematic diagram of the LLRF controller in 3^rd harmonic cavity at SSRF

Fig. 5. Flow chart of the measurement beam sign

Fig. 6. Data collected after detecting、filtering and amplification

Fig. 7. Experimental data of piezo voltage and displacement (increase or decrease)

Fig. 8. Brief schematic of the controller in tuner

Fig. 9. Tuning thresholds related to the status of motor (slow tuning) and piezo (fast tuning)

Fig. 10. Chart of the judgement on quench

Fig. 11. Delayed measurement of quench signal (Quench trigger, Quench to PLC, Quench to klystron in main RF system)(Color online)

Fig. 12. Amplitude stability comparison of cavity voltage (a) Open loop within ±5%, (b) Close loop within ±1%

Fig. 13. Output voltage of piezo

Fig. 14. Improvement of beam life(a) Not stretched, (b) Stretched