0428同步年報-2021-全

Facility Status 095 if two RF stations are engaged and their bandwidths are both high ( i.e. , with high integral gain), the interactions between these two stations through the beam would result in greater instability and reduced maximum storage beam current. If the bandwidth of one of the controllers is set to a low frequency ( i.e. , substantially lower than that of the synchrotron frequency), then that LLRF controller would not react to the disturbance caused by the beam. Therefore, the effect from the interaction between the two RF stations would cease and the maximum current could be increased slightly. When operating both digital controllers at high bandwidth, the maximum beam current was only approximately 400 mA. Such instability from the beam–LLRF interaction is a key concern for high bandwidth operation at 500 mA. To increase the maximum storage beam current, the loop gains were lowered by adjusting the attenuation of the RF signals from the transmitted power Pt of the cavity or the RF output power sent to the RF transmitter. However, this method had limited effect due to the resolution of the analog-to-digital and digital-to-analog components. In an alternate method, the phase angle between the PI output signal PI out -the signal just after the output of the PI control circuit, and the digitized Pt signal was adjusted. If the phase of Pt leads at PI out is maintained, then improvements in instability can be realized through high bandwidth Fig. 3 : Spectra of the cavity gap voltage at a 30-mA beam current under the operation of the analog and digital LLRF systems. and high-beam current operation. Although such improvements have been clearly documented, their reasons and underlying mechanisms must still be elucidated through the examination of control theory. We performed a long-term stability test with a high bandwidth, and the recording is displayed in Fig. 4 (see next page). The DLLRF system maintained a 500 mA-stable operation without any trips for approximately 19 hours and finally tripped because of the occurrence of an earthquake. The spectra from the beam position monitor near SRF #2 are displayed in Fig. 5 (see next page). Compared with the analogue system, the DLLRF system had a stronger capacity to suppress the sidebands of 60-Hz noise and their high- order harmonics, maintaining these values below -70 dBc in the DLLRF system. The spectra analyses revealed that the DLLRF system exhibits high suppression capacity for 60-Hz harmonics. The long-term stability test with the DLLRF for the stored beam current of 500 mA at the TPS was also successful. The GUI for the remote control is under development. For the third RF plant, the LLRF system will be the digital type. After evaluations of the long-term stability and reliability of the DLLRF system, the analog LLRF system for the other two RF plants will be replaced with DLLRF-type systems in the future. (Reported by Fu-Yu Chang, Zong-Kai Liu and