0428同步年報-2021-全

094 ACTIVITY REPORT 2021 -70 dBc. However, for the storage ring operation with the DLLRF system, several difficulties have been encountered because of the high bandwidth of the digital controller and the heavy-beam-cavity–LLRF interaction, which may result in an oscillation of the accelerating field. The operation parameters for each RF station, therefore, must be tuned for stable operation under the heavy-beam-cavity–LLRF interaction. A long-term stability test for the DLLRF system was performed in October 2021. Under appropriate operational parameters, the TPS DLLRF system exhibited stable operation at 500 mA. An introduction to high-beam current operation with the DLLRF control system in the TPS storage ring is provided in this report. Two KEKB-type 500-MHz superconducting RF modules are used for the accelerating cavities in the TPS storage ring. Each possesses an individual low-level system based on the DLLRF architecture of the TPS storage ring, as displayed in Fig. 1 . The reference signal—master clock (MCLK)—from the instrument and control group is sent to each front end to create various synchronized clocks and 550-MHz local oscillator (Lo) signals. The MCLK signal is also passed into the FPGA for use in the in-phase (I) and quadrature components (Q) demodulating functions and phase synchronization. The detailed work principle can be found in Fu-Yu Chang’s work. 2 The cavity gap voltage (related to the accelerating field build in the RF cavity) is controlled by setting the amplitude Vc and station phase θ sp . Station phase refers to the phase that leads the MCLK by θ sp degrees. Thus, the actual phase of the digital-to-analog converter output is regulated by the proportional integral (PI) controller to maintain the phase angle between the MCLK and the cavity gap voltage. By regulating the individual station phase of cavities #2 and #3, the injection efficiency and power balance between the two RF stations can be optimized. Figure 2 displays the graphical user interface (GUI) from the local operation panel of an RF station. To facilitate the adjustment of the operational parameters, such as the station phase, in addition to the DLLRF-related signals, some RF-related signals, such as forward power from the RF transmitter and beam power, are also displayed on the panel through EPICS PV. The DLLRF controller has a high processing speed and fewer sources of noise (for example, those that arise from the set points and the control process), which result in higher field stability and more effective noise reduction for the cavity gap voltage. For low-beam current operation, 3 these advantages are evident. Figure 3 (see next page) displays the spectra of the cavity gap voltage at a 30-mA beam current under the operation of the analog and digital LLRF systems. The 60-Hz sideband and its higher harmonics are considerably smaller in the DLLRF system; the 3.12 kHz signal was reduced to -85.37 dBc under digital control. However, high-beam current ( i.e. , that larger than approximately 400 mA) operation instability concerns exist and may cause an oscillation of the cavity gap voltage, resulting in a beam trip. A series of studies on the instability caused by LLRF- beam-cavity interaction at different gain settings was conducted. The maximum storage current decreased when the bandwidth of both digital controllers was set to a frequency near that of the synchrotron. Thus, when the TPS was operated under high current, the DLLRF controller bandwidth had to avoid the synchronization frequency. To maintain noise suppression, the digital controller should be operated at high bandwidth. However, Fig. 2 : RF GUI for the TPS storage ring.