NSRRC Activity Report 2022

Facility Development and Status 091 Fig. 7 : Proportions of downtimes of the TPS accelerator in 2022. (62.17 hours in total) Phase Drift Compensation Loop for a Radiation Frequency System of TPS Booster Ring I n synchrotron radiation light sources, the energies of electrons are provided by a radiation frequency (RF) system, which consumes substantial power. In 2015, members of the RF group worked to reduce this power consumption and develop an economic operation for RF systems in the Taiwan Light Source (TLS) booster ring. 1,2 In 2018, this economic operation system was implemented in the Taiwan Photon Source (TPS) booster ring. 3 The standard operation condition of the TPS is currently top-up mode operation with a 500-mA beam current of multiple bunches but with a single bunch of 3 mA in the middle of clear bunches (also known as hybrid-mode operation). Because of the high injection efficiency and long lifetime (over 8 h) during normal user operation, the injection period accounts for a small portion of the total time required to maintain a beam current fluctuation of less than 5 mA. Thus, the energy-saving operation is expected to reduce the power consumption of the booster ring considerably. Briefly, electron beams are injected from the booster ring to the storage ring to replenish the lost beam every 242 s. Thus, the booster ring operates only during the 2-s injection period and then rests for the remaining 240 s. At the beginning of the energy-saving operation for the TPS booster ring, only the magnets of the booster ring are powered off during the 240-s resting time. However, because the TPS booster RF system consumes 60 kW of electricity, the system should be operated in the power- saving mode. Problem of Digital Low-Level RF During Economic Operation Economic operation is realized using an energy-saving module, which controls the anode voltage and cathode current of the klystron to operate at a high level in the injection mode and at a low level in the standby mode. During switching between these two modes, the instant phase jump caused by the change in the klystron cathode current is beyond the compensation capacity of the digital low-level RF (DLLRF) system. Compared with an analog low-level RF system at the TLS or the TPS, the DLLRF control system has a wider bandwidth and a faster feedback response for improved feedback performance. Thus, the DLLRF control system can sense the aforementioned instant phase jump and consequently drive the proportional- integral-derivative (PID) controller to reach saturation rapidly and thereby trigger RF interlocking to protect the system. Figure 1 (see next page) depicts the phase jump as a digital-to-analog converter (DAC) output phase. To maintain a constant gap voltage phase, which is depicted as an analog-to-digital converter (ADC) phase in Fig. 1 , the DLLRF system must compensate for phase differences up to approximately ±85°, similar to the DAC phase behavior during mode switching. As displayed in Fig. 2 (see next page), this phenomenon enables the RF trip to occur smoothly. In Fig. 2 , t 1 is the preparation zone for the klystron to increase the cathode current (I cc ) for pulling the maximum power up by increasing the anode voltage, so as anode current (I-anode) raising, whereas t 2 represents the first ramp of the gap voltage, which fails in this case. This failure clearly occurs as a result of the oscillation of