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内容記述 |
LIPAc (the Linear IFMIF Prototype Accelerator), the accelerator that can provide the proton and deuteron accelerated up to 4.5 and 9 MeV with 62.5 and 125 mA, respectively, in CW (continuous wave), is being constructed in Rokkasho, Japan. The RF (Radio Frequency) system of LIPAc consists of three accelerating components: RFQ (Radio Frequency Quadrupole), Re-buncher cavities, and superconducting cavities. The tetrodes amplification system and solid-state amplifiers are adopted and RF control modules, called LLRF (low-level RF), are synchronized via the White Rabbit system. Until the end of June of FY2024, beam commissioning for the intermediate stage of the accelerator construction, which consists of all LIPAc accelerator components except for the superconducting cryomodule, was performed. As the result, we demonstrated and studied the beam acceleration up to 5 MeV and transport in 8.75% duty cycle pulsed operation with 119 mA beam current. The core topic of beam commissioning was the preparation to the operation with the superconducting linac, including the checkout of the other accelerating components with arbitrary thermal loading enhanced by the beam loading. One of the most interesting and challenging parts is the RFQ-RF system. We confirmed the improvement of the system coming from the efforts paid since the previous commissioning in the noise environment mitigation, stability of the tetrode, and the feedback process and utilities of the RF control system. Now we are working on several topics to be considered for the updates addressed in the future commissioning. The dissipation of the RF loading in circulators and its compensation is the core challenge in targeting the really high duty cycle beyond 50 %. Especially for the RFQ RF control, the effect of different thermal cycles among circulators, and input couplers will limit the duty cycle. Furthermore the effect propagates to the other RF stations via RFQ cavity itself. So, we are working to analyze the stripline circulator behavior and tuning method to mitigate the impact of the heating transient. The multipactor effect and induced plasma in the RF input coupler, and its control of the eight parallel RF inputs are also a topic to be studied. The transient and dependency on the duty cycle may disappear by replacing the RF coupler, therefore we prioritize the study of the multipactor by simulations for some coupler models and testing another coupler model in the high power (> 200 kW) transmission test bench. |
