Active Phase Drift Compensation in Master Oscillator to Phase Reference Line Connection for the European Spallation Source

The European Spallation Source in Lund, Sweden, is planned to be the world’s most powerful pulsed neutron source. The machine includes a 600 m long proton accelerator with almost 300 RF resonator cavity structures and beam diagnostic devices, a five-ton tungsten target wheel, and fifteen instruments for science known as experimental stations. An essential requirement for ESS is to ensure a precise phase synchronization of the subsystems. The phase accuracy requirements are $0.1^{10}$ for the short term, 0.1° for the long term between adjacent outputs, and 2.0° for the long term between any two outputs. This paper covers the design and installation of the active phase drift compensation system in RF connection from the Master Oscillator to the Phase Reference Line for the European Spallation Source. The system stabilizes phase simultaneously at 352.21 MHz and 704.42 MHz reference frequencies. It also demonstrates simultaneous group delay drift reduction from 18.1 ps to 0.71 and 0.87 ps at 352 MHz and 704 MHz, respectively, without noticeable degradation of the residual phase noise. The drift suppression factor equals about 20 and is considered an excellent result, considering the system’s complexity and the necessity of measuring phase at a distant cable end. The system was implemented in the ESS facility and has been successfully operated there since 2021. Further tests are planned in 2024.