Phase Reference Line for the European Spallation Source

The European Spallation Source (ESS) in Lund, Sweden, is based on a 600 m long linear proton accelerator (LINAC) with over 155 RF resonator structures, both normal and superconducting, operating respectively at frequencies of 352 MHz and 704 MHz. The accelerator will also be equipped with more than 150 various proton Beam Diagnostic (BD) devices. This paper covers the design and installation of the RF synchronization system in the ESS accelerator tunnel to provide phase reference signals for over 300 RF accelerator subsystems with accuracy reaching 0.1° at both operating frequencies. The described system consists of a Phase Reference Line (PRL), an entirely passive system based on a single 1-5/8” coaxial rigid line installed at the tunnel ceiling and supporting systems such as power amplifiers and temperature and gas pressure control systems. The 580 m long PRL was designed to distribute both reference frequencies from a Master Oscillator to 56 tap points in the tunnel. Each tap point contains a broad-band, adjustable directional coupler, passive diplexer, and a configurable set of power splitters to feed 3 or 6 signal outputs with equalized power levels. The entire PRL is temperature stabilized ($+/-0.1$ $\operatorname{deg} \mathrm{C}$) and includes an inner-line gas pressure stabilization to assure phase synchronization precision. This contribution covers the concept of the PRL, technical assumptions, the design, the installations, and current performance test results.