Master Oscillator and Phase Reference Line Design for the PIP-II Linac

TL;DR

The paper addresses robust RF phase reference and clock distribution for the PIP-II linac across 162.5, 325, and 650 MHz. It proposes a phase-averaging reference line with forward–reflected phase aggregation and active recirculation, driven by a temperature-stabilized master oscillator and large-diameter Heliax cabling to minimize drift. The method uses ADS-based simulations and bench measurements of inter-station ripple alignment, along with a three-source cross-correlation phase-noise measurement to validate oscillator performance. Results indicate sub-femtosecond-level phase stability over realistic temperature ranges and phase-noise performance meeting accelerator requirements, supporting reliable synchronization of RF, LO, and LLRF subsystems. The work demonstrates a scalable approach to high-precision RF distribution in long accelerator tunnels, with implications for future high-frequency linac facilities.

Abstract

The phase averaging reference line system provides the RF phase reference, LO and clock signals to the LLRF and other accelerator sub-systems. The PIP-II linac has RF systems at three frequencies - 162.5 MHz, 325 MHz and 650 MHz. A temperature-stabilized, low-phase-noise oscillator is used as the master oscillator. Phase reference signals at 162.5 MHz, 325 MHz, and 650 MHz, along with LO signals at 182.5 MHz, 345 MHz, 670 MHz and LLRF clocks at 1320 MHz and 1300 MHz, are generated in temperature-controlled RF modules at each frequency section. A phase reference from each module travels to the next section, where it is doubled to produce required frequencies. The reference also travels alongside the accelerating cavities in the tunnel, allowing cavity probe and reference cables to temperature track and reduce measurement errors from temperature changes or phase drift.

Figures (6)

• Figure 1: Schematic of 2-station forward-reflected phase aggregated reference line.
• Figure 2: ADS simulation for the reference line
• Figure 3: Approximate lengths of the 162.5MHz, 325MHz & 650 MHz sections
• Figure 4: Approximate lengths of the 650 MHz section of the reference line.
• Figure 5: Relative phase deviation with respect to station 1 as a function of electrical length change for the 162.5 MHz, 325 MHz, 650 MHz, and 1300 MHz lines.