RF synchronization and phase recovery using a White Rabbit network for the Large Hadron Collider (LHC) at CERN [Oral Presentation]

• Arthur Spierer (CERN)

During LHC operations, RF signals are crucial not only for accelerating cavities but also as a clock for experiments, beam instrumentation and the new Crab cavities. We propose a highly scalable method for synchronizing RF generation across the LHC complex, enabling automatic phase recovery. In the upcoming long shutdown (LS3), scheduled in the period 2026-2028, the analog RF distribution will be replaced by a White Rabbit (WR) network. This network will provide sub-nanosecond clock synchronization, temperature compensation and the RF over Ethernet (RoE) protocol, which facilitates the transmission of Frequency Tuning Words (FTW) and phase information. In this presentation, we will explore the system architecture and the mechanism used for generating and recovering the RF phase in distributed nodes.

High-precision clocks and triggers for longitudinal beam measurements in high energy synchrotrons [Oral Presentation]

• Giulia Papotti (CERN)

High energy synchrotrons, like the Super-Proton Synchrotron (SPS) and the Large Hadron Collider (LHC) at CERN, require high-precision beam synchronous triggers for longitudinal measurements, e.g. to acquire bunch profiles from a wall current monitor pickup. The observation triggering scheme is based on the General Machine Timing (GMT) followed by dedicated trigger units counting the revolution and RF frequency clocks to allow synchronisation to the exact RF bucket. Combined with a programmable fine delay, accurate triggers can be placed at any azimuthal position with a resolution of about 20 ps. The SPS installation is described in detail, including its additional flexibility for measurements at injection, extraction and bunch rotation. The performance and limitations of such an implementation are analysed in terms of jitter with respect to the beam, delay variation due to cable lengths between the clock generation and acquisition with a sweeping revolution frequency. Such varying delays can be compensated with the White Rabbit (WR) technology, and highlights from the recent upgrade in the SPS are presented.

Measurements of the PSB LLRF multi-harmonic beam loading compensation system [Oral Presentation]

• Mariangela Marchi (Sapienza Universita e INFN, Roma I (IT))

The Proton Synchrotron Booster (PSB) produces a variety of proton beams covering a very large longitudinal parameter space, which are accelerated up to 2 GeV. This low-energy regime requires a significant frequency sweep of the RF system. Finemet-based cavities provide this large frequency range without the need of a tuning loop, but they introduce a significant longitudinal broadband impedance. An ensemble of digital feedback loops therefore compensates the beam-induced voltage at the revolution frequency harmonics. This contribution focuses on measurement techniques used to precisely characterise the signal processing of these loops. First comparisons with beam-based measurements of the transient and steady-state response of the full system are presented. These measurements contribute to a detailed understanding of the LLRF system, necessary to accurately model its behaviour. Moreover possible improvements to the cavity voltage amplitude and phase calibrations in the presence of strong beam loading are highlighted.

Beam-Based Voltage Calibration for Double-Harmonic RF Systems in the CERN Super Proton Synchrotron [Poster]

• Leandro Intelisano

Accurate knowledge of the voltage and phase in an RF cavity gap is essential to preserve beam quality and to achieve efficient, precise real-time correction with LLRF feedback. Voltage calibration using longitudinal phase-space tomography is a well-established beam-based technique that has demonstrated remarkable precision in determining the RF voltage experienced by a particle bunch. In a double-harmonic RF system, beam-based voltage calibration involves minimizing a four-dimensional parameter space that depends on the phase-voltage parameters of both RF systems. This process can be computationally challenging, and it is often more practical to perform a sequence of two-parameter voltage measurements, referencing the higher-order cavity system with respect to the main one. The Super Proton Synchrotron (SPS) at CERN is equipped with 200 MHz and 800 MHz cavity systems, that operate in phase at the bunch position for a non-accelerating bucket. In this context, the latest beam-based voltage calibration campaign conducted in the SPS will be presented, comparing different approaches.

HL-LHC RF distribution over White-Rabbit, the WR2RF and eRTM modules [Poster]

• Gregoire Hagmann (CERN)

The Hi-luminosity Large Hadron Synchrotron (HL-LHC) is an upgrade of the LHC which aims to increase the instantaneous luminosity by 5 to 7.5-fold with respect to the LHC nominal value. During LS3 (2026-2028), Super-conducting crab-cavities will be installed around the ATLAS (point1) and CMS (point 5) experiments which are located several kilometres away from the existing main RF system (point 4). The RF signal distribution for RF users (Experiments, Beam-Instrumentation) will also be upgraded, with use of WR2RF modules, on the RF user side. The crab-cavities LLRF and WR2RF module are synchronized to the main RF system through a White-Rabbit (WR) network. The WR network is used for both clocks and RF synchronization between RF stations. The LLRF electronics is using fixed frequency sampling and processing clocks, and the clocks are reconstructed locally from the WR data stream and a low noise PLL. The plans of the RF distribution, clock generation and the main hardware modules involved will be presented with results from prototype tests.