Development of a Low-Level RF (LLRF) System for Phase-Locked Loop (PLL) Control of a Superconducting RF Cavity

About the project

About the Project Project Background The superconducting radio-frequency (SRF) cavities made of bulk niobium (Nb) are used in charged particle accelerators for more than 50 years. They exhibit a million times lower power dissipation than ones made of copper, and allow to accelerate charges particle with high acceleration field up to ~50 MV/m. However, the cost of Nb and liquid helium required for operation are increasing over recent years dramatically, and the performance of the Nb cavities reached the limitation in Nb physics properties. A way to improve is to employ copper (Cu) cavities coated with superconducting thin films (TF). Cu is a material with lower cost and higher thermal conductivity than Nb. The most promising TF materials are Nb3Sn, NbTiN, V3Si, …, that have higher super-conducting parameters such as critical temperature (Tc), critical magnetic field (Bc), etc. Further improvement can be achieved by employing multilayer structures of superconducting and insulating layers. Applying this technology should allow accelerators to reduce their operating power by a factor of 2 and/or increase the gradient to up to 100 MV/m in CW or long pulse machines. To facilitate this TF-SRF technology, the CI collaborating multidisciplinary team of ASTeC, Lancaster University and now Manchester is focused on the following technologies: copper cavity surface polishing, material science development (TF deposition and characterisation), superconducting property evaluation of deposited TF and, finally, an RF testing at cryogenic conditions. In Aug. 2025 this team, for a first time in the world, the bulk Nb cavity was coated with Nb3Sn thin film and send to HZB for RF testing (in a frame of IFAST collaboration). This has placed Cockcroft in a world leading position at the forefront of sustainable RF research. Daresbury Laboratory (DL), home for CI, is a unique place where all these technologies are developed in present, and the only one in UK. Only a few places worldwide have integrated deposition teams, surface/RF characterisation and SRF integration all in one institute. A key aspect of our approach is a suite of novel characterisation facilities with fast sample turn around allowing CI to optimise key deposition parameters in a few weeks that would take other groups years. This team has the leading role of thin film SRF workpackages in international projects such as ARIES (2017-2021), IFAST (2021-2025), iSAS (2023-2027), EPITA (2025-2029). Aim of project to develop all technologies for producing TF SRF cavities and test them at real accelerator conditions at DL, e.g.: CLARA beam loaded test. While thin-film Nb cavities are now fairly common, a beam test of a novel material TF cavity would be a major leap forward in development of these systems further cementing the CI team as a trailblazer in this critical technology. A long-term ambition of this project are producing acceleration structures for UK-XFEL, ISIS-II, participating in the international projects, e.g.: FCC, ILC, etc. Background and Motivation Superconducting radiofrequency (SRF) cavities are central to modern particle accelerators, providing high accelerating gradients with low power dissipation. A key next step in the development of thin film SRF cavities is a test with beam, to show a full cryomodule can operate in an accelerator environment in a stable manner. Such a system however needs a LLRF system that can cope with some of the unique challenges of frequency tuning the more brittle materials required. The SRF group at Daresbury Laboratory, lead by Lancaster University is developing novel longitudinally split cavities and has developed methods for rapid prototyping. To exploit their performance fully, precise control of amplitude and phase stability is essential. Low-Level RF (LLRF) systems play a critical role in stabilising cavity fields against perturbations such as Lorentz force detuning, microphonics, and beam loading. At the core of LLRF operation lies the Phase-Locked Loop (PLL), which ensures synchronisation between the cavity resonance and the reference oscillator. Current LLRF solutions face challenges in latency, noise resilience, and adaptability under dynamic operating conditions. Research Objectives The proposed project aims to develop, model, and experimentally validate an advanced LLRF system dedicated to PLL-based control of SRF cavities. Specific objectives include: − Modelling and Simulation – Develop mathematical and computational models of the cavity – PLL interaction, capturing detuning dynamics and noise sources. − System Design – Architect both a Digital and Analog LLRF control system. Exploring FPGA/DSP-based implementations with high-speed ADC/DAC interfaces as well as IQ solutions. − Stability and Control Strategies – Investigate advanced control algorithms (e.g., adaptive loop filters, predictive controllers) to minimise phase error under microphonics and beam loading conditions. − Prototype Development – Build and test hardware prototypes. − Validation – Benchmark the proposed system against existing LLRF approaches, evaluating performance in terms of phase stability, loop bandwidth, and robustness. Proposed Scheme of Work Initially student will be introduced to existing low power RF system in CI bunker that has been designed and build prior PhD project started. This will allow him/her to familiarise with operation of such systems. Then he/she will be involved in further development of this system. Also, all PhD students attend a standard CI 2-year course of lectures on all aspects of charge particle accelerators. Funding and eligibility: Upon acceptance of a student, this project will be funded by the Science and Technology Facilities Council for 3.5 years. This consists of a tax free stipend at UKRI rates, university fees at the home (UK) rate, plus support for travel to conferences and workshops. A full package of training and support will be provided by the Cockcroft Institute, and the student will take part in a vibrant accelerator research and education community of over 150 people. Contact for further information: Dr. Harry Marks h.marks@lancaster.ac.uk How to apply: Apply at the Cockcroft Institute PhD webpage . For full consideration for funded awards, please apply by Jan 31st 2026. Anticipated Start Date: October 2026 for 3.5 Years

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