New paper on femtosecond-resolution beam diagnostics

A new study from the University of Liverpool’s QUASAR Group, with collaborators from the MAX IV Light Source, describes a pioneering design for a “bunch-length monitor” capable of measuring ultra-short electron bunches with femtosecond-level resolution, using spatial images of Coherent Transition Radiation (CTR). The work is published in the open-access journal Instruments under the title Designing a Femtosecond-Resolution Bunch Length Monitor Using Coherent Transition Radiation Images.

The broadband coherent imaging system installed and operational in the MAX IV SPF.

The authors, led by QUASAR and EuPRAXIA-DN Fellow Ana Guisao-Betancur and Dr Joseph Wolfenden, Plasma Accelerator Research Fellow in the QUASAR Group, present the conceptual design and first demonstration of a CTR-based imaging system at the MAX IV short-pulse facility (SPF), intended to monitor the longitudinal profile (bunch length) of high-energy electron beams.

Their approach uses a broadband spatial imaging system. When a relativistic electron bunch crosses a boundary (a metallic foil), it emits CTR; by focusing this radiation with mirrors onto a camera, the spatial image distribution can be analysed to infer the bunch length. This avoids the complexity of full spectral measurements or disruptive beamline devices.

In initial tests at SPF (operating at 3 GeV), the device captured CTR images for bunches in the 35–100 fs FWHM range at ~100 pC charge. Analysis of the spatial profile (e.g., width and intensity) showed trends consistent with independent reference measurements using a transverse deflecting cavity (TDC), indicating the potential of CTR imaging as a diagnostic tool.

As accelerator technology advances, especially in areas such as compact plasma-based accelerators or next-generation free-electron lasers (FELs), there is a growing demand for reliable, high-precision diagnostics of ultrashort electron bunches. Traditional tools like TDCs are powerful but often costly, bulky, and disruptive to the operation of the beamline for users and experiments.

The CTR-imaging technique described here offers a compelling alternative. It could provide single-shot, minimally invasive, and compact bunch-length diagnostics. For facilities where footprint and flexibility are paramount (e.g., plasma accelerators or compact FELs) such a monitor could become an essential tool for real-time beam control and optimisation. By lowering the instrumentation barrier, this method may help accelerate the adoption of advanced accelerator technologies in academic, medical, and industrial settings.

The authors are clear: the current prototype is a proof-of-concept. While the initial results are promising, further development and refinements are needed before the monitor can deliver high-accuracy, fully reliable measurements.

Key next steps include, improving the imaging optics to better resolve spatial features in the CTR image, and expanding the system’s spectral bandwidth to increase sensitivity to shorter bunches. Work is also under way developing advanced image-analysis methods utilising machine-learning-based techniques to extract bunch profile information from CTR images.

If these challenges are overcome, the method could evolve into a robust, widely deployable diagnostic, enabling real-time monitoring of ultrashort electron bunches in cutting-edge accelerators.

Dr Joseph Wolfenden, who has led this work over the past 7 years, says, “This represents a significant milestone toward more compact, flexible, and accessible beam diagnostics. It aligns strongly with our groups broader research goals: reducing the size, cost, and complexity of accelerators whilst maintaining performance.”

Further information:

Guisao-Betancur, A., et al. “Designing a Femtosecond-Resolution Bunch Length Monitor Using Coherent Transition Radiation Images”, Instruments, 9(4), 29 (2025). https://doi.org/10.3390/instruments9040029