A novel beam diagnostic instrument developed by researchers in the University of Liverpool’s QUASAR Group has achieved a major milestone: full operational approval for use in the Large Hadron Collider (LHC) at CERN. The instrument, known as the Beam Gas Curtain (BGC) monitor, has now been cleared for continuous operation (~2,000 hours per year) in the world’s most powerful particle accelerator.
“This is a tremendous achievement for our collaboration,” said Professor Carsten P. Welsch, Head of the QUASAR Group who has led this development for almost 20 years. “Seeing a device that began as a concept explored by several generations of our PhD students now operating at the heart of the LHC is truly inspiring. It demonstrates the power of long-term innovation, teamwork, and persistence.”
The BGC monitor was conceived, developed, and refined over a decade within the QUASAR Group, part of the University’s Department of Physics and the Cockcroft Institute. The system was designed to meet one of the toughest challenges in modern accelerator physics: how to measure the properties of very high-energy particle beams without disturbing them.
In a paper just published in Physical Review Research, Liverpool physicists and their GSI and CERN collaborators report the first-ever full-cycle, non-invasive beam emittance measurements at the LHC using the Beam Gas Curtain technique. The instrument works by creating an ultra-thin, supersonic sheet of neon gas – a “curtain” – that interacts with the circulating proton or lead ion beam. The resulting faint flashes of fluorescence light are captured by a sophisticated optical system, revealing precise information about the beam’s size and quality throughout the full acceleration cycle.
Unlike existing instruments that require dedicated calibration time or interrupt normal operation, the BGC can continuously monitor the beam profile and emittance from injection at 450 GeV up to the LHC’s top energy of 6.8 TeV – all while physics experiments are running.
The system was tested extensively at the Cockcroft Institute before installation at CERN. Its performance has exceeded expectations, delivering high-precision, non-invasive measurements for both proton and heavy-ion beams. As the Physical Review Research paper shows, its results agree closely with independent LHC diagnostics such as the Beam Synchrotron Radiation Telescope and emittance scans at the ATLAS and CMS experiments.
“Having our monitor now fully integrated into daily LHC operations is a real ‘wow’ moment,” said Dr Hao Zhang, one of the leading experts in the team. “It is the culmination of years of development, from vacuum compatibility studies and optical design to software integration and on-site commissioning.”
With the BGC now approved as a permanent part of the LHC’s beam instrumentation, it paves the way for similar systems in other major research facilities, including the European Spallation Source in Sweden, the Electron Ion Collider in the USA, and even medical accelerator applications.
The research has been supported by the Science and Technology Facilities Council (STFC) through the High-Luminosity LHC (HL-LHC) UK programme and the LIV.DAT and LIV.INNO Centres for Doctoral Training, as well as the Cockcroft Institute.
“This achievement shows how university-based innovation can directly shape the tools that keep the world’s largest scientific instruments running,” added Professor Welsch. “It is a very proud moment for Liverpool and for all the students and researchers who contributed to this remarkable journey.”
Further information:
O. Sedláček et al., “Full-cycle, noninvasive emittance monitoring with the beam gas curtain monitor at the LHC”, Physical Review Research 7, 043144 (2025). DOI: 10.1103/5ggy-f8lm
Photographs and animation of the BGC functioning principle: https://hilumilhc.web.cern.ch/article/beam-gas-curtain-installed-large-hadron-collider