World’s first crabbing of a proton beam

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The first prototype crab cavities being assembled during summer 2017 (Image: Julien Ordan/CERN)

Experts from the QUASAR Group, working closely with colleagues from around the world at CERN in Switzerland, have contributed to a technology breakthrough that promises to revolutionise the world of high energy particle physics. The results achieved last week are a crucial step towards the future development of the world' highest energy particle accelerator, the Large Hadron Collider, and the further unlocking of the secrets of the universe.

Our very own Dr Lee Carver contributed to successful tests of a new technology called ‘crab cavities’. The measurements demonstrated that these cavities can be used to rotate a beam of protons. This is crucial for the enhancement of the LHC's performance. 

The world's first beam tests on 23 May 2018 lasted for more than 5 hours and were carried out at an operating temperature of 4.2 K with a single proton bunch accelerated first to 26 GeV and containing between 20 and 80 billion protons, almost the intensity of the LHC bunches. The crab cavities were powered to about 10% of their nominal voltage in these first tests. The “crabbing” was observed using a special monitor to observe the tilt along the length of the bunch. 

The crab cavity technology promises to revolutionise the world of high energy particle physics as the implementation of these cavities is critical to the delivery of the next level of energy and luminosity of the LHC, paving the way to increasing our understanding of exotic particles, such as the Higgs Boson. Liverpool makes key contributions to the High Luminosity (HL) upgrade programme of the LHC, including novel beam diagnostics R&D and beam dynamics simulations. The latter have proven instrumental for the successful measurements last week. 

STFC’s Executive Director of Programmes Professor Grahame Blair said: "This is a fantastic result for CERN and for all the collaborators involved.  I am particularly happy that UK university teams, working with staff at STFC national laboratories and with the international community, have made such a key contribution. This demonstrates the value of our world class university-based programme in accelerator physics working alongside our detector and analysis teams in mapping out fully the structure of nature at the LHC energy scale; every bit of luminosity will count in this endeavour."

The HL-LHC, which will be commissioned after 2025, will increase the luminosity of the LHC by a factor of five to ten. Luminosity is a crucial indicator of a collider’s performance: it gives the number of potential collisions per surface unit over a given period of time. In other words, the higher the luminosity, the higher the number of collisions and the more data the experiments can gather. This will allow researchers to observe rare processes that occur beyond the LHC’s present sensitivity level. Physicists will also be able to perform precise studies of new particles observed at the LHC, such as the Higgs boson. The newly developed crab cavities will play an important role to increase the luminosity. 

The UK is one of the main partners in HL-LHC with University of Manchester leading the UK collaboration and Lancaster University being UK project manager and key contributor to the original design of crab cavities. The QUASAR Group's research is led by Professor Carsten Welsch, who said: "This fantastic result will help make the LHC an even better accelerator in the future. It was made possible by close collaboration between UK institutions contributing to the High Luminosity upgrade and CERN."

In the coming months, the cavities will be commissioned to their nominal voltage of 3.4 million volts and will undergo a series of tests to fully validate their operation for the HL-LHC era. A total of 16 such cavities will be installed in the HL-LHC – eight near the ATLAS experiment and eight near CMS.