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ESS cavity milestones at STFC's Daresbury Laboratory

Experts in accelerator cavity testing are celebrating a milestone at STFC’s Daresbury Laboratory, with the receipt, testing, assessment and delivery of high-beta superconducting cavities destined for the European Spallation Source (ESS) in Sweden now underway.

A crucial part of the ESS jigsaw

When complete, the ESS will be a powerful neutron accelerator and will in effect be a giant neutron microscope, which will have huge implications for new medicines, technology and much more.

Built in Europe, the 88 high-beta superconducting cavities are a crucial part of the ESS.  The ESS will have 21 cryomodules, each containing four of these high-beta cavities. The cavities will determine how fast the protons will travel along the linear accelerator. For optimum results, the ESS team hope to accelerate the protons up to ~90% of the speed of light when it is brought in to action in a few years' time. In order to achieve this, the superconducting cavities will need to be in full working order and as clean as possible when they are fitted. This is where the work of STFC’s Daresbury Laboratory comes in.

Clean and efficient cavities

In order to test the cavities at STFC, a purpose-built vertical test facility called SuRFLab has been created. This includes a large cryostat housed in a well that is shielded by a concrete bunker. This elaborate-sounding set up allows special cavity inserts, which hold up to three cavities at a time, to be safely lowered in and cooled to less than 2 Kelvin using helium. Once they are cooled, a Radio Frequency (RF) amplifier is used to provide power to ensure the cavities can produce sufficiently high electric fields to accelerate the particle beam.

In the final accelerator, this beam of particles will be fired into a large rotating Tungsten target to generate an intense beam of neutrons. Each cavity needs to achieve the necessary accelerating voltage using a minimal amount of RF power. If the accelerating voltage is not achieved within RF power limits, the 1.2 m long, >90 kg cavity will typically require re-cleaning and preparing in a semi-conductor class cleanroom and re-testing. Once verified, all test and verification documentation is prepared and approved by STFC and ESS. The cavity is then ready for shipping to CEA Saclay, a French partner on the project, for them to integrate into final-stage accelerating cryomodules.

Each round of testing takes approximately two weeks for the three cavities in the test facility and another three simultaneously being prepared. A small number of cavities have also been tested at DESY,  a German accelerator centre and ESS partner, in parallel in order to mitigate schedule impacts; the STFC project team thank the DESY partners for their assistance. The team at Daresbury Laboratory is now on track to complete the testing and assessment of six high-beta cavities per month, until early 2023.

Shipping to their next stage of build

Once the cavities pass their rigorous tests, they are carefully shipped. Even the shipping has involved the design of specially made freight containers to ensure the cavities aren’t damaged or moved in transit as even small changes can affect the effectiveness of the vessels.

To date, ten qualified cavities – those which have completed the testing and assessment process – have been delivered to CEA Saclay. Of those, four are already being assembled into the first high-beta cryomodule at CEA Saclay and will be ready for final testing as a complete module by the end of the year.


Helium efficiency as standard

Cavity testing traditionally uses large amounts of liquid helium (LHe) to fully cool the cavities to their operating temperature. LHe is a finite and costly resource. The innovative new vertical test facility at Daresbury Laboratory has been designed to use up to 70% less LHe than traditional test facilities by using the existing cavity helium jacket as a more efficient method for cooling on the test stand. They also recycle the helium gas which is boiled-off during test, so that it can be reused. Each of these provide significant environmental and energy savings over the life cycle of the testing to be completed. Speaking about the work completed so far, Peter McIntosh, STFC’s Project Sponsor, said: “It’s fantastic to see how the incredible hard work of the Daresbury Laboratory team is having such a significant impact in enabling the earlier start of high-beta cryomodule assembly at CEA in France while ensuring the ESS project itself can achieve its ultimate energy expectations.”

An evolving facility

The test facility is based within Daresbury’s Superconducting Radio Frequency Laboratory (SuRFLab) and while it is a major project, there is much more still going on in the facility. New clean rooms and testing infrastructure are being designed for a proposed extension to the facility. This is in preparation for future work on the PIP-II project at Fermilab in the USA – another international proton accelerator project that will be accessing the strong expertise and capabilities within STFC’s Daresbury Laboratory for various critical work strands in the coming years.

To find out more about the ESS, and the science and research it will support, visit the ESS website.

To find out more about the PIP-II projects and the science and research it will support, visit the PIP-II website.

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Last updated: 01 November 2021


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