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Qualifying HL-LHC magnets and cavities at Uppsala University

Qualifying HL-LHC magnets and cavities at Uppsala University
The Gersemi vertical cryostat. (Image: CERN)

Uppsala University in Sweden has been a valuable partner to CERN since the Laboratory’s foundation. In the 1950s, Uppsala, having just constructed its own cyclotron, contributed to the development of the very first accelerator at CERN, the Synchrocyclotron. In the 1980s, CERN assisted Uppsala in constructing a proton and heavy-ion accelerator and cooler-storage ring named CELSIUS and, in the mid-2000s, Uppsala assisted in the development and operation of the CLIC CTF3 test facility at CERN. Now, Uppsala University is upgrading its FREIA Laboratory, initially constructed for the ESS project, to test superconducting magnets and crab cavities for the HL-LHC.

Uppsala University established the FREIA Laboratory for instrumentation and accelerator development in 2011. It is equipped with a horizontal cryostat called Hnoss, a cryomodule test stand for superconducting cavities, and a vertical cryostat called Gersemi. In Nordic mythology, Hnoss and Gersemi are daughters of the goddess Freia.

A unique feature of Gersemi is its double functionality for both cavity and magnet testing. Cavities are tested in liquid helium at 2 K and sub-atmospheric pressure, while magnets are tested at 2 K and atmospheric pressure. Magnets create a magnetic field that can magnetise any metallic parts around the cryostat, such as reinforced concrete. Since superconducting cavities are very sensitive to magnetic fields, this puts substantially different requirements on the functionality of the cryostat in its two modes of operation.

An HL-LHC crab cavity being prepared for testing. (Image: CERN)

Gersemi uses different inserts for cavity and magnet testing, and has an active earth-magnetic-field compensation system to shield superconducting cavities, monitored by a prototype 3-axis magnetic sensor produced in collaboration with UK company Bartington Instruments Ltd.

The Gersemi vertical cryostat was installed and commissioned during 2019. During the summer of 2020, a first HL-LHC prototype crab cavity was sent from CERN, installed into Gersemi and cooled down to 2 K. An extensive testing period followed, supported under the EU-funded ARIES project Transnational Access scheme, in which the cavity reached an electric field of 4.6 MV. This was more than 1.2 MV above the nominal design value.

“We overcame a lot of issues and passed plenty of milestones, including mechanical, vacuum, cryogenics and radiation shielding issues,” said Akira Miyazaki, the Superconducting Radio Frequency (SRF) researcher responsible for the test. “We are now firmly on the starting line of the cavity business!”

Simultaneously, preparations for testing an HL-LHC orbit corrector magnet were ongoing. Two power converters and energy extraction units developed by CERN were sent to Uppsala and, on 23 June 2020, the first positive results were announced.

After completing the crab cavity test, the magnet was installed into Gersemi and cooled down, first to 4 K and then to 2 K. An extensive testing period was performed at both temperatures to commission the complete set-up for superconducting magnet testing. Many small and not-so-small problems had to be fixed, both on the cryostat hardware and on the testing hardware and software. On 1 April 2021, the system was finally ready for the first powering of the cold magnet at 4 K. Two weeks later, the magnet was cooled down to 2 K and successfully powered again. “After encountering difficulties for a few weeks, even months, I am happy to announce that a superconducting magnet has been powered for the first time in the FREIA lab,” said magnet test engineer Kévin Pepitone. “All systems responded as expected.” The LHC superconducting orbit corrector magnet was powered to a current close to the nominal current, and a field of 2.4 T was produced in Gersemi.

An LHC superconducting orbit corrector magnet being prepared for testing. (Image: CERN)

The successful commissioning of the new equipment at Uppsala establishes the FREIA Laboratory as an important complement to the SM18 test facility at CERN, in time for the testing of new HL-LHC components.

In addition to the current tests of superconducting magnets at Gersemi, Uppsala and CERN have started a new collaboration project that will use new manufacturing technologies to produce an innovative new type of magnet, a so-called canted-cosine-theta design. The basic idea, which consists of combining two solenoids slightly canted in opposite directions, originated in the 1960s. It is only nowadays with accurate computer-aided manufacturing that it has become feasible to industrialise it. Uppsala University and Linnaeus University will provide skills development to three participating companies in Sweden to develop the technology to manufacture the magnet. The goal is to develop a prototype magnet that, in the future, can replace existing dipole orbit corrector magnets in the LHC when they reach the end of their lives. A major requirement is to make it plug-in compatible with the existing orbit correctors, limiting the design choices of current, quench protection, overall dimensions and connections. The design work on the superconducting cable and magnetic layout has started. The powering tests of the magnet will be performed at Gersemi.

Preliminary design of the canted-cosine-theta magnet. Above, the winding of the superconducting wire. Below, the magnetic field strength, maximum 3.1 T. (Image: CERN)

This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under GA No. 730871.