CERN Accelerating science

A big step towards the superconducting magnets of the future

Last April, the FRESCA2 dipole magnet reached a field of 14.6T. This field value sets a new world record for dipole magnets with a free aperture, and breaks the old record established in 2008 of 13.8T by LBNL with the HD2 dipole magnet.

The development of magnets with fields beyond 10T started in Europe in 2004 with the FP6-CARE-NED project where the basic technologies were developed and specifically the Nb3Sn conductor which is the workhorse for the HL-LHC 11 T magnet, the LHC luminosity upgrade programme and baseline option for the more powerful 16T magnets for the Future Circular Collider study.

“FRESCA2 has already played an important role in the development of the new magnets for the High Luminosity LHC and will soon help develop the next generation of magnets." says Gijs de Rijk, head of the FRESCA2 programme.

The FRESCA2 dipole magnet design and construction was started in the framework of the FP7-EuCARD-HFM project in 2009 and has been co-financed by HL-LHC. The FRESCA2 magnet is much larger than a LHC magnet, measuring 1.5 m in length and 1 m in diameter. This allows the magnet to have a large aperture, measuring 10 centimetres, so that it can house the cables being tested, as well the sensors to monitor their behaviour. 

The FRESCA2 magnet before the start of the tests. (Image: Maximilien Brice/CERN). 

The magnet is the outcome of a successful collaborative effort between CERN and CEA-Saclay. The technology developments for FRESCA2 were essential for the new Nb3Sn magnets of HL-LHC. Formed by the superconducting niobium-tin compound and cooled to 1.9 kelvin (-271°C), it had already reached a field of 13.3 teslas in August 2017. Then, with a modification of the mechanical pre-stressing, it started a new series of tests in April before reaching its record intensity.

FRESCA2 will also be used to test coils formed from high-temperature superconductors. The goal is to test not only the maximum electrical current but also study in depth the effects of so high magnetic fields and the behaviour of the coil. Results from these measurements feed current efforts to design high-field magnets for future energy-frontier colliders. 

The magnet was tested to the nominal operating field, and achieved 13.3T in August 2017 after a very rapid training of 5 quenches. As a second step, the mechanical preload was increased and the magnet was retested in April 2018 to explore the ultimate operating limit. In this configuration FRESCA2 reached a maximum bore field of 14.6T at a temperature of 1.9K with additional 6 training quenches. The tests are currently being performed in the new purposely built test cryostat of the SM18 cryogenic test station at CERN.

This result is a major milestone in the progression towards high field accelerator magnets beyond HL-LHC. The future of FRESCA2 is to provide background fields for tests of cables and small coils, a new facility that will provide unique test capabilities.

Athena Papageorgiou Koufidou, Livia Lapadatescu (CERN)
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A new step towards successful MgB2 superconducting coils

On 7th March the INFN-LASA laboratory completed the construction and successfully tested a superconducting coil in MgB2 (Magnesium di-Boride based conductor) to be used in a high order corrector magnet. Development of MgB2 coils for a Round Coil Superferric Magnet corrector was launched in the framework of the HL-LHC project IR magnets. This design, proposed in the 70s by Russian scientists, allows to create any multipole with the same round coil through a three dimensional shaping of the iron; the low curvature radius of the coil allows using MgB2 superconductor. 

Application of this design to the HL-LHC high order correctors started in 2014 by our late lamented Giovanni Volpini with CERN support. Computations showed that this option had a lower efficiency in terms of longitudinal space, and therefore the classical superferric option with Nb-Ti and standard two dimensional iron shaping was retained; INFN-LASA built three correctors based on this design in the past two years, and two more types are being built in collaboration with industry. Nonetheless, the MgB2 RCSM prototype magnet has remained in the development line to explore the manufacturing aspects of this design, and to have a MgB2 corrector available for high energy accelerators, a prima for this technology.

A single coil, wound with conductor produced by Columbus Superconductors (Genova), was tested in LASA without the iron yoke; this coil is the active part of the RCSM magnet, which will be assembled and tested in September 2019. The coil, cooled at 4.2 K with liquid helium, reached without any training quench the specification current (“ultimate current”, 160 A), passing also the stability test of one hour at the ultimate current. The coil was then energized to larger currents to investigate the limiting current, which resulted in 243 A; this is 73% of the intersection of the load line with the critical current for virgin, not-degraded, conductor. It should be noted that this current limit was reached without intermediate quench (no training).

Image Credit: INFN/LASA

“The test result is beyond our expectations,” says Massimo Sorbi from INFN-LASA. “MgB2 virgin conductors are very prone to degradation when they are wound and manipulated with even better procedures used for the other common superconductor magnets based on NbTi. Our cryogenic test was also complemented with the measurement of the thermal contraction of coil (literature is lacking of this data regarding MgB2 coils), which will enable a better design of the mechanical structure for the final magnet.” “This is a relevant technological spin-off of the HL LHC project”, says Ezio Todesco, in charge of HL-LHC IR magnets, “enabled by the synergy between INFN-LASA and CERN”.

Image Credit: INFN/LASA

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