CERN Accelerating science

 16.2 T peak field reached in RMC racetrack test magnet
 by Luca Bottura, Juan Carlos Perez, Paolo Ferracin, Gijs de Rijk (CERN)
The RMC_03 racetrack test magnet. Credits: CERN

This September, experts from the CERN Magnet Group in the Technology Department celebrated the achievement of a 16.2T peak field in the Racetrack Model Coil (RMC). This is twice the nominal field of the LHC dipole and the highest field ever reached with this configuration. The result, which pushes forward existing boundaries for high-energy accelerators, is the product of a successful cooperation between several R&D programmes within the physics community.

Tested in the CERN SM18 vertical test station at a temperature of 1.9 K, the RMC, which consists of two racetrack coils,  trained up to a maximum current of 18.5 kA, within less than 5% of the projected critical current of the cable. Based on the calculation of the field, this current corresponds to peak fields of 16 T on the 33-turns Powder-In-Tube (PIT) coil and 16.2 T on the 35-turns Rod Restack Process (RRP) coil. Three major ingredients made this achievement possible. First, such high fields are only possible thanks to the use of Nb3Sn, a intermetallic and brittle compound which withstands a much higher magnetic field intensity compared to the previously-used Nb-Ti alloy.  Secondly, RMC uses new technologies that allow the coil to resist increasingly high electromagnetic forces. An example of this is the “bladder-and-keys” structure developed at LBNL (USA). The third and perhaps most important ingredient was the close relationship with European and overseas R&D programmes, which joined efforts and synergies to push through existing technology barriers.

The RMC result feeds in a larger objective shared by most high-energy accelerator projects: reaching high magnetic fields to permit higher beam energies – in the case of dipoles - or to squeeze the beam in the experiments, which is the case for high-gradient quadrupoles. RMC-type tests are now a part of the technology programme that supports the EU-funded EuroCirCol design study, which, in turn, is part of the Future Circular Collider study. This aims to be a conceptual design study for a post-LHC research infrastructure focuses on an energy frontier 100 TeV circular hadron collider. The test setup and measurement provide evidence for the feasibility of a 16 T dipoles based on low-temperature Nb3Sn superconductors. For this reason, the personnel in charge of setting up the testbed, working with industry and performing the test, are also working for the FCC 16 T magnet R&D programme.

RMC tests are a major step for many other R&D projects. In fact, they serve as technology support for the new High Luminosity LHC Interaction Region quadrupole QXF and for the 11 T dispersion suppressor dipoles. Finally, RMC is using wires and cables of the same class as those being used to build FRESCA2, a 13 T dipole magnet with a 100 mm aperture that will be used to upgrade the CERN cable test facility (FRESCA). FRESCA2 coils are currently under construction and will be ready for testing by summer 2016.