To 20 Tesla and beyond: the high-temperature superconductors

Prototype “Roebel” cable based on the high-temperature superconductor ReBCO (rare-earth barium-copper oxide) is being used to wind a demonstration accelerator dipole at CERN as part of the EuCARD-2 project. (Image: H Barnard/CERN)

Future high-energy accelerators will need magnetic fields of 20 Tesla and above. In order to achieve this level of performance, a new technological leap is required after niobium-titanium (NbTi) and niobium-tin (Nb3Sn) technologies have reached their practical performance limits. The magnets of the future will most probably be manufactured from high-temperature superconductors (HTS).

These materials are thus named because they exhibit superconducting behaviour at higher temperatures than niobium-titanium and niobium-tin, which are both known as low-temperature superconductors (LTS) and must operate at extremely low temperatures to reach and retain their superconductive state. In the LHC, the NbTi magnets are cooled to 1.9 Kelvin ( –271.3 °C) using liquid helium as a coolant. Above a critical temperature and above a critical magnetic field, the superconductors fail to maintain their superconductive state, and stop operating correctly (they are said to undergo a “quench”). This is very undesirable behaviour for a magnet, because it results in the appearance of voltage and a rapid temperature increase, requiring quick detection so that the current can be turned off.

High-temperature superconductors exhibit very different properties from those of classical LTS. “High-temperature superconductivity was discovered more than 30 years ago, but only recently has the community made major steps forward,” says Glyn Kirby, lead engineer for HTS development in the Technology department.

Not only can HTS conductors retain superconducting behaviour up to around 100 Kelvin, but they can also bear a magnetic field much higher than 20 Tesla, which is the main factor of interest for the accelerator magnets of the future. Because the critical temperature is so high, the material has a very large operating margin, which is beneficial to avoid quenching and to increase the reliability of the magnet.

“For the moment, this technology is significantly more expensive than niobium-titanium and niobium-tin superconductors,” says Gijs De Rijk, deputy group leader of the Magnets, Superconductors and Cryostats group in CERN’s Technology department. “However, the raw material is not the dominant factor in the overall cost of the conductors, meaning they will potentially become less expensive when manufactured in large quantities,” he adds.

To explore the use of high-temperature superconductors in high field accelerator magnets for future particle accelerators, in 2013 CERN partnered with a European particle accelerator R&D project called EuCARD-2. The project involved 40 partners from 15 European countries including CEA (FR), KIT (DE), University of Geneva (CH), University of Twente (NL) and Bruker HTS (DE). The aim of the project was to develop an HTS accelerator-quality demonstrator magnet, called Feather2, able to produce a standalone field of 5 Tesla, and between 17 and 20 Tesla when inserted into the Fresca2 high-field magnet. The first Feather2 magnet was built using an initial version of HTS conductor based on tapes of rare-earth barium-copper-oxide (generally referred as ReBCO). This was tested during the summer and achieved a standalone field of over 3 Tesla. The next magnet, based on high-performance ReBCO tape, is expected to exceed the 5 Tesla target by a significant margin, possibly approaching a field of 8 Tesla.

“Understanding and solving the issues related to the use of ReBCO wide tapes require novel approaches when it comes to cabling, coil-winding and magnetic field quality,” explains Kirby. “These require significant research and development before application in an accelerator becomes feasible, but HTS could revolutionise accelerator technology, and it is expected to have a major impact on other fields such as medical applications, magnets for high-field science, magnetically-levitated trains, space travel and fusion energy,” he concludes.

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Initial-development coil of new high-temperature superconductor (HTS) magnets based on ReBCO material (Image: Robert Hradil, Monika Majer/ProStudio22.ch)

This text is published on the occasion of the conference EUCAS 2017 on superconductors and their applications​.