High-temperature superconductors (HTS) may be a driver of the next technological evolution of particle accelerators and REBCO (rare-Earth barium copper oxide) is the material that currently holds the largest potential.
Thanks to the financial drive of the research in innovative fusion reactors, coated conductors (CCs) — thin ribbons with an HTS coating produced in kilometre-long lengths — are now widely available from many suppliers. While it is well known that HTS materials are very promising for developing high-field magnets, it is less known that they can play an important role in improving the performance of radio frequency (RF) systems.
An R&D programme to verify the use of HTS coating for beam screens was initiated in 2015 at CERN, in collaboration with several European Institutes [1], in the framework of the FCC-hh study. The success of this R&D programme has offered a wealth of new data on the RF properties of the novel REBCO coated conductors, together with the development of the technologies to apply the tapes to (almost) any surface (Figure 1). Currently, this approach is well ahead of dedicated coating technologies of REBCO on substrates of interest, so much so that with this technology in hand, new projects have quickly been identified.
The RADES (Relic Axion Detection Exploratory Setup) study, dedicated to the search of galactic halo axions, which is typically performed using copper RF cavities inside a strong magnetic field as sensors, rapidly saw the advantage that HTS cavities would bring, increasing the sensitivity of the detectors thanks to the higher conductance compared to copper, while working flawlessly inside a strong magnetic field (Figure 2).
The physics results of the first data-taking run with a cavity covered with HTS CCs were recently published. The prototype cavities are now routinely showing Q values a factor of more than four better than copper.
A similar technology was also adopted by the axion physics group CAPP in Korea, whose first HTS cavities also show large improvement factors over copper.
Leveraging CERN expertise in X-band RF technology, employed for example in the CLIC study, HTS is also being investigated for its potential for high-gradient applications. Indeed, HTS could be the basis of a “third way” between classical normal-conducting RF structures, typically made of copper, and traditional superconducting structures, typically made of niobium cooled at liquid-helium temperatures.
HTS structures could work at liquid-nitrogen temperatures, and if their power dissipation compared to copper is low enough as to compensate the inefficiency of the cryoplant that cools them down, one could find an ideal compromise in terms of wall-plug power consumption, which is a key discriminating factor for future accelerators. The inconvenience is that practically no data exist on the behaviour of HTS at high gradient RF.
A study [2] was thus initiated at CERN to explore the potential of HTS, under the aegis of the Linear Collider study and with funding from the I.FAST Innovation Fund that supports improving the sustainability of particle accelerator technologies. The study has also received the support of the SLAC National Accelerator Laboratory in the US, which has a high-gradient cryogenic test facility perfectly adapted for the purpose.
SLAC’s C3 study aims at a copper collider cooled at 77 K, for which further refinements in the power efficiency would be a natural development. Initial tests focused on a coated sample disk (Figure 3), either coated with tapes by CSIC-ICMAB or with a full coating process by the company CERACO, and were presented at the recent IPAC’24 Conference, displaying power losses 10-15 times better than copper.
These first low-gradient results encourage further pursuit of studies by rising the power levels and most importantly further developing the coating technique. KCT, another partner in the IIF-funded collaboration together with CERN and CSIC-ICMAB, will develop wider tapes to coat the entire surface of the test disk.
On-going work also includes coating of an RF cavity developed by SLAC and manufactured in segments, perfectly adapted to be coated with HTS tapes. The results from this endeavour are expected in the coming months.
[1] CSIC-ICMAB, UPC, ALBA, IFAE, CNR-SPIN, TU-Wien, Roma Tre University.
[2] “HIGHEST”, High-Temperature High-Gradient Superconductors. Institutes and partners: CERN, CSIC-ICMAB, KCT.