Advances in the study of HTS superconductors for the beam impedance mitigation in CERN-FCC: the thermal runaway problem

In CERN Future Circular Collider (FCC-hh), a possible next-generation high-energy hadron-hadron collider, the center-of-mass collision energy will be of 100 TeV, with opposite proton beams of 50 TeV steered in a 100-km circumference tunnel by 16 T superconducting magnets. The synchrotron radiation, emitted by the beams, is absorbed by a beam-facing screen held at 50 K. The surface impedance of this screen has a strong impact on the beam stability, and copper at 50 K allows only tight beam stability margin. This has motivated investigating the possibility of high-temperature superconductors (HTSs) coatings on the beam screen internal surface, as a possible solution. In this communication, we will briefly review the general theory of the surface resistance of HTS in high field, low frequency regimes and will present specific calculations for REBCO commercial tapes that represent one of the possible envisaged solutions. The possible thermal runaway problems arising using REBCO tapes are then discussed. In particular, the upper limit for the transverse thermal resistance that guarantees thermal stability is quantitatively determined as a function of the REBCO superconducting properties at FCC operating conditions.