HTS coatings for impedance reduction of beam-induced RF image currents in the FCC

The FCC-hh presently under study at CERN will make use of 16 T superconducting dipoles for achieving 100 TeV p-p center-of-mass collision energy in a 100 km ring collider. A copper coated beam screen, like in the LHC, is envisaged to shield the 1.9 K dipole cold bores from the 28 W/m/beam of synchrotron radiation. Operating temperature should be in the 50 K range, as best compromise temperature in order to minimize the wall-plug power consumption of the cryogenic system. However, preliminary studies indicate that copper at 50 K might not provide low enough beam coupling impedance in the FCC-hh. It has then been proposed to reduce the beam impedance by a thin layer of a High-Temperature Superconductor (HTS), which will thus effectively shield the beam-induced RF image currents. Purpose of this paper is to define the basic requirements for an HTS film in the RF field induced by beam image currents and exposed to a high magnetic field, and to identify the best candidate materials and coating processes.