Evaluation of the surface impedance of ReBCO coated conductors and requirements for their use as beam screen materials for the FCC-hh

<!--HTML-->The Future Circular Hadron Collider (FCC-hh) is a proposal for a 100-kilometre-long particle accelerator with collision energy substantially higher than previous colliders to expand the Large Hadron Collider (LHC) research.<br>The selection of a low surface impedance beam screen coating material for such a high-energy collider has become of fundamental importance to ensure sufficiently low beam impedance and guarantee stable accelerator operation at high beam currents. Currently, copper is the baseline coating material. In comparison to it, only high-temperature superconductors (HTSs) in the form of ReBCO coated conductors (ReBCO-CCs) have a lower surface impedance and would provide for higher margins of beam stability at the expected operating temperature (40 - 60 K). Whilst theoretical studies of HTSs under FCC-hh conditions have proved encouraging, no experimental data of ReBCO-CCs subject to FCC-hh conditions exists to confirm these predictions.<br>This thesis provides two main contributions: First, it presents the experimental set-ups designed and constructed to measure the surface impedance of ReBCO-CCs, on the basis of dielectric resonator structures of Hakki-Coleman and parallel plate type under AC and DC magnetic fields, photon irradiation, all at cryogenic temperatures, to mimic the environment of the beam screen in the FCC-hh. We also developed DR$\Omega$MS, an automated data acquisition software, and ARPE, a post-processing algorithm to accurately extract the parameters of the dielectric resonators -resonance frequency and quality factor- from the S-parameters of the vector network analyser.<br>The second contribution consists in the results from experimentally-measured surface impedance of ReBCO-CCs from six different manufacturers. We present surface impedance as a function of temperature and under strong magnetic fields in the gigahertz range. Complementary to this, we analysed the non-linear behaviour of ReBCO-CCs in the presence of a magnetic field superimposed on radio-frequency electromagnetic fields with amplitudes comparable to the ones generated by proton bunches in the FCC-hh. Finally, we studied the effect of photon irradiation on the surface resistance in-situ and ex-situ, at the ALBA Synchrotron Light Source.<br>Extrapolating from our data to 16 T and 1 GHz demonstrates the possibility of lowering the surface resistance of the beam screen by up to two orders of magnitude by using ReBCO-CCs instead of copper while the surface reactance is at the same order of magnitude. We conclude that ReBCO-CCs are suitable candidates to significantly reduce the resistive wall impedance in the FCC-hh.