Evaluation of the nonlinear surface resistance of REBCO coated conductors for their use in the FCC-hh beam screen

Abstract

To assess the feasibility of using high-temperature superconductors for the beam screens of future circular colliders, we have undertaken a study of the power dependence of the microwave surface resistance in state-of-the-art REBCO coated conductors at about 8GHz and 50K. We have employed a dielectric resonator to produce radio-frequency electromagnetic fields on the surface of the coated conductors having amplitudes similar to those generated by proton bunches circulating in the vacuum chamber of the proposed hadron-hadron Future Circular Collider at CERN. We show that surface resistances in REBCO coated conductors without artificial pinning centers are more affected by a radio-frequency magnetic field than those containing nano-inclusions. Despite that, at 8GHz, 50K, and 9T, most REBCO coated conductors studied outperform copper in terms of surface resistance, with the best sample having a 2.3mΩ surface resistance while being subject to an RF field 2.5 times stronger than that in the FCC-hh. We also extrapolate the measured data to 16T and 1GHz, the actual FCC-hh dipole magnetic field, and mid beam frequency spectrum, demonstrating the possibility of lowering the surface resistance of the vacuum chamber by up to two orders of magnitude compared to copper. Further, we discuss the correlation between the time structure of the electromagnetic fields provided by vector network analyzers compared to the proton bunches' time structure in the collider and present the effect of low alternating magnetic fields on vortex displacement and the possibility of demagnetization of superconducting samples.

Evaluation of the impact of REBCO-coated conductors on the resistive wall impedance of the FCC-hh

“…This is due to the small dependence of the surface impedance on the applied magnetic field at low frequencies. This is not true at high frequencies, therefore the large difference observed between injection and collision at the single bunch frequency [8].…”

“…For the REBCOcoated conductors, we have assumed R s ∝ f 2 and X s ∝ f , for the surface resistance and surface reactance, respectively, estimated within the framework of the two-fluid model surface impedance of superconductors [13]. At 50 K and 8 GHz, we have used experimental data of R s = X s = 7.8 mΩ for Cu and R s = 0.23 mΩ and X s = 10 mΩ for REBCO-CC [8].…”

“…On the contrary, in the case of the REBCO-CC, the change in the surface impedance due to an externally applied magnetic field cannot be neglected. Table 1 summarises the values for REBCO-CC and Cu used in this study [6].…”

“…A hybrid structure made of HTS and Cu will be required to keep the magnetic field quality within specifications [3]. Extensive studies of its surface impedance at or close to the working conditions of the FCC-hh have been performed [4,5,6,7,8,9]. Recently the data obtained has been used to calculate the RW impedance of a round pipe fully coated with REBCO-CC [9].…”

Accurate Determination of Dielectric Properties in Small, High-Permittivity Dielectric Cylinders

“…Hakki-Coleman shielded dielectric resonators have been commonly used to measure the surface impedance of small superconducting samples. Nowadays, they are key in the determination of microwave properties in High-Temperature Superconductor-Coated Conductors (HTS-CC), which are being considered as possible substitutes of copper in the beam screen of the Future Circular Collider (FCC) (hadron collider) [1], [2], [3]. HTS-CC are currently available in tapes having several mm in width (typically 12 mm) and over hundredths of meters lengths.…”