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 8 GHz and 50 K. We have employed a dielectric resonator to produce radio-frequency (RF) 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 future circular collider Hadron-Hadron (FCC-hh) at CERN We show that surface resistances in REBCO coated conductors without artificial pinning centers are more affected by a RF magnetic field than those containing nano-inclusions. Despite that, at 8 GHz, 50 K, and 9 T, most REBCO coated conductors studied outperform copper in terms of surface resistance, with the best sample having a 2.3 mΩ 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 16 T and 1 GHz, the actual FCC-hh dipole magnetic field, and the 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.
Highlights
We have employed a dielectric resonator to produce radio-frequency (RF) 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 future circular collider Hadron-Hadron (FCC-hh) at CERN We show that surface resistances in REBCO coated conductors without artificial pinning centers are more affected by a RF magnetic field than those containing nano-inclusions
Even though these previous works on coated conductors have shown that their small-signal surface resistance is significantly below that of Cu, there are other aspects related to their surface impedance that may compromise their use in the FCC-hh beam screens (BSs), [19,20,21,22,23,24,25]: its dependence on radio-frequency (RF) strength under large applied DC magnetic fields, and the appearance of temperature effects, including self-heating, resulting from the application of large RF fields
The results shown in figure 6(a) are for REBCO-CCs that contain nano-inclusions as artificial pinning centers (APC), as indicated in table 1
Summary
We calculate the expected azimuthal RF magnetic field strength HRF,FCC on the surface of the inner FCC-hh BS, which will be used as the reference value for the following experiments. A dielectric resonator (DR) cavity [26, 27] has been used to measure the quality factor Q0 as a function of applied RF power. To investigate the RF surface magnetic field strength dependency of the surface resistance, it is necessary to specify the relationship between the electromagnetic fields in the DR (equations (S1) and (S2) in the supplementary material (available online at stacks.iop.org/SUST/35/025015/mmedia)) and the VNA output power Pout This can be done by relating the stored energy W, the unloaded quality factor Q0, the dissipated power Pdiss, and the electromagnetic fields inside the resonator, which gives the peak surface RF magnetic field strength. Where G is the geometric factor of the upper (or lower) wall and RS is defined as the average surface resistance of the samples under test. The frequency sweep was set to be centered at the resonance frequency with a span factor ten times the 3 dB bandwidth
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