Accurate determination of dielectric properties in small, high-permittivity dielectric cylinders

We describe a method for performing cryogenic permittivity and loss tangent measurements of small (few mm), high-permittivity dielectric cylinders used in surface impedance measurement of superconductors. We combine the use of sapphire and PTFE supports to hold the dielectric under test and provide good thermal conductivity to the cold head. Additionally, the sapphire support is used to facilitate the measurement of the cavity's metal loss and the assessment of its contribution to the overall resonator loss.Authors wish to acknowledge the support provided by CERN under Grants FCC-GOV-CC-0072/KE3358 and FCC-GOV-CC-0210/KE4945. NT acknowledges MSCA-COFUND-2016-754397 for the PhD grant.Peer ReviewedPostprint (author's final draft

Potential impedance reduction by REBCO coated conductors as beam screen coating for the future circular hadron collider

The Future Circular Collider study creates a conceptual design for a post-LHC particle accelerator using 16T superconducting dipoles to achieve collision energies of up to 100TeV in a 90km circumference ring. A copper-coated beam screen, similar to that used in the LHC, is planned. However, the undertaken research indicates that copper at the high working temperature of 50K has a strong influence on the accelerator's performance, particularly at injection energy. In this work, we relate the experimentally determined properties of REBCO-coated conductors with their potential performance in the FCC-hh beam screen. Specifically, we use a round pipe approximation to demonstrate that a beam screen coated with a combination of REBCO and copper can have a much lower resistive wall impedance than one using only copper. The reduction is substantial (several orders of magnitude), and is observed in both the longitudinal and transverse wall impedance. Such a reduction has important effects on beam stability, operating costs, potential reduction in beam screen size, and lowering the stringent specifications of the 16T magnets required for the Future Circular Hadron Collider.The authors acknowledge the support and samples provided by Bruker HTS GmbH, Fujikura Ltd, SuNAM CO Ltd SuperOx, SuperPower Inc. and Theva Dünnschichttechnik GmbH. UPC funding was also provided through the Unit of Excellence María de Maeztu MDM2016-0600. N. Tagdulang acknowledges MSCA-COFUND-2016-754397 fot hte PhD Grant.Peer ReviewedPostprint (published version

Determination of pumping and dynamic vacuum properties of conductive quaternary alloy of TiZrVAg non-evaporable getter

Non Evaporable Getter (NEG) coating has been employed extensively in the particle accelerator especial-ly where the vacuum conductance of the vessel is se-verely restricted and ultra-high vacuum condition is required. NEG coating will significantly reduce the outgassing rate and provides active pumping surface for H2, CO and CO2. In addition, it has been proven that NEG coated surfaces have a very low secondary electron yield, as well as low photon and electron stimulated desorption yields. However, the existing NEG film increases the RF surface resistance of the beam pipe. In order to increase NEG coating conductivity, at ASTeC, in the past several years, the alternative NEG composition have been studied by adding more con-ductive element such as Cu, Au, Al and Ag. In this study, we report on the photon stimulated desorption, activation temperature and surface resistance from room temperature to cryogenic temperature for a new NEG quaternary alloy of TiVZrAg as function of the film composition.Peer ReviewedPostprint (published version

RF characterisation of new coatings for future circular collider beam screens

For the future high energy colliders being under the design at this moment, the choice of a low surface impedance beam screen coating material has become of fundamental importance to ensure sufficiently low beam impedance and consequently guaranteed stable operation at high currents. We have studied the use of high-temperature superconducting coated conductors as possible coating materials for the beam screen of the FCC-hh. In addition, amorphous carbon coating and laser-based surface treatment techniques are effective surface treatments to lower the secondary electron yield and minimise the electron cloud build-up. We have developed and adapted different experimental setups based on resonating structures at frequencies below 10 GHz to study the response of these coatings and their modified surfaces under the influence of RF fields and DC magnetic fields up to 9¿T. Taking the FCC-hh as a reference, we will show that the surface resistance for REBCO-CCs is much lower than that of Cu. Further we show that the additional surface modifications can be optimised to minimise their impact on the surface impedance. Results from selected coatings will be presented.Work supported by CERN under Grants FCC-GOV-CC-0210 (KE4945/ATS), FCC-GOV-CC-0209 (KE4946/ATS) and FCC-GOV-CC0208 (KE4947/ATS). ICMAB funding through RTI2018-095853-B-C21 SuMaTe from MICINN and co-financing by the European Regional Development Fund, 2017-SGR 1519 from Generalitat de Catalunya, and COST Action NANOCO-HYBRI (CA16218) from EU, the Center of Excellence award Severo Ochoa CEX2019-000917-S. UPC funding through the Unit of Excellence Maria de Maetzu MDM2016-0600. N. Tagdulang and A. Romanov acknowledge MSCA-COFUND-2016-754397 for the PhD grant.Peer ReviewedPostprint (published version

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

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.The authors acknowledge the support and samples provided by Bruker HTS GmbH, Fujikura Ltd, SuNAM CO Ltd SuperOx, SuperPower Inc. and Theva Dünnschichttechnik GmbH. This work was supported by CERN under Grant Nos. FCC-GOV-CC-0072/KE3358, FCC-GOV-CC-0153/KE4106 and FCC-GOV-CC-0208/KE4947/ATS. UPC funding was also provided through the Unit of Excellence María de Maeztu MDM2016-0600. N Tagdulang and A Romanov acknowledge MSCA-COFUND-2016-754397 for the PhD Grant. ICMAB authors acknowledge RTI2018-095853-B-C21 SuMaTe from MICINN and co-financing by the European Regional Development Fund; 2017-SGR 1519 from Generalitat de Catalunya and COST Action NANOCO-HYBRI (CA16218) from EU, the Center of Excellence award Severo Ochoa CEX2019-000917-S.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Performance of high-temperature superconducting REBCO coated conductors under synchrotron irradiation for future circular colliders

The influence of medium-to-high energy synchrotron radiation (SR) (≈10–100 keV) produced by the ALBA Synchrotron Light Source on state-of-the-art REBCO coated conductors (CCs) has been studied to assess the feasibility of using high-temperature superconductors for the beam screen of future circular colliders. Long-term irradiation studies were conducted with ex-situ surface resistance testing by using a dielectric resonator. In addition, a cryogenic test system was established for in-situ measurements of the critical temperature and surface impedance of REBCO-CCs during synchrotron irradiation, with intensities similar to or above those generated by proton beams circulating in the vacuum chamber in the future circular collider hadron–hadron design. It is shown that the SR impact does not introduce any macroscopic defects that permanently alter the critical temperature or surface impedance of REBCO-CCs. However, the most significant effect of SR is a transient increase in the REBCO’s surface impedance. This effect is likely caused by heat, as the material returns to its original impedance values once the radiation exposure stops. The correlation between the time structure of the SR and the possibility of suppressing the Cooper pairing mechanism is also discussed

Algorithm for resonator parameter extraction from symmetrical and asymmetrical transmission responses

We describe an algorithm capable of extracting the unloaded quality factor and the resonant frequency of microwave resonators from vector S-parameters. Both symmetrical (Lorentzian) and asymmetrical (Fano) transmission responses are supported. The algorithm performs an adaptive outlier removal to discard measurement points affected by noise or distortion. It removes the effects caused by imperfections in the device (such as modes with close resonance frequencies or stray coupling between the resonator ports) or the experimental setup (such as lack of isolation or dispersion in the test set and cables). We present an extensive assessment of the algorithm performance based on a numerical perturbation analysis and the evaluation of S-parameter fitting results obtained from network analyzer measurements and resonator equivalent circuits. Our results suggest that uncertainty is mainly caused by factors that distort the frequency dependence of the S-parameters, such as cabling and coupling networks, and is highly dependent on the device measured. Our perturbation analysis shows improved results with respect to those of previous publications. Our source code is written in Python using open-source packages and is publicly available under a freeware license.This work was supported in part by CERN under Grant FCC-GOV-CC-0210 (KE4945/ATS) and Grant FCCGOV-CC-0209 (KE4946/ATS) and in part by UPC funding through the Unit of Excellence María de Maeztu under Grant MDM2016-0600. The work of Nikki D. Tagdulang was supported by MSCA-COFUND-2016-754397.Peer ReviewedPostprint (author's final draft