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

A la portada: logo Alba synchroton(English) 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. 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. 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 DROMS, 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. 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. 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.(Español) El Future Circular Hadron Collider (FCC-hh) es una propuesta para un acelerador de partículas de 100 kilómetros de longitud con energías de colisión significativamente superiores a los colisionadores anteriores para expandir la investigación del Large Hadron Collider (LHC). La elección de un material de revestimiento de baja impedancia superficial para el interior del tubo de vacio se ha vuelto de fundamental importancia para garantizar una impedancia de haz suficiente baja. Esto permite un funcionamiento estable del acelerador con corrientes de haz altas. Actualmente el cobre es el material de revestimiento de referencia. En comparación con eso, solo los superconductores de alta temperature en forma de conductores recubiertos con ReBCO (ReBCO-CCs) tienen una menor impedancia superficial. De esta manera, prometen mayores márgenes de estabilidad del haz a la temperatura esperada de funcionamiento (40 - 60 K). Mientras los estudios teóricos de superconductores de alta temperatura en condiciones FCC-hh han sido prometedores, no existen datos experimentales de conductores recubiertos con ReBCO sujetos a condiciones FCC-hh para confirmar estas predicciones. Esta tesis aporta dos contribuciones principales. Primero, presenta las condiciones experimentales diseñadas y construidas para medir la impedancia superficial de ReBCO-CCs en base a estructuras de resonadores dieléctricos de tipo Hakki-Coleman y placa paralela bajo campos magnéticos AC y DC, irradiación de fotones, todo ello a condiciones criogénicas temperaturas, para imitar el entorno del tubo de vacio en el FCC-hh. También desarrollamos DRΩMS, un software de adquisición de datos automatizado, y ARPE, un algoritmo de posprocesamiento para extraer con precisión los parámetros del resonador dieléctrico -frecuencia de resonancia y factor de calidadde los parámetros S del analizador de redes vectoriales. La segunda contribución consiste en los resultados de las mediciones experimentales de impedancia de superficie de ReBCO-CC de seis fabricantes distintos. Presentamos la impedancia superficial en función de la temperatura bajo fuertes campos magnéticos en el rango de los gigahercios. Complementariamente, analizamos el comportamiento no lineal de ReBCO-CCs en presencia de un campo magnético superpuesto a campos electromagnéticos de radiofrecuencia con amplitudes comparables a las generadas por haces de protones en el FCC-hh. Finalmente, estudiamos el efecto de la irradiación de fotones sobre la resistencia de la superficie in-situ y ex-situ, en la fuente del luz del sincrotrón ALBA. La extrapolación de nuestros datos a 16 T y 1 GHz demuestra la posibilidad de reducir la resistencia superficial del tubo de vacio de hasta dos órdenes de magnitud mediante el uso de ReBCO-CC en lugar de cobre, mientras que la reactancia superficial está en el mismo orden de magnitud. Concluimos que los ReBCO-CCs son canditatos adecuados para reducir significativamente la impedancia de la pared resistiva en el FCC-hh.Teoria del senyal i comunicacion

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

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

Coated conductor technology for the beamscreen chamber of future high energy circular colliders

The surface resistance of state-of-the-art REBa2Cu3O7-x coated conductors has been measured at 8 GHz versus temperature and magnetic field. We show that the surface resistance of REBa2Cu3O7-x strongly depends on the microstructure of the material. We have compared our results to those determined by the rigid fluxon model. The model gives a very good qualitative description of our data, opening the door to unravel the effect of material microstructure and vortex interactions on the surface resistance of high temperature superconductors. Moreover, it provides a powerful tool to design the best coated conductor architecture that minimizes the in-field surface resistance. We have found that the surface resistance of REBa2Cu3O7-x at 50 K and up to 9 T is lower than that of copper. This fact poses coated conductors as strong candidate to substitute copper as a beamscreen coating in CERN's future circular collider. To this end we have also analyzed the secondary electron yield (SEY) of REBa2Cu3O7-x and found a compatible coating made of sputtered Ti and amorphous carbon that decreases the SEY close to unity, a mandatory requirement for the beamscreen chamber of a circular collider in order to prevent the electron-cloud phenomenon.Peer ReviewedPostprint (author's final draft

Contactless electrical resistance of 2D materials using a rutile resonator

Measuring the electrical surface resistance of 2D materials without contact can provide a method for obtaining their intrinsic characterisation. The aim of this paper is to show that a rutile dielectric resonator (RDR) can be used to measure the electrical surface resistance of conducting coatings deposited on substrates, at the resonance frequency. Moreover, it is known that the substrate exerts a strong influence capable of intrinsically modify the properties of the 2D materials, as found in graphene. The RDR method is used for different samples of metals (Cu, Mo, Ti, brass), carbon nanotubes (bucky paper), a film of compacted graphene flakes, a film of compacted graphene oxide flakes and graphene obtained by CVD on different substrates (SiO2/Si, quartz and PET). The results show that reasonable values can be obtained for thin conducting materials with a thickness of not less than a few micrometers. In the case of graphene grown on a substrate, the presence of graphene is clearly detected but the resistivity value cannot be extracted.Peer ReviewedPostprint (author's final draft

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

Small footprint evaluation of metal coatings for additive manufacturing

Rutile resonators have been used in the past to characterize high-temperature superconductors. In this work we describe a novel use of this type of resonator: the characterization of surface resistance in metal coatings deposited on plastic substrates at 9.1 GHz. A very small sample surface (about 1 cm 2 ) is required due to the high permittivity of rutile. We describe the use of the resonator to analyze pairs of small samples and to analyze the homogeneity of a sample set diced from a larger metallized surface. We also show a novel experimental method to determine the geometrical factors in dielectric resonators.Peer ReviewedPostprint (published version

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

RF characterisation of laser treated copper surfaces for the mitigation of electron cloud in accelerators

In accelerator beam chambers and RF waveguides, electron cloud and multipacting can be mitigated effectively by reducing the secondary electron yield (SEY). In recent years, it has been established that laser surface structuring is a very efficient method to create a copper surface with SEY close to or even below unity. Different laser pulse durations, from nanoseconds to picoseconds, can be used to change surface morphology. Conversely, the characteristics that minimise the SEY, such as the moderately deep grooves and the redeposited nanoparticles, might have unfavourable consequences, including increased RF surface resistance. In this study, we describe the techniques used to measure the surface resistance of laser-treated copper samples using an enhanced dielectric resonator with 12 cm diameter sample sizes operating in the GHz range. The quantification basis lies in a non-contact measurement of the high-frequency losses, focusing on understanding the variation of surface resistance levels depending on the specifics of the treatment and possible post-treatment cleaning procedures

Coated conductor technology for the beamscreen chamber of future high energy circular colliders

The surface resistance of state-of-the-art REBa2Cu3O7-x coated conductors has been measured at 8 GHz versus temperature and magnetic field. We show that the surface resistance of REBa2Cu3O7-x strongly depends on the microstructure of the material. We have compared our results to those determined by the rigid fluxon model. The model gives a very good qualitative description of our data, opening the door to unravel the effect of material microstructure and vortex interactions on the surface resistance of high temperature superconductors. Moreover, it provides a powerful tool to design the best coated conductor architecture that minimizes the in-field surface resistance. We have found that the surface resistance of REBa2Cu3O7-x at 50 K and up to 9 T is lower than that of copper. This fact poses coated conductors as strong candidate to substitute copper as a beamscreen coating in CERN's future circular collider. To this end we have also analyzed the secondary electron yield (SEY) of REBa2Cu3O7-x and found a compatible coating made of sputtered Ti and amorphous carbon that decreases the SEY close to unity, a mandatory requirement for the beamscreen chamber of a circular collider in order to prevent the electron-cloud phenomenon.Peer Reviewe