Table of contents

The 2021 Joint Cryogenic Engineering Conference (CEC) and International Cryogenic Materials Conference (ICMC) events were held from July 19 through July 23 virtually, using the Whova platform. As at past conferences, the international scope of these meetings was strongly maintained with 23 countries being represented by 641 attendees who gathered virtually to enjoy the joint technical programs and industrial exhibits. In total, 224 papers were submitted for publication of which 215 are published in these conference proceedings.

The program for the joint conferences included a total of 429 presentations organized into 69 sessions - plenary, oral, poster and awards. Four plenary talks gave interesting in-depth updates and overviews on exciting topics with titles: i) "The Promise of Superconducting Quantum Information Processing", ii) "The Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE)", iii) Airbus SAS talk, "ASCEND - A First Step Towards Cryogenic Electric Propulsion for Aircraft?", and iv)) "ITER Cryogenic Systems - Scale, Complexity, and Innovation". Virtual attendance for each of the plenary talks was excellent from ∼ 140 to 280. The attendees also convened for three special Joint CEC-ICMC Symposia organized: i) Hydrogen Technologies for Transportation, ii) Superconducting Quantum Systems, and iii) SRF Materials and Systems. Twelve additional curated Focus Sessions were included in the programming, emphasizing high-quality invited talks. And this included 4 topics new to the conference: i) Topological Materials for Electronics, ii) 3D Printing Materials, iii) Mechanical Properties of HTS Wires and Cables, and iv) LTS and HTS Cables for Fusion. In addition to the Joint Session Hydrogen Technologies for Transportation with 4 Sessions and 9 talks, an extra Focus Session on Transportation was held again this year consisting of 6 Sessions and 35 talks. Contributed papers covered a wide range of topics including many aspects and advances in cryogenics and superconductors, along with their applications.

Full Preface information is available in the pdf.

Detailed Awards information is available in the pdf.

ICMC Board of Directors list is available in the pdf.

The ICMC Technical Editor list is available in the pdf.

The Acknowledgments list is available in the pdf.

List of Materials Index and Subject Index are available in the pdf

The Author Index is available in the pdf.

All papers published in this volume have been peer reviewed through processes administered by the Editors. Reviews (2 required per paper) were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

• Type of peer review: Single anonymous

• Conference submission management system: Morressier

• Number of submissions received: 57

• Number of submissions sent for review: 57

• Number of submissions accepted: 55

• Acceptance Rate (Number of Submissions Accepted / Number of Submissions Received X 100): 96.5%

• Average number of reviews per paper: 2.25

• Total number of reviewers involved: 59

• Format quality control: In addition to the technical reviews, all papers went through an initial quality control review process handled via the peer review system.:

• Contact person for queries:

Name: Annett Cady

Affiliation: Centennial Conferences

Email: cecicmc@centennialconferences.com

The tensile properties of XM-19 austenitic stainless steels with thick plates (30 mm thickness), extra-thick plates (100 mm thickness), and rectangular bars (square of 14.3 mm) were examined at cryogenic temperatures. Influence of thermomechanical treatments on the steels was demonstrated. Even though their 0.2 % proof stresses were over 1200 MPa at 4.2 K, there were variations in strength depending on location. Namely, the 1/2t specimen (the position of the half in the thickness) of extra-thick plate showed lower 0.2% proof stress, elongation, and reduction of area at 4.2 K than the 1/4t (the position of the quarter in the thickness) and the thick plates. The effect of grain size on the 0.2% proof stress was clearly described by the Hall-Petch relationship for the materials.

The effects of grain size and strain rate on the low-temperature tensile properties of ferrite-austenite duplex stainless steel were investigated. The coarse- and fine-grain specimens exhibited a grain size of approximately 20 and 8 μm, respectively. At 77 K, the fine-grain specimen exhibited a higher strength and elongation than the coarse-grain specimen. The work-hardening rate at 77 K in both specimens leveled off or increased slightly at late stage of deformation although the work-hardening rate was lower than the true stress. This characteristic work-hardening behavior is caused by the deformation-induced martensitic transformation of metastable austenite. Grain refinement stabilized austenite phase and enhanced the elongation of the material, resulting in the better low-temperature tensile properties of fine grain. The elongation decreased remarkably with an increasing strain rate at 77 K independent of grain size. The characteristic work-hardening was not detected at a high strain rate, indicating that the deformation-induced martensitic transformation did not occur. The strain rate affects the frequency of deformation-induced martensitic transformation, resulting in a change in elongation. Grain refinement effectively improves the low-temperature tensile properties of duplex stainless steels; however, these properties are strongly influenced by the strain rate, where a high strain rate causes a low elongation.

Friction stir welding (FSW) is a solid state joining process that uses the heat generated by the friction of a rotating tool and the base material to join materials together. Due to the fact that the material is never melted, and that extensive plastic deformation is introduced in the weld seam, a unique set of properties is achieved. The technique has been extensively used to join aluminium and aluminium alloys, but very few developments are reported on high strength austenitic stainless steel, which is the material of choice for many high energy physics and fusion magnets. This paper contains a comprehensive microstructural and mechanical characterization, including at cryogenic temperature, of an 8 mm thick high strength austenitic stainless steel plates. The steel grade is the high alloy version of AISI 316LN (identified as 1.4429 or -X2CrNiMoN17-13-3 according to European standards). Special attention was given to cryogenic elastic – plastic J – integral testing of the weld seam. To the authors' knowledge, this is the first time fracture toughness at cryogenic temperature on friction stir welded 1.4429 has been measured.

The purpose of this study is to investigate the change in dielectric performance in the direction of parallel to layers due to the difference in γ-ray irradiation temperature, irradiation atmosphere, and absorbed dose. γ-ray was irradiated using a ⁶⁰Co source at room temperature, in liquid nitrogen, and in liquid nitrogen temperature gas atmosphere in the absorbed doses of 0, 5, and 10 MGy, respectively, and the dielectric breakdown tests were conducted at room temperature using a high-voltage breakdown tester. To investigate the mechanism of the change in dielectric strength, the density and electron spin resonance (ESR) measurements of the specimens were carried out. After irradiation at room temperature, the dielectric strength increased slightly with the increase in the absorbed dose. The reason for this may be that the molecular density increased due to the promotion of cross-linking reaction by recombination of molecules and reaction of unreacted epoxy groups by γ-ray irradiation. After irradiation in liquid nitrogen, the dielectric strength decreased significantly at 10 MGy, while after irradiation in liquid nitrogen temperature gas atmosphere, the dielectric strength slightly decreased with the increase in absorbed dose. These results suggest that, at low temperature, the freezing of molecular motion limits the movement of radicals generated by irradiation, and that the decomposition reaction is dominant against the crosslinking reaction.

Epoxy-based composites exhibit mechanical compatibility at cryogenic temperatures. Owing to these properties, composites based on epoxy are used as electrical insulators in high temperature superconducting (HTS) power applications. However, the inevitable presence of voids in solid insulators, triple points, and airgaps at high-voltage conductor-insulator interfaces increase electric fields locally. The intensified electric field around these defects and interfaces is the main cause of partial discharge (PD), which is a dielectric challenge for numerous power applications including HTS cables. Recently, electret has been introduced as a promising solution to mitigate PD activities caused by voids and triple points. In this work, we intend to report the PD mitigation performance of electrets fabricated from epoxy resin, which is suitable for cryogenic power applications.

The interlaminar shear properties of epoxy impregnated coils are critical parameter to the design of the high field superconducting magnets. The existing test methods of the interlaminar shear strength of the composite insulation systems for the superconducting magnet coils in cryogenic temperature were discussed. The insulation system of the niobium titanium superconductor coated with woven glass fiber reinforced polymer coil were fabricated by vacuum pressure impregnation with IR-3 and CTD-101K resins. Short beam shear tests of the impregnated coil were performed at room temperature and liquid nitrogen temperature (77K). The results show that the interlaminar shear strength of the samples used IR-3 resin are better than CTD-101K both at room and cryogenic temperature. A detailed observation of the failed specimens was also investigated to verify the failure mechanisms by optical microscopy.

The study of the electrical properties of materials at cryogenic temperature is significant for the design of insulation structures of superconducting equipment. Here, epoxy resin was combined with a small amount of KH650 modified SiC nanoparticles. When the dosage of SiC nanoparticles increased from 0.1% to 0.3%, the DC breakdown strength of the composite increase from 83.93 kV/mm to 97.51 kV/mm at room temperature and from 90 kV/mm to 120 kV/mm at 77 K. Furthermore, we used differential scanning calorimeter (DSC) and dynamic thermomechanical analysis (DMA) to analyze the effect of low-content SiC nanoparticles on the thermal behavior of epoxy resin.

The epoxy resin is one of the critical components of large-scale superconducting magnets which find wide applications in magnetic confinement fusion, high energy accelerator and magnetic resonance imaging. In general, magnetic confinement fusion, high energy accelerator and magnetic resonance will be used at cryogenic temperatures. An approach with respect to real-time monitor of epoxy curing and cryogenic process is practically significant for magnetic reliability, maintainability as well as future mass production. Fiber Bragg Grating (FBG) sensors are excellent candidates of this measurement for their anti-electromagnetic interference characteristics and high sensitivity. The aim of this work is to use the embedded Fiber Bragg Gratings (FBG) to obtain real-time strain response of curing epoxy, and the strain change of the epoxy at cryogenic temperature from 4.2K to 300K. An FBG pre-tensioning device was designed to keep the FBG embedded in the epoxy in a stretched state during the epoxy curing process. Moreover, strain gauge sensors were also embedded to monitor the curing process at the same time. There are subtle differences in strain values obtained by the two sensors, which illustrate higher sensitivity of FBGs. This work verifies the reliability of FBG monitoring the strain response of epoxy during curing process as well as the cryogenic service.

For conduction-cooled superconducting magnets to be truly effective, it is necessary to have good thermal conduction from the cooling source through the insulating epoxy resin to the magnet winding. We have developed epoxy resins filled with thermally conductive nanoparticles that have significantly increased thermal conductivities while retaining most of their dielectric and mechanical strengths. Base resins, including CTD101K, were filled with alumina, aluminum nitride, or diamond nanoparticles. A wire-based 3ω method was developed to measure the thermal conductivity of the nano-filled epoxies over the temperature range from 4 K to 300 K. A unique aspect of this experimental setup is the 10 μm diameter Invar wire heating element, which serves as a sensitive thermometer down to 4 K. Measurements show the filled resins have improved thermal conductivity, especially at temperatures of 15 K and higher.

Superconducting magnets are critical components in particle accelerators and are used to generate and sustain the large magnetic fields needed for High Energy Physics programs. One significant issue with current epoxy insulated Nb₃Sn magnets is the long training process required before stable magnet performance can be realized. It is believed that training can be significantly reduced by addressing magnet quenching through improvements in the epoxy electrical insulation. In this work, two approaches for insulation modification have been undertaken: (1) addition of thermally conductive fillers to help with quench management and (2) development of insulation resins with high strain capability at cryogenic temperatures. This paper will discuss the characterization of these insulation systems to verify their performance prior to evaluation in subscale Nb₃Sn canted cosine theta accelerator dipole magnets.

For the operation of high-temperature superconducting (HTS) power cables in liquid nitrogen (LN₂) at high voltage levels, there is a need for reliable and cost-effective insulating materials. This article investigates the dielectric losses in poly-propylene laminated paper (PPLP- Sumitomo) which is used as a cold dielectric for HTS cables. To calculate the dielectric losses, an experimental setup was developed to measure the relative permittivity of PPLP at different temperatures from 65 to 300 K at various frequencies. Using an in-house developed experimental setup, the dielectric loss for different layers of PPLP, at various temperatures and operating frequencies was estimated to evaluate one of the criteria of cold dielectric material and the same was compared with the data at room temperature. The temperature was reduced to 65 K using a vacuum pump-assisted sub-cooling system. The effective relative permittivity increases marginally for subcooled LN₂ impregnated PPLP, however there is reduction in tan delta loss which in turn reduces the dielectric loss and is useful for high voltage HTS based AC cables. This paper provides a comparative study on PPLP performance variation with respect to operating temperature and frequency.

We have developed a test facility dedicated to thermal conductivity k(T) measurements at low temperature. These experimental devices allow the test of four samples simultaneously during each run. The measurements are performed using steady-state axial heat flow method with a careful control of heat leaks to the surrounding. The apparatus was successfully used for measuring k(T) (Temperature T in the range: 1.5 K - 10 K) of niobium and other materials used for the fabrication of Superconducting Radio Frequency (SRF) cavities. We have tested the following samples: 1) niobium samples from different supplier (as received or/and after various high temperature Heat Treatments (HT)), 2) thermally sprayed copper coatings. The resulting experimental data are presented and compared to the experimental results previously reported by other groups and theoretical values. As expected heat treatment have a strong impact on k(T) and in particular on the phonons peak at T∼2K. More precisely, HT @ 1200°C with Ti gettering improves the Nb Residual Resistance Ratio (RRR) by a factor of 3 and consequently k(T). Finally, the correlation between the niobium RRR and the thermal conductivity. at T=4.2 K is confirmed in good agreement with the Wiedemann-Franz law

In space cryogenic missions, the thermophysical properties of materials must be studied and measured to establish a highly reliable cooling chain. An Al-alloy is a primary material in cryogenic thermal structural designs and is particularly used for providing high thermal conductance of thermal shields and thermal straps. We measured low-temperature properties (the thermal conductivity and the electrical resistivity) of several Al-alloys and found that A6063 had a lower thermal conductivity than that reported previously. We also found that the Al-alloy ST-60, which has equivalent thermal conductivity as pure aluminum, has higher thermal conductivity than A6061 and A6063, and the material is a good candidate for the space cryogenic design at the temperature range between 4K and 300K. These results are critical for the thermal study of upcoming next-generation space astronomy missions, such as LiteBIRD, SPICA and ATHENA.

Propellant migration in microgravity is a critical challenge for in-space fuel storage and transfer. Origami fuel bladders are a new design solution to prevent cracking in polymeric materials within the cryogenic regime. This paper explores manufacturing techniques for producing origami bladders as well as the limits of possible thicknesses for origami bladders. Origami bladders are manufactured from polyvinylidene fluoride (PVDF) and polyethylene terephthalate glycol (PETG). Hand folded bladders follow a Kresling bellows origami pattern while the thermoformed samples are prepared using a mold of the Kresling bellows and an EZFORM LV 1827 thermoformer is used to shape the polymeric film. Compressive fatigue testing is carried out within liquid nitrogen to confirm the sample can survive deformation at cryogenic temperatures. Hand folded origami bellows specimens survived 3300+ cycles with minimal signs of degradation, while similar PVDF specimens cracked in fewer than 10 cycles.

Polymer-matrix composites play an important role as packaging material in superconducting equipment. The research on thermal conductivity of composites by combining computer numerical simulation and reference data can not only save the expenses but also shorten the period of material development. Herein, the internal heat flow of silicon carbide/aluminum nitride/Epoxy composites was simulated by COMSOL finite element software. And the thermal conductivity calculated by the finite element software is compared with the existing thermal conductivity mechanism. Meanwhile, the influence of the fraction, gap distance, interfacial thermal resistance, etc. on the thermal conductivity of composites was analyzed. In short, it can provide a reference for the thermal conductivity design of polymer composites.

Thermal insulation plays a major role in the use of cryogenic systems. Due to its superior thermal insulation property, Multi-layer insulation materials (MLI) are widely used in various cryogenic systems, such as superconducting magnets, space propellant storage, satellites and so on. Thus, it is crucial to estimate and measure the insulation performance of it. This paper has carried out a numerical simulation of the performance of one kind of MLI through a modified Layer-by-Layer model. The thermal insulation propriety of the test specimen is also measured by a newly designed measurement system, which is based on the steady-state axial heat flux method. Finally, the simulation and test results are compared and discussed.

Niobium Tin cable is being used in the high field magnets in the Divertor Tokamak Test Facility under construction by ENEA in Italy. Kiswire Advanced Technology was selected to provide 55 tonnes of high-performance Niobium Tin wire for the 18 Toroidal Field Coils. The conductor design is based on a 0.82mm Nb₃Sn strand similar to that provided for the ITER TF coils. H. C. Starck Solutions provided the pure Nb rod and Ta sheet for their conductor design. The enhanced strand requirement has both a minimum current (320 A at 12 Tesla) and maximum hysteresis loss (± 3T cycle 1000mJ/cc) and a Cu/Non-Cu of 1:1. To achieve the current and hysteresis loss reliably, the niobium rod must maintain its shape throughout the drawing process. The n value requirement (>20) is a good indication of niobium filament uniformity while the RRR requirement (>100) is a good indication of tantalum barrier performance. The HCSS rod supplied to KAT provided consistently high Ic (>320A at 12 Tesla), low hysteresis loss and the Ta sheet provided very good RRR values. We report here the consistency of more than 11 tonnes of HCSS Nb rod properties for chemistry, hardness, grain size and wire performance.

The ITER project is ongoing in France, and it is expected that a lot of high energy neutrons will be generated, and some neutrons will reach to superconducting magnets. To keep the safe and stable operation of the superconducting magnets, the neutron irradiation data must be piled up and the mechanisms of the irradiation effect must be investigated. A 15.5 T superconducting magnet and a variable temperature insert were installed in a radiation control area at Oarai centre in Tohoku University, and the superconducting properties of the irradiated Nb₃Sn wires have been evaluated. The critical current was improved once and drastically dropped after irradiation of 10²³ n/m², and the critical temperature degraded monotonically, and the critical magnetic field decreased around 17 T after the irradiation of 10²³ n/m². The mechanism of the irradiation effect was discussed based on a flux pinning concept.

In the push to develop high power electric aircraft, superconducting technology promises to significantly reduce mass and volume of motors and generators. However, challenges related to AC-loss and thermal management are a significant factor in preventing the proliferation of aerospace superconducting technologies. Increasing the resistance of the metal matrix stabilization has only gone so far in reducing coupling currents for higher frequency applications. In this research, Multiphysics simulations of a single composite filament were used to investigate stability decreases when using very high thermal conductivity electrical insulator (CsI) or metal-to-insulator transitioning material (V₂O₃) to replace the slightly resistive metal matrix typically used for a low AC loss MgB₂ composite wire. The insulators separate the MgB₂ filaments entirely, only allowing transient current sharing to occur with the high purity Nb diffusion barrier or with the metallic state V₂O₃. These simulations show that for these very low AC-loss composites at 20 K, instability will become a major issue due to reductions in current sharing. With higher electrical conductivity metal-to-insulator materials, higher thermal conductivity impregnation materials, and thicker metallic diffusion barriers it may be possible to find a reasonable balance between AC-loss and stability.

The performance and cost of Bi-2212/Ag wire is limited by the large area fraction of Ag matrix (∼3:1) that is required in the conventional oxide-powder-in-tube fabrication process. An alternative process is being developed in which fine-powder Bi-2212 is uniaxially compressed to form a bar tetrahedral bar with a Ag-alloy foil cladding. The fine powder naturally textures under compression (aligns the a-b planes perpendicular to the direction of compaction). Earlier work demonstrated that a symmetric billet of trapezoidal-cross-section textured-powder (TP) bars draws conformally so that the local area ratio is preserved.

The BaZrO₃/YBa₂Cu₃O₇ (BZO/YBCO) interface has been found to affect the vortex pinning efficiency of one-dimensional artificial pinning centers (1D-APC) of BZO. A defective BZO/YBCO interface due to a lattice mismatch of ∼7.7% has been blamed for the reduced pinning efficiency. Recently, we have shown incorporating Ca_0.3Y_0.7Ba₂Cu₃O_7-x spacer layers in BZO/YBCO nanocomposite film in multilayer (ML) format can lead to a reduced lattice mismatch ∼1.4% through the enlargement of lattice constant of YBCO via Ca diffusion and partial Ca/Cu replacement on Cu-O planes. In this work, the effect of this interface engineering on the BZO 1D-APC pinning efficiency is investigated at temperatures of 65-81 K through a comparison between 2 and 6 vol.% BZO/YBCO ML samples with their single-layer (SL) counterparts. An overall higher pinning force (F_p) density has been observed on the ML samples as compared to their SL counterparts. Specifically, the peak value of F_p (F_p,max) for the 6% BZO/YBCO ML film is about ∼ 4 times of that of its SL counterpart at 65 K. In addition, the location of the F_p,max (B_max) in the ML samples shifts to higher values as a consequence of enhanced pinning. For the 6% BZO/YBCO ML sample, a much smaller "plateau-like" decrease of the B_max with increasing temperature was observed, which is in contrast to approximately linear decrease of B_max with increasing temperature in the 6% SL film. This result indicates the importance of restoring the BZO/YBCO interface quality for better pinning efficiency of BZO 1D-APCs especially at higher BZO doping concentration.

Microstructural analysis of the BaZrO₃ (BZO)/YBa₂Cu₃O₇ (YBCO) interface has revealed a highly defective and oxygen deficient 2-3 nm thick YBCO column surrounding the BZO one-dimensional artificial pinning centers (1D-APCs). The resulting semi-coherent interface is the consequence of the ∼7.7% BZO/YBCO lattice mismatch and is responsible for the low pinning efficiency of BZO 1D-APCs. Herein, we report an interface engineering approach of dynamic Ca/Cu replacement on YBCO lattice to reduce/eliminate the BZO/YBCO lattice mismatch for improved pinning at a wide angular range of the magnetic field orientation. The Ca/Cu replacement induces a local elongation of the YBCO c-lattice near the BZO/YBCO interface, thereby ensuring a reduction in the BZO/YBCO lattice mismatch to ∼1.4% and a coherent BZO/YBCO interface. This has resulted in enhanced pinning at B//c-axis and a broad angular range of B-field orientation. For example, the 6 vol.% BZO/YBCO film with interface engineering exhibits F_p ∼158 GN/m³ at 65 K and B//c-axis, which is 440% higher than the ∼36.1 GN/m³ for the reference 6% BZO/YBCO sample, and enhanced J_c and F_p in a wide angular range up to ∼ 80°. This result illustrates a facile scheme for engineering 1D-APC/YBCO interface to resume the pristine pinning efficiency of the 1D-APCs.

The cost and complexity of large, high-field superconducting magnet modules and related subsystems comprise 30% to 60% of the fusion reactor core capital cost. The strategic plan for the U.S. burning plasma research, the Fusion Energy Sciences Committee Report (FESAC) "Power the Future: Fusion and Plasmas'', and 2021 NASEM report "Key Goals and Innovations needed for a U.S. Fusion Pilot Plant" recommends that the U.S. pursue innovative science and technology to enable construction of a Fusion Pilot Plant (FPP) that produces net electricity from fusion at reduced capital cost. To achieve this, a novel combination of lower-cost high temperature superconductors (HTS) in cable configurations with co-wound reinforcement for higher current density are being investigated using a simplified construction strategy to produce compact stable coils. They would be capable of generating 20 T at up to 10-20 K. Small-scale, inexpensive test coils and prototypes will help develop each feature and validate cabled conductor design models. The near term goal is to validate engineering approaches, scientific models and fabrication capabilities applicable to fusion reactor development such as U.S. fusion nuclear science facility (FNSF), sustained high-power density tokamak facility (SHPD) and FPP designs. The design options include lower-cost, high-strength, quench resistant REBCO or Bi-2212 cables in an all metal coil design that simplifies HTS coil construction and quench protection system, with co-wound reinforcements that integrate stress management in HTS cable design and provides thermal mass to help prevent quench damage.

Liquid Nitrogen (LN2) is used as the cryogen and is part of the dielectric insulation system in most high temperature superconducting (HTS) power cables. However, in electric propulsion systems for transportation, using LN2 is not feasible due to asphyxiation concerns in the event of a leak in a confined space. Gaseous helium (GHe) has been proposed as an alternate cryogen to relieve this asphyxiation concern. GHe has benefits such as a wider operating temperature range and versatility for a centralized cryogenic cooling system for multiple devices which could potentially reduce the overall weight of the superconducting system. The drawback is that GHe has a lower dielectric strength compared to LN2, which limits its use to low-medium voltage applications. A hybrid cryogen HTS cable was proposed using LN2 as the dielectric media and GHe as the cryogen of an HTS cable to solve the limitations and issues associated with each cryogen and explained in this paper. A 1-m prototype cable that used LN2 as the cryogen and dielectric was fabricated and dielectric breakdown measurements were performed on it to establish a baseline on what to be expected from the hybrid cryogen HTS cable.

Large electric transportation systems such as electric aircraft and electric ships have been designed with multiple high temperature superconducting (HTS) power cables as part of superconducting power distribution network, and a good electric insulation for the power cables is a critical design requirement needed to be considered. This is because terminations in HTS cable represent a location of significant electrical and thermal stresses and needs to be addressed to reduce the substantial heat load that might be introduced into the cryogenic system. The use of a warm dielectric termination is seen as a potential solution to reduce the electrical standoffs required as part of the design and allow for a high-power dense termination design. A warm dielectric termination design using commercially off-the-shelf ceramic breaks as electrical breaks is explored and withstand and partial discharge measurements are performed to determine the viability of the design concept.

For practical operations of high-temperature superconducting (HTS) devices, we have proposed a monitoring and diagnosing system to detect the degradations of superconducting properties before normal transitions occur locally in the HTS winding. The proposed system estimates the distributions of critical current indirectly from measurement results of the active power components of the Poynting vector around the monitoring HTS devices conducting ac currents by means of pick-up coils pairs mounted surrounding the HTS devices. For this purpose, it is necessary to solve the inverse problem. In order to estimate the critical current distribution, therefore, it is first necessary to be able to calculate the measurement signal distribution from the critical current distribution of the monitoring coil. This paper first proposes the calculation method of signal distributions measured by our system, next reports the results of the experiments and analysis, which were carried out to verify the validity of the calculation method. A pancake coil wound with Bi-2223 multifilamentary tape exposed to a uniform AC magnetic field was assumed as a monitoring coil in the experiments and analyses. The good agreement between the measured signals obtained from the experiments and the numerical analysis demonstrates the effectiveness of the proposed calculation method of measured signals.

Design, fabrication, and testing of cryogenic cables with thermal performance considerations is presented in this work. Cables were designed with commercial off-the-shelf materials for low-GHz operation in a small form factor. Thermal and microwave simulations were performed during the design of cables. Fabrication processes were developed to create features in Cu-cladding and to successfully deposit thin-film metal in large step-height regions. Post-fabrication processes such as laser machining and connector assembly were used to cutout and interface with fabricated cables. Microwave performance was measured at room temperature and at 4.2 K with comparable results to simulations performed. Cables designed and fabricated in this work provide a solution to thermal and spatial limitations while achieving targeted microwave performance at cryogenic temperatures.

This paper presents the lessons learned from tests of first module of superconducting cryogenic bypass line (BPL), a part of the international Facility for Antiproton and Ion Research (FAIR) SIS100 cryogenic system, currently under construction in Darmstadt, Germany. Design, manufacturing, and installation of the superconducting cryogenic bypass line is a part of a Polish in-kind contribution to the FAIR project, realized by the Wroclaw University of Science and Technology. The main goal of the tests was to check the superconducting, Nuclotron type busbar system containing four pairs of busbars, transferring 13.2 kA pulsing current with the ramp rate of 28 kA/s. The mechanical stability of the busbars, especially at the connection region, was investigated with the use of vibration sensors and cameras located inside the vacuum space. The tests revealed insufficient mechanical stability of the busbars in the connection area due to pulsing Lorentz forces, and necessity of additional supports and clamps. Results of the tests were presented and discussed. The conclusions can be significant not only for the bypass line design, but also for design of the busbar connections in the superconducting magnets.

A high-field winding can be fabricated from a cable of non-insulated REBCO tapes stacked face-to-face without twisting. If the cable is oriented within each turn of a winding so that the tape face is closely parallel to the magnetic field at its location, the supercurrent capacity of that cable is enhanced ∼3x greater than in a transverse or twisting orientation. This concept for a conformal winding was presented in a previous paper pertinent to the body winding of a REBCO based high-field dipole. Strategies are presented and simulated for how the same orientation can be sustained in the flared ends of a high-field hybrid dipole.

Conductor on a rounded core (CORC^®) cables with current densities beyond 300 A/mm^-2 at 4.2 K, and a capacity to retain around 90 % of critical current after bending to a diameter of 3.5 cm, make them a strong candidate for high field power applications and magnets. In this paper, we present a full 3D-FEM model based upon the so-called H-formulation for commercial CORC^® cables manufactured by Advanced Conductor Technologies LLC. The model presented consists of tapes ranging from 1 up to 3 SuperPower 4mm-width tapes in 1 single layer and at multiple pitch angles. By varying the twist pitch, local electromagnetic characteristics such as the current density distribution along the length and width are visualized. Measurements of macroscopical quantities such as AC-losses are disclosed in comparison with available experimental measurements. We particularly focused on the influence of the twist pitch by comparing the efficiency and performance of multiple cables, critically assessing the optimal twist pitch angle.

A better understanding of the interaction between three phases is required when developing superconducting cables for high voltage AC systems. With a particular focus on the energy losses of real power transmission cables, in this paper we utilize the so-called H-formulation of Maxwell equations to devise a 2D model for superconducting triaxial cables. The major aim of this model is to comprehend and reproduce the experimental observations reported on the first triaxial prototype cable developed by SuperOx and VNIIKP. The computationally modelled and prototyped cable is made of up to 87 tapes of 4 mm width SuperOx tape arranged across the three phases. Our computational results are compared to the experimental measurements performed by VNIIKP with the electrical contact method, showing a high degree of accuracy over the outer phase of the cable, whilst revealing technical issues with the experimental measurements at the inner phases. Thus, in consultation with VNIIKP it has been concluded that for the actual experimental measurement of the AC losses at the inner phases, and consequently of the overall cable, a sophisticated calorimetric setup must be built. Still our model is capable to provide an independent assessment of the VNIIKP-SuperOx cable design, by investigating the magnetic profiles per phase in the time domain. In this sense, we confirm that the unbalanced arrange of currents and distancing between the phases affirmatively lead to no magnetic leakages, and therefore to an adequate balance of the cabling inductance.

Structural Finite Element Analysis was performed to study the delamination behaviour of layer laminated REBCO tapes using a Solid-Shell element under various loading conditions, including (1) transverse tensile delamination, (2) shear delamination and (3) peeling delamination. A Solid-Shell element is well suited to simulate REBCO tape because the material and thickness of all the layers can be specified within a single element. The results obtained from the models utilizing Solid-Shell elements are compared to a more computationally intensive method known as Cohesive Zone Method (CZM), which has successfully modelled the initiation and propagation of REBCO's delamination [1]. The results show that the stress state of REBCO tape in transverse tensile and shear directions can be accurately captured by the proposed method employing Solid-Shell elements if the transverse tensile and shear delamination stress is measured experimentally for a specific REBCO and used as failure criteria. In this work, the failure criteria adopted from previous experimental studies are 50 MPa in the transverse tensile direction and 10 MPa in the shear direction. This work provides a time-efficient modelling tool to predict and mitigate delamination in REBCO tapes used in complex systems.

High temperature superconducting (HTS) magnetic bearing can operate in the harsh environment of low temperature and negative pressure with low loss and self-stability. Therefore, it has been gradually applied in flywheel energy storage, cryogenic liquid pump, cold compressor and some other cryogenic rotating machinery. The load-carrying performance of HTS magnetic bearing is affected by the cooling temperature of HTS directly. In this paper, the linear model is used to modify the temperature dependence to evaluate the influence of cooling temperature on load-carrying performance quantitatively, and the results are analyzed and displayed from the aspects of maximum levitation force, levitation stiffness and hysteresis effect. The research results of this paper have practical significance for the design and safe operation of HTS levitation system.

If a cold source such as liquid H₂ or cryogenic fuel is available, cryogenic and high-temperature superconductivity technologies are promising to significantly increase performance of electric propulsion systems. A first study for aeronautic applications shows that the power density of electrical components could be multiplied 2 to 3 times to reach more than 30kW/kg for electric motors and power electronics, the weight of cables can be considerably reduced, and the efficiency increased by more than 50% with a voltage between below 500 V for multi-megawatt applications. With ASCEND, AIRBUS intends to demonstrate the potential and feasibility of a cryogenic and superconducting powertrain to breakthrough aircraft electric propulsion performance.

The paper is a review of the opportunities and challenges of cryogenic power devices of electric aircraft, and the ongoing research and development efforts of the government agencies and the industry. Liquid Hydrogen (LH2) and Liquefied Natural Gas (LNG) are compared to support high temperature superconducting (HTS) and normal metal devices, respectively. The power devices were assumed to operate at the normal boiling point of the fuel used. The efficiencies of the electrical devices are estimated based on state-of-the-art technology. The mass flow rates and total fuel requirements for both the cryogenic fuels required to maintain the operating temperatures of the devices were simulated using thermal network models. A twin-aisle, 300 passenger aircraft with a 5.5 h flight duration was used for the models. The results show that the required masses of LH2 and LNG are 744 kg and 13,638 kg, respectively for the cooling requirement. The corresponding volumes of LH2 and LNG required are 9,760 and 30,300 L, respectively. In both cases, the estimated mass of the fuel needed for the aircraft is more than what is needed to maintain the cryogenic environment of the power devices. It was concluded that an electric aircraft with LNG cooled normal metal devices is feasible. However, an aircraft with HTS devices and cooled with LH2 is more attractive if the ongoing R&D efforts on HTS devices and LH2 infrastructure are successful. The emission reductions would be substantially higher with LH2, particularly when H2 is produced using renewable energy sources.

With the advancements in Electric Vehicle (EV) technology, more and more EVs are entering into service and simultaneously the number of charging stations are also increasing. These charging stations are connected to the EVs for a small duration of time. Hence, as the number of EVs increases, the simultaneous charging of multiple EVs generates a peak active power demand for this duration. The available peak load compensation technologies such as fywheel storage, pumped hydro storage or Battery Energy Storage System (BESS) take a long time to respond and are not very efcient. With an operational efciency exceeding 90 % and quick reaction time (< 1 second), a Superconducting Magnetic Energy Storage (SMES) system can be a viable solution for this scenario. A SMES system generally consists of a superconducting coil system being charged during the low load/no load period and discharged during the peak demand, with the help of a power conditioning unit (PCU). This paper describes the development of various sub-systems of the SMES-PCU, such as a three-phase rectifer, bidirectional chopper unit, three-phase inverter and the controllers with a description of various modes of operation and sizing of components. The paper also includes the simulation of overall SMES-PCU with suitable assumptions, to present the operational characteristics of the integrated system.

In the framework of the German project TELOS (Thermo-Electrically Optimised Aircraft Propulsion Systems) a high-temperature superconducting 40 MVA DC demonstrator busbar for hybrid-electric propulsion systems for aircraft has been developed. The design current for a temperature below 25 K is 13.3 kA and the rated voltage is 3 kV. The 2-pole busbar contains 2 stacks of REBCO coated conductors which are supported by a 3D-printed structure allowing compensation of thermal length changes of the superconductor. It fits in a cryostat tube with an inner diameter of 25 mm. A special focus has been put on low-resistive joints that are necessary to connect single elements of the busbar system. The special layout of the joints allows an effective current redistribution between the different tapes in a stack. We present results for the test of the DC busbar demonstrator in liquid nitrogen at 77 K. The design current for this temperature is 3.3 kA which corresponds to a rated power of 10 MW. We applied currents up to 3.5 kA and measured the I-V characteristics and contact resistances of 90° and 180° joints in a virgin and in a strained state thus simulating thermal length changes. We also present results of Lorentz-Force tests with short AC current pulses up to 20 kA to demonstrate the viability of the design for application with currents up to 13.3 kA.

The High Temperature Superconducting (HTS) synchronous motors offer considerably higher torque and power densities along with higher efficiencies for strategic and critical industrial applications. The most widely explored airgap based topology for HTS synchronous motors has conventional copper windings in stator and HTS pole coils in the rotor. The shape of these rotor pole coils affects various electrical parameters of the motor. In this paper, a detailed investigation has been carried out on effect of shape of HTS pole coil on airgap magnetic field and rated open circuit characteristic (OCC) of an 8 MW HTS synchronous motor along with its other parameters like pole coil excitation current, peak magnetic field inside the motor, maximum perpendicular magnetic field on HTS tape and operating temperature of HTS pole coils through FEM based electromagnetic analysis. Based on the results of electromagnetic analysis, the optimum shape of HTS pole coil has been finalised.

Liquid nitrogen is the foremost coolant when it comes to cooling HTS power devices due to its abundance and since it is more economical than other cryogens. Sub-cooled LN₂ is preferred over saturated LN₂ as it ofers substantial advantages in terms of performance. However, in the cases where it is not possible to cool HTS power cables using sub-cooled LN₂, saturated LN₂ is the only option. Additionally, even when sub-cooled LN₂ is used for long length cables, the heat-in-leaks along with other core losses result in rise in temperature and an unavoidable two-phase fow afects the cooling characteristics of the cryogen. This paper deals with the modeling and numerical analysis of a two-phase fow through an HTS power cable cryostat to understand the temperature profle, the pressure drop per unit length, and required pumping power for circulation of cryogen.

The increasing need for higher power density and closer integration of power subsystems consisting of superconducting motors/generators, degaussing coils, energy storage modules, and cables leads one to consider the merits of refrigerating the associated power electronics down to cryogenic temperatures. High temperature superconducting (HTS) components combined with cryogenic power converters resulting in high power density power conversion systems will have a significant effect on several industrial, commercial, transportation, and renewable energy applications. Cryogenic power converters provide promising benefits over their room temperature counterparts in terms of reduced size and weight due to increased power density, improved efficiency, switching speed, and reliability. Integration could result in significant weight and space savings for the overall system. In this paper, a conceptual design study on the wide-bandgap-based (especially SiC and GaN) MW-class power inverter/converter is reported. Based on the total power loss of the designed converter, different cryogenic cooling strategies are proposed. The cooling power requirements, cooler mass, the cooler cost is evaluated based on the operating temperature of MW-class power converter. Finally, a power density comparison for different types of conventional power applications and HTS applications together with MW-class power converters is presented. The future of the cryogenically cooled power electronics system together with superconducting power devices is described for large-scale applications such as for future electric aircraft.

The cryogenic operation of power converters integrated with the superconducting electric machines can leverage the lower parasitic, lower core material, and insulation requirement to reduce weight and increase power density in aerospace applications. To that end, Gallium Nitride High Electron Mobility Transistor (GaN HEMT) has recently proven to be the most viable option for the cryogenic converter operation. However, the topology selection and optimization are greatly influenced by the current/voltage rating of the power devices. Although series connected HEMTs can increase the blocking voltage, the dynamic voltage imbalance issue stemming from device parameter fluctuations, the larger number of required isolated gate driver makes the need for single high blocking voltage devices more compelling. This is even more pronounced in GaN due to its higher operating frequency which stems from the lower input capacitance. Although 900V cascode GaN HEMTs are now commercially available, single-chip high voltage HEMTs are attractive due to their zero reverse recovery charge, and lower package bonding parasitics. In this paper, a 1200V enhancement-mode GaN HEMT from GaNPower International has been characterized at cryogenic temperature. A 63% decrease has been demonstrated in the on-resistance, R_ds(on) from room temperature to -150°C. The threshold voltage shows a 1.2x increase from room temperature to cryogenic temperature (-178°C). The transconductance (G_fs) shows a 1.34x increase for a temperature difference of 175°C. The results translate to a lower conduction loss and a faster switching speed and unfold further possibility of GaN in cryogenic converter applications.

Cryogenic power electronics is an emerging technology to achieve high efficiency and high power density. As the key element of the power electronics system, the device performance should be evaluated under cryogenic temperatures. To demonstrate the differences between different materials and technologies, cryogenic temperature performance comparisons among silicon (Si) metal–oxide–semiconductor field-effect transistor (MOSFET), Si insulated-gate bipolar transistor (IGBT), silicon carbide (SiC) MOSFET, Cascode gallium nitride (GaN), and GaN high-electron-mobility transistor (HEMT) from different perspectives are made, which includes on-state resistance, threshold voltage, turn-on and turn-off times, turn-on and turn-off switching losses. The normalized values are adopted and results are drawn in the same pictures to demonstrate the differences. The results demonstrate that traditional Si devices have better performances under low temperatures. The SiC MOSFET is not suitable for cryogenic applications due to its increased on-state resistance and switching loss. Cascode GaN and GaN HEMT are promising candidates for cryogenic applications with reduced conduction loss and switching loss.

Modern transportation applications including electric vehicles (EVs), more electric aircraft (MEA) and electric comprise of different power conversion systems. Reduced weight and improved semiconductor efficiency are the major challenges towards their improved overall performance. To overcome them, cryogenic operation of power electronics converters (CPEC), electric motors, cables, and energy storage components has been under research and evaluation. CPEC are being investigated to drive superconducting machines (SCM) because of their lightweight structure and improved efficiency. CPEC can offer improved switching performance, reduced conduction losses and therefore increased efficiency; because of reduced on-resistance and increased carrier mobility of active devices. This article will discuss how can the benefits of lower operating temperature (< 93 K) be utilized in determining the most suitable DC-AC converter configuration. The paper compares three basic converter topologies: (a) two level voltage source inverter (2L-VSI), (b) three level t-type neutral point clamped inverter (3L-TNPC) and (c) two-level current source inverter (2L-CSI). Analysis is provided based on volume of passive components, electromagnetic interference (EMI) spectra and number of active devices, where all components considered are compatible with low temperatures. It can be observed that utilization of cryogenic temperature affects the volume of passive components, and therefore the overall selection of converter.

The Linac Coherent Light Source (LCLS) is an X-ray science facility at SLAC National Accelerator Laboratory. The LCLS-II project (an upgrade to LCLS) is in its final stages, and the LCLS-II-HE (High Energy) project is another upgrade to the facility enabling higher energy operation. An electron beam is accelerated using superconducting radio frequency (SRF) cavities built into cryomodules. It is planned to build twenty-five modules total for the LCLS-II-HE project. Fourteen of these modules are planned to be assembled and tested at Fermilab. Procurements for production modules have been awarded, and parts are arriving. The design of the LCLS-II-HE modules is similar to the module design used in LCLS-II. The major cryomodule design change from LCLS-II is the nominal gradient required per cavity, increasing from 16 to 21 MV/m. The additional gradient yields additional heat loads. The first LCLS-II-HE cryomodule, referred to as the verification cryomodule (vCM), was assembled using parts from an LCLS-II module with new SRF cavities. The assembly is complete, and this module is currently being tested at Fermilab. The vCM cavities perform well, exceeding the required gradient and quality factor specifications. The first article module assembly is planned to start later in 2021. The status of the production as well as some of the design and assembly changes will be presented.

The need for larger mK cooling platforms is being driven by the desire to host ever growing numbers of cryogenic qubits in quantum computing platforms. As part of the Superconducting Quantum Materials and Systems Center at Fermilab funded through the Department of Energy under the National Quantum Initiative, we are developing a cryogenic platform capable of reaching millikelvin temperatures in an experimental volume of 2 meters diameter by approximately 1.5 meters in height. The platform is intended to host a three-dimensional qubit architecture based on superconducting radiofrequency accelerator cavity technologies. This paper describes the baseline design of the platform, along with the expected key performance parameters.

Quantum computers (QC) have the potential to efficiently solve problems currently unfeasible on even the fastest generation of classical computers. The building block of a QC is a quantum bit (qubit). Encoding and reading qubit states coupled via high-Q resonant cavity modes is a solution to maintaining qubit states; however there is a need for simple, scalable and robust fabrication techniques capable of realizing high density cavity arrays. RadiaBeam is developing a novel approach utilizing metal additive manufacturing(AM) using both laser and electron beam powder bed fusion. Using a 6GHz quarter wave resonator (QWR), we fabricated several niobium and titanium alloy QWR cavities and characterized their superconducting RF performance. In this letter, we provide the details of the first 3D-printed qubit cavities design and fabrication, and compare their Q-factors, measured at dilution fridge temperatures, against the machined Nb resonators.

Additive manufacturing is recognized as a potential technology to design and create complex geometries as well as a fast track to build prototype components. Different materials are possible to use, depending on the specific requirements of an application. Superconducting applications like magnets or rotating machines are demanding for the structural components. Either high and/or cyclic mechanical loads can be one of the limiting factors in design. In the cryogenic temperature regime austenitic steels are used due to the mechanical performance and the machinability. In this work 316L austenitic steel samples were produced using laser powder bed fusion, also known as Selective Laser Melting (SLM). Followed by different heat treatments to systematically influence to the microstructure evolution. Beside mechanical properties also thermal properties as heat capacity or expansion are investigated. Having the cryogenic application in mind the tests are conducted at room temperature, liquid Nitrogen and liquid Helium temperature. The measured mechanical and thermal properties are compared to industrial cast austenitic steels to qualify the overall performance of the additive manufactured samples.

The use of additive manufacturing and cryogenic cooling enables engineers to build electric machines with higher power densities than previously seen. Both the new manufacturing technologies and very low temperatures have a significant impact on the mechanical properties of materials. In the present work, a rotordynamics analysis of a permanent magnet synchronous motor concept from the AdHyBau project, funded by the German Federal Ministry for Economic Affairs and Energy, is conducted with an emphasis on the influence of additive manufacturing and cryogenic temperatures on vibrations and natural frequencies.

Recent advances with additive manufacturing (AM) technologies using various materials allow them to be considered for the manufacture of precise and complicated metal parts of the magnet coils of high field accelerator magnets from aluminum bronze, titanium alloy, stainless steel, etc. The 3D printing technology is also being used to fabricate prototype models of real parts to test and optimize their geometry. This paper discusses the designs of the complex stress-management coil parts developed at Fermilab, their fabrication using AM technologies, and quality control methods and results.

Thermal emissivity is an important parameter in characterizing the thermal radiative properties of materials and a critical factor in radiative heat transfer. The radiation of an actual object is different from that of the blackbody: its radiation energy varies irregularly with wavelength and temperature. To investigate the thermal radiative properties, thermal emissivity could be measured at cryogenic temperatures. In this paper, measuring methods and factors affecting the thermal emissivity of metals and infrared coatings at low temperatures were comprehensively reviewed. Two existing methods for measuring thermal emissivity at low temperatures, the calorimetry and radiometry methods, were described. Moreover, this work also discussed several factors such as material properties, surface temperature, and surface conditions. Testing devices for measurement of emissivity at low temperatures based on calorimetry and radiometry were reviewed and analyzed emphatically.

With the application and development of low-temperature superconductivity technology, the research on electrical properties of insulating materials under different low-temperature conditions has attracted more and more attention. According to the breakdown characteristics of materials in low-temperature environments, a test device for the discharge breakdown characteristics of insulating materials in vacuum low-temperature environments is designed. This device can meet test of the gaseous and solid insulating materials in different low temperature and vacuum conditions. Compared with the traditional low temperature electrical test device, it has the advantages of convenient replacement, short test cycle and multi-functional testing. The finite element simulation software is used to simulate the performance of the cooling device of gas and solid insulating materials, and the effects of liquid helium cooling and refrigerator cooling on the temperature distribution of the device are analyzed and compared. Finally, the flow field analysis of the cooling system defines the internal flow characteristics of liquid helium under liquid helium cooling, which provides theoretical guidance for the design of the low-temperature breakdown test device.

In order to determine the influence of process parameters including nitrogen and oxygen flow rate on the structure and electrical transport characteristics of zirconium oxynitride(ZrO_xN_y) thin films, the ZrO_xN_y films were prepared on sapphire substrates by RF reactive magnetron sputtering deposition technology. The crystal orientation and morphology of ZrO_xN_y films prepared at different nitrogen and oxygen flow rate were characterized by XRD and SEM, respectively. The electric transport behavior of ZrO_xN_y films at 300K to 3K was measured by PPMS. The results show that the insulativity of ZrN films is enhanced with the increase of nitrogen flow rate in sputtering atmosphere. With the increase of oxygen flow rate in sputtering atmosphere, the insulativity of ZrO_xN_y film is enhanced.

High-emissivity coatings are commonly applied in satellite detectors and large-scale cryogenic systems. Accurate and quick measurement of emissivity is in high demand for practical applications at cryogenic temperatures. In this paper, a method for the measurement of the total hemispherical emissivity from 50 K to 300 K was presented based on the static calorimetric method. A two-stage GM cryocooler was used as a cooling source in the measuring device without assumption of cryogenic liquids. The thermal emissivity of a black carbon coating was measured by monitoring temperatures timely in the device. The tested results exhibited good accordance with previous reports. This cryogenic measurement device has made it a strong candidate for the radiative heat dissipation at cryogenic temperatures. Design details of the experiment and measurement results will be presented.

In our previously published work, we have reported colossal magnetoresistance, Andreev oscillations, ferromagnetism, and granular superconductivity in oxygen-implanted carbon fibers, graphite foils, and highly oriented pyrolytic graphite (HOPG). In this follow-up research, more results on these oxygen-implanted graphite samples are presented. We show results from transport measurements on oxygen-implanted diamond-like carbon thin coatings, amorphous carbon films, and HOPG. Significantly, a three-order magnitude drop in the electrical resistance of the oxygen-implanted diamond-like carbon films is observed at the 50 K temperature that we have previously reported for the transition to the superconducting state. Below 50 K, the films' resistance oscillates between the high and low resistance states, less when the sample is under a transverse magnetic field. This metastability between the insulating and superconducting-like states possibly reflects the evolution of the amplitude for the superconducting order parameter also known as the longitudinal Higgs mode. Transitions to low resistance state and metastability are also observed for amorphous carbon films. Finally, the HOPG samples' resistance have a thermally activated term that can be understood on the basis of the Langer–Ambegaokar–McCumber–Halperin model applied to narrow SC channels in which thermal fluctuations can cause phase slips. We also find that in oxygen-implanted carbon materials, the electron charge and spin correlations do not compete and their interplay rather facilitates the emergence of high-temperature superconductivity, and thus, additional unexpected effects like Heisenberg spin waves and magneto-structural transitions are observed.

Gamma radiation effects on superconducting microwave transmission line structures, which may find use in future radiation challenging environments, such as satellites or accelerators have been investigated. Two versions of weakly coupled through-type Nb microstrip transmission line resonator material stack-ups were explored. In one version, the Nb signal trace was encapsulated with 20 µm of HD-4110 and in the other version the top Nb was not encapsulated. Exposure of the resonators to gamma radiation was performed for 28 days at room temperature in a sealed vacuum chamber using Co-60 as the gamma radiation source. The quality factors of the resonators were extracted at various cryogenic temperatures below the critical temperature of Nb and resonant frequencies up to 20 GHz. A large dose of gamma radiation used in this work showed a small change in the Nb superconducting properties.