High current MgB2 joints made by IMD process

Superconducting joints between single-core MgB2/Ni and MgB2/Nb wires made by the Internal Magnesium Diffusion (IMD) process have been manufactured in a “scarf joints” architecture. Joints with an angle of 15 degrees have reached the current carrying capacity of only 6.7% of critical current measured for MgB2/Ni wire. The superconducting connection of MgB2/Ni wires is difficult to prepare due to interaction of Mg particles with Ni sheath and penetration of Mg-Ni between the joined wires, which resulted in no superconducting current path. It was found that addition of boron powder between the joined wires allows creation of sufficient superconducting current path. It was shown that usage of inert metallic sheath, optimized pressing mode and sufficient amount of added boron are decisive for well-connected superconducting wires made by “scarf joints” architecture. Joined MgB2/Nb wires have large transport currents up to 60 % of critical current of used wire. Consequently, this technique can be applied for MgB2 coils working in persistent mode.

I. Hušek, P. Kováč, T. Melišek and M. Hain, Transport currents and micro-structure of superconducting joint between MgB2/Ni and MgB2/Nb wires made by IMD process, Ceramics International, Volume 49 issue 7 (2023) 11178-11183.

Schematic description of the preparation of a “scarf joint” between two MgB2 wires (a, b, c) and a SEM micrograph of the longitudinal cross-section of the joint (d).


AC losses in superconducting round cables with alternating and coinciding winding directions

Traditionally, the HTS tapes in round cables are wound following an alternating sense of helicity. Then, the tape edges are raised due to the Poisson effect, causing local surface irregularity and pressing into subsequent layers. We have found recently that this effect is less deteriorating in the case of coinciding lay angles because then the tape edges create imprints that are parallel to the tape length. However, it was not clear how such arrangement will influence the AC loss. The investigations by experimental testing of small laboratory models and numerical modelling revealed that the impact of geometry is found only in low AC magnetic fields. There, the lowest AC loss was found for the cable consisting of pairs of stacked tapes. On the other hand, at the conditions expected in the winding of a coil generating magnetic field the AC loss of different arrangements approached the same values.
In the preparatory work for experimental verifications it was necessary to study in detail the limits of mechanical loads inevitably experienced by tapes during the cabling process, and the role of non-uniformities as sources of excessive degradation of critical current.

Ries, R., Hlaváč, D., Solovyov, M., and Gömöry, F.: Induced delamination in REBCO coated-conductor tape by a scratch line and bending, Physica C 613 (2023) 1354358.

Mošať, M., Šouc, J., Ries, R., and Gömöry, F.: Longitudinal critical current profiles in coated conductors examined by transport and magnetization measurements, IEEE Trans. Applied Supercond. 33 (2023) 6602205.

Gömöry, F., Šouc, J., Godár, M., Hintze, C., and Grosse, V.: Analysis of critical current fluctuations as a means of checking the quality of high-temperature superconductor tape slitting, Supercond. Sci Technol. 36 (2023) 054001.

Kujovič, T., Ries, R., Mošať, M., and Gömöry, F.: The critical current of REBCO coated conductors subjected to a mechanical loading at varying angles, IEEE Trans. Applied Supercond. 33 (2023) 6601004.

Solovyov, M., Šouc, J., Kujovič, T., Frolek, L., and Gömöry, F.: Magnetization AC losses in multilayer superconducting round cables with coinciding and opposite lay angles, Supercond. Sci Technol. 36 (2023) 034001.

Ries, R., Gömöry, F., Mošať, M., Kujovič, T., Hintze, C., and Gil, P.: Effect of off-axis bending on microstructural and transport properties of coated conductor tape, Supercond. Sci Technol. 36 (2023) 014006.

Current density distributions resulting from numerical simulations for cables with coinciding and opposite tape winding directions, in external magnetic field of 12 mT and 120 mT applied perpendicular to the cable axis.



Superconducting motors for electric and hydrogen-powered aircrafts

Commercial aviation is a growing source of greenhouse-effect emissions (CO2, NOx, H2O at high altitude). Indeed, the International Air Trasport Association (IATA) set 2050 as goal of complete emission-free aviation in the western world. Superconducting electric propulsion motors enable zero-emission electric and hydrogen-based aircrafts. The latter are more promising, since liquid hydrogen can cool down superconducting parts down to 20 K. In our institute, we built a unique experimental set-up to measure the self-heating power of REBCO superconducting windings in motor stators down to 20 K. This quantity should be kept as low as possible in the motor, in order to keep the low required temperatures (between 20 and 40 K). Unexpectedly, measurements showed that for the measured coil, residual substrate magnetism is dominant, becoming a warning to tape producers. We also worked on concepts to reduce cross-field demagnetization effects in rotors made of REBCO superconducting stacks of tapes, which act as strong permanent magnets.

Kováč, J., Kopera, L., Pardo, E., Melišek, T., Ries, R., Berberich, E., Wolfstädler S., and Reis, T.: Measurement of AC loss down to 25 K in a REBCO racetrack coil for electrical aircraft motor, Sci Reports 12 (2022) 16454.

Dadhich, A., Li, S., Solovyov, M., Šouc, J., Mošať, M., and Pardo, E.: Reducing cross-field demagnetization of superconducting stacks by soldering in pairs, Supercond. Sci Technol. 35 (2022) 115001.


Magnetic field penetration in superconducting thin films for radiofrequency accelerating cavities

The Department of Superconductors participates in the European project I.FAST and is involved in tasks aimed at the development and optimization of new types of superconducting thin film coatings for radio frequency resonant cavities, which are used to accelerate and form elementary particle beams in accelerators. Our task is focused at complex characterization of the superconducting properties of the developed thin films and multilayers. Studying the surface defects of thin Nb films, we were able to confirm the connection between the field of the first penetration of the magnetic flux into the superconducting layer and the magnetic field enhancement near the defects. We also found that the magnetic field enhancement factor, calculated for the typical surface defects using Finite Element Method (FEM), is a much better indicator of the first penetration field value than the commonly used average surface roughness. While the surface roughness is affected by all the defects occurring at the surface of the surface superconducting film, calculating the amplification factor allows identifying the defects causing the highest local field amplification that are thus the most critical for the start of the magnetic field penetration.

Ries, R., Seiler, E., Gömöry, F., Medvids, A., Onufrijevs, P., Pira, C., Chyhyrynets, E., Malyshev, O.B., and Valizadeh, R.: Surface quality characterization of thin Nb films for superconducting radiofrequency cavities, Supercond. Sci Technol. 35 (2022) 075010.

Ries, R., Seiler, E., Gömöry, F., Medvids, A., Onufrijevs, P., Pira, C., Chyhyrynets, E., Malyshev, O.B., Valizadeh, R., Leith, S., and Vogel, M.: Numerical calculation of magnetic field enhancement and impact of surface defects on premature entry of magnetic field in thin Nb films for SRF cavities, IEEE Trans. Applied Supercond. 33 (2023) 3500405.

a) The modeled pit defect with diameter d = 0.6 micron, height h= 1 micron and zero edge curvature radius. A dashed curve represents a pit using curvature radius c = 1 micron. b) Magnetic field distribution around a pit defect, calculated using a 2D Comsol model considering d = 0.6 micron, h = 1 micron and c = 0.1 micron.



Superconducting Magnetic Flux Pumps

High temperature superconducting (HTS) flux pumps are promising devices in order to inject large DC currents into the winding of superconducting machines or magnets in a contactless way. The superconducting dynamo, as a type of flux pump with simple structure and easy maintenance has become very popular during the recent years. Using the dynamos, employing troublesome brushes in HTS machines or bulky currents leads with high thermal losses will be no more required. The working mechanism of HTS dynamo in open- circuit mode and with transport current is complicated and not fully investigated yet, despite several explanations and models that have been proposed. Recently, we published the first three-dimensional (3D) computer modeling of HTS flux pumps, revealing the distribution of screening currents for the first time. We have also published our modelling results of the charging process of a superconducting coil, which approaches our research to superconducting power applications.

Ghabeli, A., Pardo, E., and Kapolka, M.: 3D modeling of a superconducting dynamo-type flux pump, Sci Rep. 11 (2021) 10296.

Ghabeli, A., Ainslie, M., Pardo, E., Queval, L., and Mataira, R.: Modeling the charging process of a coil by an HTS dynamo-type flux pump, Supercond. Sci Technol. 34 (2021) 084002.


Improvement of superconducting properties of thin films via laser annealing

In the framework of the EC project ARIES we have been studying the superconducting and structural properties of superconducting thin films annealed post-deposition with the help of the laser beam. Development of this technology is focused on optimization of the superconducting coating on the inner walls of the radiofrequency resonant cavities. Our experimental investigation confirmed that the laser post-annealing has a high potential for improvement of the superconducting properties of the deposited films. However, the laser beam intensity has to be carefully optimized, otherwise new surface defects can be created and degradation of the parameters is very likely to occur.

Ries, R., Seiler, E., Gömöry, F., Medvids, A., Onufrijevs, P., Pira, C., Chyhyrynets, E., Malyshev, O.B., and Valizadeh, R.: Improvement of the first flux entry field by laser post-treatment of the thin Nb film on Cu, Supercond. Sci Technol. 34 (2021) 065001.

Leith, S., Vogel, M., Fan, J., Seiler, E., Ries, R., and Jiang, X.: Superconducting NbN thin films for use in superconducting radio frequency cavities, Supercond. Sci Technol. 34 (2021) 025006.

SEM micrographs of: a) surface of the Cu substrate, b) surface of the superconducting (Nb) thin film before the laser irradiation, c) – f) surface of the superconducting thin film after the laser irradiation (laser beam intensity increasing from c) to f)).



Advanced Superconducting Motor Experimental Demonstrator (ASuMED)

Air traffic is expected to grow worldwide after the Covid crisis, causing a significant increase in the global emissions. Thus, the ACARE Flightpath 2050 from the EU seeks reductions of CO2 by 75 %, NOx and particulates by 90 %, and noise by 65 %. Distributed electric propulsion can achieve these goals thanks to both higher efficiency and, more importantly, the possibility to drastically improve the overall aircraft aerodynamics. Both batteries and fuel-based turbine generators can provide the electric power. The goal of the Horizon 2020 project ASuMED is to construct a 1 MW full superconducting experimental motor, to be tested in laboratory conditions. The motor uses stacks of high-temperature REBCO tapes in the rotor as strong permanent magnets and REBCO windings as efficient stator. Maybe our most important result for 2020 is the modeling cross-field demagnetization of high-temperature superconducting REBCO stacks on the rotor for the relevantly high number of tapes (100) and up to 2 million cycles. Such high number of cycles is essential, since typical ripple fields of 1000 Hz frequency reach 2 millions of cycles in just 33 minutes of flight. We also developed strategies to reduce the energy loss in the stator below 0.04 % of the total power.

Dadhich, A. and Pardo, E.: Modeling cross-field demagnetization of superconducting stacks and bulks for up to 100 tapes and 2 million cycles, Sci Rep. 10 (2020) 19265.

Dadhich, A., Pardo, E., and Kapolka, M.: Time constant of the transverse-field demagnetization of superconducting stacks of tapes, Supercond. Sci Technol. 33 (2020) 065003.

Li, S., Kováč, J., and Pardo, E.: Coupling loss at the end connections of REBCO stacks: 2D modelling and measurement, Supercond. Sci Technol. 33 (2020) 075014.

Benkel, T., Lao, M., Liu, Y., Pardo, E., Wolftädler, S., Reis, T., and Grilli, F.: T–A-formulation to model electrical machines with HTS coated conductor coils, IEEE Trans. Applied Supercond. 30 (2020) 5205807.



Ultralight superconducting MgB2 wires



3D modeling and measurements of demagnetization of superconducting bulk and stack of tapes with magnetic cross-field ripples



Superconducting cables for large magnetic systems



Hiding objects in AC magnetic fields of power grid frequency by two-shell ferromagnetic/superconducting cloak



Dense MgB2 superconducting layer made by diffusion process (IMD)



AC loss modelling for HTS coils



Rutherford MgB2 cable + Low AC loss cable produced from transposed striated CC tapes



Magnetic cloak from superconducting/ferromagnetic composite