The development of reliable jointing technique is of great importance for the future of applied superconductivity, especially for superconducting magnets working in persistent mode. Superconducting joints with termination architecture between single-core not reacted ex-situ MgB 2 /Fe wires were manufactured and tested. I -V characteristics of joints were measured at 4.2 K and 20 K for variable external fields and compared with I -V of joined wire. The ratio of joints critical current to that of parent conductors ( I c-joint / I c-wire ) has been evaluated and discussed. It was found that I c-joint with termination architecture may be even larger than I c-wire at high external field, but decreasing for lower fields. I c-joint / I c-wire = 0.8 was measured for the best joint at 20 K and low external field 1.5 T, which is promising for functional persistent switch.

This work presents a comprehensive 3D numerical model of MgB2 multi-filamentary superconducting wires using the Finite Element Method (FEM) software, COMSOL Multiphysics (R) 6.0. The study aims to investigate the electro-thermal behavior of MgB2 composite wires during standard transport measurements at various initial temperatures under subcooled water ice conditions. By solving a series of partial differential equations governing heat transfer and dynamic current transport, the model provides detailed insights into the wire's performance. The simulation results are rigorously compared with experimental E-I characteristics measured for 6-filament MgB2 wires with internal copper stabilization. This comparison validates the model and highlights its capability to predict the behavior of superconducting wires under cryogenic conditions. The findings offer valuable data on the current distribution, ohmic losses, and overall thermal stability of the composite wires, contributing to the advancement of cryogen-free superconducting technologies. This study bridges the gap in the literature regarding the electrothermal dynamics of MgB2 wires cooled by subcooled water ice, providing a foundation for further research and practical applications in high-field generation devices.

The Rutherford cable here was made of 12 single-core MgB(2)in-situ wires with an NbTi resistive barrier and a CuNi outer sheath. The uniformity of the wires and cables was analyzed using x-ray microtomography. The critical currents of the wire and cable samples were measured at 4.2 K in the outer fields from 4.0 to 8.0 T, and at 15 K-25 K in fields from 1.0 to 5.5 T. Magnetization AC losses were measured at temperatures of 20 K-40 K, external fields of 0.001-0.1 T and frequencies of 72 and 144 Hz. The obtained results show high engineering current density and reduced losses in the MgB2 cable, which can be especially interesting for windings of superconducting motors or generators.

One of the objectives of the SCARLET project is to develop and industrially manufacture superconducting MgB2 cables cooled by liquid hydrogen. The ex situ powder-in-tube MgB(2 )wires manufactured by ASG are considered for the cable design that can carry DC current of 20 kA. These braided superconducting wires, containing brittle filaments, require high current. Thus, the study of the electro-mechanical properties of MgB2 wires is crucial for the cable design and its functional use. Superconducting wires have to withstand all the stresses applied during the cabling process, installation, and operations at the temperature of around 20 K. Hence, several configurations of MgB2/Ni/Monel composite wires have been subjected to detailed electrical and mechanical characterizations, which allow the estimation of the stress limits during the manufacturing of the designed cable. These experiments demonstrated that the maximal tensile stress applied to the wire at room temperature should be below 180-200 MPa, and safety bending observed for the outer filament strains was below 0.3%-0.35%. It is also revealed that the limit of acceptable torsion (expressed by the twist pitch to wire diameter L- t/d w) is affected by the filament architecture and wire diameter. This limit should be above 100 for 1 mm wire and above 150 for 1.53 mm wire.

The effects of the interface angle and applied deformation on the transport current and structure of scarf architecture joints between single- and multi-core MgB2 wires are studied in this paper. The transport currents of the joints were measured at 4.2 K, external magnetic fields of 2-8 T, and joint's resistances between 27 and 42 K. Resistive transitions and critical currents of the prepared joints were compared with the transition and critical current of the unjointed MgB2 wires. The interface structure of joined in situ wires was analysed using optical microscopy. Increasing the interface area between the joined wires and optimizing deformation by pressing allows to enlarge the superconducting current path. In-field transport currents of up to 73% of wire's I-c were measured for joined single-core MgB2/Fe wires. In the case of six-filament wires, the maximal transport current up to 53% of wire's I-c was reached. The results show that the presented joint technique could potentially be used for superconducting MgB2 coils in a persistent mode.

The combination of liquid hydrogen and superconducting cables presents a unique opportunity to distribute both hydrogen and bulk electricity in the same infrastructure. In particular, liquid hydrogen around 20 K is ideally suited for cooling the MgB 2 superconductor, resulting in a compact power cable that also leaves sufficient place for the hydrogen flow. Such a hybrid system operating in the MVDC range at 25 kV and 20 kA constitutes one of the main goals of the European project SCARLET. After a description of the rationale and benefits of the electricity – hydrogen system, various possible applications and a first distribution system are presented. Furthermore, the different cable components already designed are discussed along with the research challenges and general strategy for the development.

Highly sub-cooled water ice, with temperatures as low as 10-20 K is not commonly utilized and, as a result, its fundamental properties remain generally unknown. Therefore, the thermal characteristics of water ice have been thoroughly reviewed in the scientific literature and compared with the performance of other solid materials, which can be potentially used as a cooling media in superconducting applications. The effectiveness of water ice as a cooling agent was demonstrated through experimental measurements of the temperature and the magnetic background field effects on the critical current of small MgB2 solenoid immersed in water ice with temperatures ranging from 10 K to 36 K and external magnetic fields from 0 to 6 T. Increase of the solenoid's temperature was observed when the transport current exceeded the critical threshold, which is determined by the conventional criterion of 1 mu V/cm. The obtained results confirm that sub-cooled water ice is a promising, cost-effective, and safe coolant suitable for superconducting systems.

Magnetization AC losses of fine-filamentary MgB 2 wires with resistive CuNi sheaths were measured. The effects of varying the number of filaments (114–342, corresponding to effective filament diameters of 14–20 μ m), twist pitch (10–30 mm) and outer sheath material on the total AC loss were studied. For a better understanding of individual loss contributions, the effects of varying applied temperature, magnetic field, and frequencies were examined. It is found that hysteresis loss per volume decreases with the reduced filament size and that coupling current losses play a dominant role. The effect of decoupling by twisting was clearly observed for the smallest twist pitches. Considering the possible degradation of transport currents by twisting, AC losses were also normalized by the critical currents of the same wires. While twisting to short pitch decreases losses significantly, it apparently does not reduce the transport current. Consequently, the fine-filamentary MgB 2 wires with resistive CuNi sheath examined in this paper are excellent candidates for future low loss applications. Unlike ReBCO tapes, round MgB 2 wires enable easy single strand twisting, and the braiding or cabling, of wires into a variety of specific shapes and diameters.

A small-sized coil of helically wound Bi-2223/Ag tape was measured in liquid/solid nitrogen (LN 2 /SN 2 ), and also in water (H 2 O) ice at external fields of 0–8 T and in a temperature range of 10–77 K. This work is especially focused on the coil stability for current amplitudes above the critical current criterion of 1 µ V cm −1 . While the E–I characteristics measured under the critical current criterion did not show any substantial variances at these different cooling conditions, significant differences were observed above the critical current magnitude, mainly upon cooling by solid nitrogen and water ice. The results confirm improved thermal stability for the coil measured in sub-cooled water ice compared to solid nitrogen. Consequently, cooling by water ice could be interesting for future applications of high-temperature superconducting coils.

In this paper, thermal stability of two 6-filament MgB2 wires with resistive CuNi sheath has been examined by DC transport measurements at the temperature of liquid He and variable external magnetic fields, and at variable temperatures of water ice under self-field conditions. Current-voltage characteristics up to the quench were performed and the operation limits of both wires under different cooling are analyzed and compared. The obtained results show that only 6 % of well conducting copper core allows to improve the thermal stability considerably, and it also limits the wire heating after the quench. MgB2 wires showed more stable operation after the quench under subcooled water ice in comparison to traditional Helium bath cooling, which offer a cheap and safe cooling mode for future devices.

Superconducting joints between single-core MgB2/Ni and MgB2/Nb wires made by an internal Mg diffusion (IMD) process have been manufactured by using scarf joints architecture. Joint's transport current were measured at 4.2 K and external magnetic fields of 2-8 T and compared with the currents of used MgB2 wires. Structure of selected joints were analysed by an optical microscope. It was found that addition of boron powder between the joined wires allows the creation of superconducting current paths. Observed Mg-Ni interaction inside the joint area reduces the creation of well-connecting MgB2 phase, and consequently, degrades the critical current of joint by 6.7% of wire's Ic. Joints between MgB2/Nb wires have no inter-metallic interactions, and consequently, large critical currents up to 60% of wire's Ic can obtained. Presented results show that wires with inert metallic sheath and added boron powder could be used for the fabrication of applicable superconducting joints and MgB2 coils working in persistent mode.

Monel-sheathed MgB 2 wires with 6, 18 and 54 filaments of diameter 0.83 mm manufactured by Sam Dong Co., Ltd, have been subjected to twisting, drawing and rolling and measurements at low temperatures. R ( T ), I–V characteristics, critical currents and stress/strain tolerances of these wires were analyzed. Low reduction of critical current (⩽10%) by twisting between 50 mm and 12.5 mm was observed, especially for the wire with 54 filaments, which also showed the best strain tolerance to tension, ∼0.43% at 4.2 K. The engineering current density of the 18-filament wire was decreased by 56% by drawing from 0.83 mm to 0.4 mm, but improved by ∼24% by rolling from 0.83 mm to 0.64 mm. A small uninsulated coil was made from wire flattened from 0.83 mm to 0.64 mm by the wind and react process, resulting in a very high winding packing factor of 0.91 and also a high winding current density of 10 4 A cm −2 at B ⩽ 5.6 T. The presented properties are interesting for possible applications of these MgB 2 wires in DC windings.

Development of reaction zones at the core Mg(Cu) - boron (B) and barrier (Ti) - sheath (Cu) interfaces of an internal magnesium diffusion (IMD) manufactured MgB2 composite wire has been studied at varying temperatures of 560-665 degrees C for 30 min and varying time periods of 30-180 min at 560 degrees C. MgB2 has predominantly formed at the Mg(Cu) - B interface with little solubility of Cu of 1-3 at%. A second phase enriched with Cu, Mg2Cu, has also formed at random locations within the MgB2 phase matrix, indicating the advantage of doping Cu in the Mg rod. However, a layer of another B-rich phase, MgB4, has also formed at a higher temperature of 665 degrees C with negligible solubility of Cu at the MgB2 - B interface. At the Ti - Cu interface, the formation of CuTi2, CuTi, Cu4Ti and Cu4Ti3 intermetallic phases took place. However, the Cu4Ti and Cu4Ti3 phases develop after annealing the wire at a relatively higher temperature of 620 degrees C and at 560 degrees C with progress in annealing time. Critical current density (J(c)) decreases with annealing time; however, the sensitivity of J(c) towards annealing time is low. Critical current density is found to be strongly dependent on the annealing temperature of the composite wires for the formation of the MgB2 superconductor phase. A maximum critical current density of similar to 2.5 x 10(4) A/cm(2) at 5 T and 4.2 K is attained for the Cu-doped MgB2 composite wire when annealed at 665 degrees C for 30 min. A maximum shift of only 0.5 K is measured for MgB2 when formed at the highest annealed temperature compared to the lowest one in this study.

Effect of wire diameter and annealing on the properties of internal magnesium diffusion (IMD)-processed MgB2 composite wires is studied by establishing a correlation with the characteristics of the interfacial reaction zones developed at Mg -B and barrier (Nb) - sheath (Al + Al2O3) interfaces. The MgB2 superconductor phase grows at the Mg -B interface along with the B-rich MgB4 phase. Formation of MgB2 prior to MgB4 is anticipated owing to a relatively lower Gibbs free energy at annealing temperatures studied. An intermetallic NbAl3 phase has developed at the barrier - sheath interface whose thickness decreases with an increase in the wire diameter. Development of NbAl3 further provides mechanical reinforcement and aids at yielding a dense superconductor phase. Resistivity - voltage characteristics of wire indicates the formation of MgB2 through a huge drop in resistivity values across the core of the wire. An annealing temperature of 645 degrees C near the melting point of pure Mg (650 degrees C) and annealing time of 120 min results in highest critical current of the MgB2 wire. Annealing time beyond 120 min has shown an impediment to the flow current due to crack propagation possibly because of accumulation of stresses within the developed superconductor phase due to grain growth. A significantly greater resistance for the wire with the smallest diameter is attributed to the formation of a thicker NbAl3 phase. However, highest engineering current density has also been attained by the wire with the smallest diameter annealed at 645 degrees C for 60 min.

A wind and react (W&R) coil of inner diameter 53 mm has been made from multi-core MgB2/Nb/CuNi wire manufactured by the internal magnesium diffusion (IMD) process. The W&R coil is wound from non-insulated rectangular wire of 1 mm2 with only 5 µm thick stainless steel foil used for interlayer insulation. The transport current performance of the coil and short wire samples was measured in a liquid He bath at external magnetic fields of 4.5–8.5 T and also in self-field conditions in sub-cooled water ice at temperatures between 33 K and 38 K. The presented MgB2 coil exhibits stable behavior at water ice cooling, and its high space factor allows a high current density of winding in comparison to the data from the already published MgB2 coils. The presented results demonstrate that MgB2 windings can be used safely in He-free conditions inside sub-cooled water ice, and this technique can be further optimized and used for future MgB2 coils.

The standard DC transport measurements of MgB 2 composite conductors were done in vacuum, low-pressure helium gas, sub-cooled solid nitrogen and water ice. The effect of these coolants on electro-thermal behaviour was studied at self-field and temperature range from 32 K to 36.5 K. The obtained results show that even small volume of Helium gas enhances the sample thermal stability during the I–V measurement in comparison to those obtained in vacuum. The cooling by solid nitrogen improves the sample stability in contrast to helium gas. The measurements performed inn water ice exhibits an excellent and stable behavior of superconducting wire carrying high transport current below and above the quench as well. Consequently, water ice shows simple, safe, and promising He-free mode of cooling for future superconducting systems.

MgB 2 wires with 114, 192 and 342 filaments of size 14–19 µ m manufactured by HyperTech Research, Inc. have been subjected to low-temperature DC measurements. R ( T ), I – V characteristics, critical currents, stress and strain tolerances of these wires differing by filament architecture and filament size sheathed by resistive CuNi alloys were measured and compared with the literature data. It was found that these fine-filamentary wires have high engineering current densities not reduced by twisting up to 10 mm, sufficient strain tolerances and therefore are promising for future applications where minimized AC losses are required due to resistive sheaths, thin MgB 2 filaments and short twist pitches.

Wind-and-react coils made using non-insulated and/or Al2O3 oxide-insulated internal magnesium diffusion-processed MgB2 wires were energized at low temperatures with the help of sub-cooled water ice. The results demonstrate thermally stable winding behaviour and current densities (>104 A cm−2 for T < 30.5–33 K) of oxide-insulated and metal-insulated MgB2 coils. The expansion caused by the water-to-ice transformation does not degrade the windings or the Al2O3 insulation. The thermometers and Hall probe are also undamaged over the full temperature range of 27–297 K. Consequently, water ice provides a cheap and safe prospective cooling mode for various superconducting windings and future He-free systems.

Superconducting joints between single-core MgB 2 /Ni and MgB 2 /Nb wires made by an internal Mg diffusion (IMD) process have been manufactured by using scarf joints architecture. Joint’s transport current were measured at 4.2 K and external magnetic fields of 2-8 T were used and compared with the currents of used MgB 2 wires. Structure of selected joints were analysed by an optical microscope. It was found that addition of boron powder between the joined wires allows the creation of superconducting current paths. Observed Mg-Ni interaction inside the joint area reduces the creation of well-connecting MgB 2 phase, and consequently, degrades the critical current of joint by 6.7 % of wire’s I c . Joints between MgB 2 /Nb wires have no inter-metallic interactions, and consequently, large critical currents up to 60 % of wire’s I c can obtained. Presented results show that wires with not reacting with metallic sheath and added boron powder could be used for the fabrication of applicable superconducting joints and MgB 2 coils working in persistent mode.

A Rutherford-type MgB2 cable has been fabricated from 12 six-core strands prepared by an internal magnesium diffusion (IMD) process with a resistive CuNi30 outer sheath. The uniformity of individual strands of diameter 0.448 mm prior to and after additional densification by rolling was studied by x-ray micro-tomography. The engineering current densities of the cable samples were measured at 4.2 K and external magnetic fields between 4.0 T and 8.0 T and compared with available published data. The presented Rutherford cable has the highest engineering current density with J (e) = 10(4) A cm(2) measured in an external magnetic field of 5.73 T. The bending tolerance of the cable shows a critical diameter of 60 mm and less gradual critical current degradation in comparison to similar cables fabricated from single-core strands. The AC loss measurements of the present cable show lower coupling losses in comparison to monolithic multicore IMD conductors. The obtained results are promising for high-current-density and low-AC-loss MgB2 superconducting cables that are suitable especially for motors and generators.