MgB₂superconducting joints for persistent current operation

Abstract

High-performance superconducting joints are essential for realizing persistent-mode magnets. Herein, we propose a concept and fabrication of such superconducting joints, which yielded reliable performance in the operating temperature range of 4.2-25 K. MgB 2 -MgB 2 joints in magnets are known to result in deterioration of localized electrical, thermal, and mechanical properties. To overcome these problems, the ends of the two wires are inserted into a pellet press, which is then filled with a mixture of unreacted magnesium and boron powders, followed by heat treatment. The critical current capacity and joint resistance were precisely evaluated by the standard four-probe method in open-circuit and by field-decay measurements in a closed-loop, respectively. These joints demonstrated up to 66% of the current-carrying capacity of unjoined wire at 20 K, 2 T and joint resistance of 1.4 x 10 −12 Ω at 4.2 K in self-field. AbstractHigh-performance superconducting joints are essential for realizing persistent-mode magnets.Herein, we propose a new process and fabrication of such superconducting joints, which yielded reliable performance in the operating temperature range of 4.2 K to 25 K. MgB 2 -MgB 2 joints in magnets, are known to result in deterioration of localized electrical, thermal, and mechanical properties. To overcome these problems, the ends of the two wires are inserted into a pellet press, which is then filled with a mixture of unreacted magnesium and boron powders, followed by heat treatment. The critical current capacity and joint resistance were precisely evaluated by the standard four-probe method in open-circuit and by fielddecay measurements in a closed-loop, respectively. These joints demonstrated up to 66% of the current-carrying capacity of unjoined wire at 20 K, 2 T and joint resistance of < 1.4 x 10 -12 Ω at 4.2 K in self-field.

Tailored joint fabrication process derived ultra-low resistance MgB2 superconducting joint

“…Once the joint temperature in the coil was stabilized at about 20 K, the background field was slowly reduced to zero. During this process, some magnetic field was trapped in the closed-loop as shown in Figure 3 [38]. The magnetic field decay line calculated using the L -R circuit time constant is also shown in Figure 3(c) using the dashed line, which matches very well with the actual decay.…”

Modeling of airgap influence on DC voltage generation in a dynamo-type flux pump

“…However, applications with windings have been impeded in part because of thermal losses at the current leads and joule loss at joints. Although there has been some progress in the superconducting joint technology [34], [35], [36], there is still a long way to develop reliable and consistent superconducting joints with the similar HTS CC characteristics such as critical current, resistance and magnetic field dependence of critical current. In synchronous motors and generators, powering the pole coils at the rotor requires slip rings or brushes, adding additional problems to the aforementioned thermal loss.…”

Niobium-titanium (Nb-Ti) superconducting joints for persistent-mode operation

“…At 0.574 µH inductance of the coil and 0.43 G field variation in 12.5 hours (Fig. 5(c)), the calculated circuit resistance according to field-decay in the L - R circuit was 3.25 × 10 −14 Ω 18–20 . This circuit resistance meets the technical requirement for a persistent-mode superconducting magnet system.…”