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.

HTS stack has high current carrying capacity which is widely used in motors or generators. Usually, the HTS tapes are stacked and soldered together at ends. Although the HTS tapes are insulated and have low loss, the soldering parts are conventional conductors with resistance. When the HTS stack is subject to a parallel ac magnetic field, the coupling currents are induced because of the soldered parts which would form current loops. And then, the coupling currents would cause coupling loss or been called resistance loss. The loss is not only related to the applied field but also depends on the number of tapes and resistance value. The minimum electromagnetic entropy production method is used to study the dependence of resistance and number of tapes on the magnetization loss systematically. A HTS stack with realistic resistance was prepared and measured to be verified the calculation. The results show that the superconductor loss increases at the beginning but then decreases later when more tapes are stacked. However, the resistance loss increases almost linearly. The resistance loss and the superconductor loss also depend on the resistance value. When the resistance is small, although there are coupling currents pass through the ending parts, the resistance value is so small that the resistance loss is very difficult to be observed. And the resistance losswould increase with the resistance linearly and the superconductor loss almost keeps constant. With the resistance increasing, there is a peak in the resistance loss and the superconductor loss drops dramatically. Later, the resistance loss decreaseswith the resistance and the superconductor loss keeps constant again, because the resistance is so large that no coupling current can pass through the soldering parts so that there is almost no resistance at all.

In high power density superconducting motors, superconducting tapes are usually stacked and connected together at terminals to improve the current capacity. When a parallel sinusoidal magnetic field is applied on this partially coupled stack, the coupling current is induced and causes additional coupling loss. Usually 3D modeling is needed to calculate the coupling loss but it takes too much computing resource and time. In this paper, a numerical 2D modeling by minimum electromagnetic entropy production (MEMEP) method is developed to speed up the calculation. The presented MEMEP model shows good accuracy and the capability to take the realistic resistance between tapes into account for coupling loss calculation with a high number of mesh element, which agrees to measurements.Thanks to the model, a systemic study of coupling loss on amplitude-dependence, frequency-dependence, resistance-dependence, and length-dependence, is presented and discussed. The results reveal the features of coupling loss which is very helpful devices with multi-tape conductors, such as the stator or rotor windings of motors.

The influence of MgB2 wires design on the magnetization AC loss was studied. AC loss in external AC magnetic field perpendicular to the wire axis was measured in the temperature range from 18 K up to 40 K and at the frequencies of 72 Hz and 144 Hz, respectively. For this purpose the experimental apparatus combining magnetization measurement system and non-magnetic vacuum vessel with two-stage crycooler for sample cooling has been used.To clarify the influence of wire architecture on the AC loss in non-magnetic GlidCop sheathed MgB2 composites experiments on a single-core, 30-filament un-twisted and also twisted samples were performed. MgB2 cables containing 7 mono-core strands and 30 filament strands were also measured. While in the cable containing single core strands the hysteresis loss was dominant, in the un-twisted wire and the cable with un-twisted filaments the coupling loss prevailed. The effect of decoupling was observed in all twisted filamentary wires. The obtained results show that in 7 strands cable the AC loss of strands is crucial to the overall AC loss of a cable. (C) 2013 Elsevier B.V. All rights reserved.

The temperature and external AC magnetic field dependence of AC magnetization losses of MgB2 wires were studied. Temperature was varied from 18K to 40K and external magnetic field of frequencies 72Hz and 144Hz from 8mT to 70mT with orientation perpendicular to the wire axis. To clarify the influence of the wire construction on AC loss, single and six filament untwisted samples of length ∼50mm were examined. For this purpose unique experimental apparatus created by the combination of original calibration-free measuring system designed for ac magnetization loss measurement and non-magnetic vacuum vessel with two-stage cryocooler for sample cooling was used. It was found, that for monofilament sample hysteretic AC losses was dominated in comparison to untwisted six-filaments sample, where coupling losses confirmed by frequency dependence were dominated.