High-Temperature Superconductors (HTS) considerably accelerate the development of superconducting machines for electrical engineering applications such as fully electrical aircraft. This present contribution is an overview of different superconducting materials that can be used as magnetic screens for the inductor of high specific power electrical machines. The impact of the material properties, such as the critical temperature ($T_c$) and the critical current density ($J_c$), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

In this study, high-density magnesium diboride (MgB₂) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (J_c-B) in MgB₂ bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and J_c-B using two methods, ex situ (sintering MgB₂ synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB₄ and MgO were found to be effective in improving the J_c-B characteristics. In the ex situ method, the degradation of MgB₂ due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the J_c-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and J_c-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (f_P) analysis revealed the possibility of refined MgO inclusions and MgB₄ phase as new pinning centers, which greatly contributed to the J_c-B properties. The contributions of the sintering conditions on f_P for both synthesis methods were analyzed.

We have succeeded in getting ¹H NMR signals at 47.8 MHz (1.12 T) from a single-sided HTS bulk magnet activated by pulse-field magnetization. Diffusion NMR using an inhomogeneous outer field was also demonstrated on the surface. This kind of NMR relaxometry can not get the information of molecular structures, which conventional NMR using a homogeneous magnetic field can acquire. However, the relaxation of nuclear spin magnetization is still observable. A higher magnetic field gradient generated by the small HTS bulk magnet, 67 T/m at a 5 mm distance from the surface, can be effectively used to observe the self-diffusion coefficient of liquids in higher spatial resolution, 44 m. The easy accessibility to the strong field gradient is one of the unique properties of compact HTS bulk magnets. We believe that single-sided (unilateral) diffusion NMR will become the new application of HTS bulk magnets.

Many papers report the AC transport current loss characteristics in HTS coils, which occur because the HTS tapes in the coil transport a current in a magnetic field generated by the currents in adjacent tapes. We previously reported our experimental results on the loss from HTS tape in an AC and DC mixed electro-magnetic condition which consists of the DC offset transport current, AC transport current and AC magnetic field. Losses were measured in the same segment by the electric and calorimetric methods, and we established the possibility of measuring the loss using the electric method in the AC and DC mixed electro-magnetic condition. Now, we report the loss characteristics with AC and DC offset transport current in an HTS double pancake coil. We measured the AC and DC loss components by a four-terminal method. The voltage from the HTS coil includes the large AC inductive voltage, and we canceled the inductive voltage using a cancel coil placed in the center of the HTS coil. We report our findings on the loss characteristics in an HTS coil with AC and DC offset current in the HTS coil.