The European DEMO, i.e. the demonstration fusion power plant designed in the framework of the Roadmap to Fusion Electricity by the EUROfusion Consortium, is approaching the end of the pre-conceptual design phase, to be accomplished with a Gate Review in 2020, in which all DEMO subsystems will be reviewed by panels of independent experts. The latest 2018 DEMO baseline has major and minor radius of 9.1 m and 2.9 m, plasma current 17.9 MA, toroidal field on the plasma axis 5.2 T, and the peak field in the toroidal-field (TF) conductor 12.0 T. The 900 ton heavy TF coil is prepared in four low-temperature-superconductor (LTS) variants, some of them differing slightly, other significantly, from the ITER TF coil design. Two variants of the CS coils are investigated—a purely LTS one resembling the ITER CS, and a hybrid coil, in which the innermost layers made of HTS allow the designers either to increase the magnetic flux, and thus the duration of the fusion pulse, or to reduce the outer radius of the CS coil. An issue presently investigated by mechanical analyzes is the fatigue load. Two variants of the poloidal field coils are being investigated. The magnet and conductor design studies are accompanied by the experimental tests on both LTS and HTS prototype samples, covering a broad range of DC and AC tests. Testing of quench behavior of the 15 kA HTS cables, with size and layout relevant for the fusion magnets and cooled by forced flow helium, is in preparation.

Although the fascinating effect of superconductivity was discovered more than 100 years ago, only a few low-temperature superconductors, especially Nb₃Sn and NbTi, have reached everyday-life use. After the discovery of superconductivity in MgB₂ in 2001, a boom appeared in interest in employing MgB₂ in commercial applications, facilitated by simple crystallographic structure, high critical temperature, low-cost raw ingredients, simple and fast production, and other advantages for moderate temperature applications. It only lacks high flux pinning and good mechanical performance. In this review, the implications of manufacturing procedures on the superconducting properties, crystal and electronic structure of MgB₂ are discussed. The advantages and drawbacks of various manufacturing processes will be presented, as well as the ways how to improve MgB₂ superconducting performance. The impact of oxygen reduction, elements doping, the introduction of defects and additives, and the effect of precursor refinement on superconducting properties are here compiled to give an overview of the progress of flux pinning enhancement in bulk MgB₂.

Critical current density, J _c, in superconductors is strongly connected with size of defects in the material. Frequently, the smaller defects, the higher J _c. In this work, we tried to reduce the size of cheap commercial boron precursor powder using high energy ultra-sonication in ethanol media. The resulting powder was then utilized in synthesizing bulk MgB₂ via sintering at 775 °C. Effect of boron powder ultra-sonication on superconducting properties of the bulk MgB₂ was studied and discussed. SEM of ultra-sonicated boron showed fine particles with sharp edges (high-energy surfaces), irregular shapes and clustering of fine particles occurred for longer ultra-sonication durations. XRD proved a high quality of MgB₂ with only small traces of MgO. Around 36% improvement in J_c at 20 K and T_c close to 39 K were observed in MgB₂ bulk prepared with boron ultra-sonicated for 15 min. Microstructure studies showed numerous nanometre sized MgB₂ grains in the bulk. Other bulks (made of boron ultra-sonicated longer, for 30, 45, and 60 min) have larger grains. It resulted in slightly lower J _c, anyway, still by 22% higher than in reference bulk. The present results demonstrate that the high performance bulk MgB₂ can be achieved without reduction in T _c via employing a cheap boron, reduced in size by high-energy ultra-sonication.

We present a model of flux pinning on large nonsuperconducting particles. It is assumed that the pins trap different numbers of vortices determined by the Poisson’s distribution function. This approach brings an additional factor into the expression for pinning force density that exponentially decays with field. The critical current density thus becomes at low fields nearly pure exponential function of magnetic field, in agreement with numerous experiments on high-$T_c$ superconductors.

Magnetic induction and current transport techniques were employed to test the electromagnetic performance of 22 samples of superconducting REBaCuO tapes of several manufacturers with the aim to select the best candidates for wiring new generation superconducting magnets for fusion reactors. An extraordinary shape of pinning force density as a function of magnetic field was observed in ‘advanced pinning, AP’ SuperPower tapes. The induced currents enabled us to classify the tapes and show the best ones, at low temperatures and high magnetic fields on the one hand and at high temperatures and low magnetic fields on the other hand. Transport current experiments confirmed the results. Moreover, they provided information on current anisotropy, which appeared to be specific for each tape type. The present results formed a basis for the ongoing study of neutron irradiation impact on the tapes’ properties. The first experiment showed that neutron irradiation by the fluence of 2.1 E22 m ⁻² either enhances, reduces, or has no effect on the tape’s properties, according to the tape pinning structure.