MgB₂ superconducting bulk materials have excellent potential for various applications. In this review, the characteristics and challenges of MgB₂ as a bulk material, and the recent research trends, such as the synthesis of dense MgB₂ bulks using high pressure process, atmospheric pressure synthesis process using liquid and gas diffusion, are introduced.

Magnesium diboride (MgB₂) has a high critical temperature (T_c = 39 K) as a metal-based superconductor and is expected to be used as a superconducting material for liquid helium-free applied equipment. The superconducting joint between MgB₂ wires is an important technology when considering applications of superconducting magnets. In this commentary, we will review the joint techniques that have been applied to MgB₂ wires, and show the results. There are several methods for producing MgB₂ wires, and various methods using wires produced by the in-situ method, ex-situ method, and IMD method were obtained from information from patents and treatises. Some typical methods and problems of some typical ones will be explained. The superconducting joint of MgB₂ wire is relatively easy to accomplish compared to that of other high-temperature superconducting wires, but I_c has not been obtained easily. This is due to the microstructure of the joint interface, and the improvement of those microstructures is considered to be important for the creation of a reliable superconducting joint for MgB₂ in the future.

Advances in applications of MgB₂ superconductor for particle accelerators are reviewed. The MgB₂ superconductor provides its unique features of much higher critical temperature and critical field than that of NbTi with a potential for the competitive cost for the production. It may contribute to expand superconducting technology applications with improved energy balance with hither temperature operation combined with conduction cooling technology. The recent advances in MgB₂ applications for particle accelerators and associated devices are overviewed in this paper.

Advances in the research and development on MgB₂ toward the application to superconducting radio-frequency (SRF) cavities for particle accelerators are reviewed. Evaluations using small flat thin film samples have shown promising results and the techniques to coat the interior of SRF cavities with MgB₂ are being developed. We will probably see some results of standard 1.3-GHz cavities coated with MgB₂ within 5 years. If MgB₂ coated cavities are successfully developed, a new class of SRF cavities operated at ~20 K with cryocoolers will be possible. Such accelerators can be used more easily at 20 K than at 4 K since higher capacity cryocoolers are available and the static heat load is more tolerable. Future applications of such accelerators include, but not limited to, environmental remediation and national security in addition to physics and material science.

MgB₂ compound is one of the low activation superconducting materials because the half-life time of the MgB₂ phase is much shorter than that of the Nb-based superconductors, such as Nb-Ti and Nb₃Sn. The natural boron powder as the boron source of the in-situ MgB₂ wire and bulk consists of two kinds of stable isotope materials, which are B-10 (¹⁰B:19.78 %) and B-11 (¹¹B: 80.22 %) isotopes. The ¹¹B isotope is a promising raw material for the further improvement of the neutron irradiation effect on the low activation MgB₂ wire. This study focused on the quality effect of the ¹¹B isotope raw powder on the superconducting properties of the ¹¹B isotope-originated MgB₂ (Mg¹¹B2) wire. We found that the nano-order particle size and the higher purification of the ¹¹B isotope raw powder would be efficient in the improvement of the superconducting properties of the low activation Mg¹¹B₂ wire.

Research and development of superconducting magnesium diboride (MgB₂) level sensor for liquid hydrogen have been conducted actively. Recently, the remarkably high performance of the external-heating-type MgB₂ level sensor has been recognized. However, the performance of the long MgB₂ level sensor based on a 100-m-long class MgB₂ wire has not yet been tested, although the test is needed to produce the MgB₂ level sensor for the large liquid hydrogen tank. Thus, we have fabricated 100-m-long class MgB₂ wires, and evaluated their superconducting characteristics. Homogeneous elongation in the total length and uniform superconducting characteristics of the MgB₂ wires were obtained. Using these MgB₂ wires, we made 500-mm-long MgB₂ level sensors and tested them. We obtained good level-detecting characteristics from those MgB₂ level sensors. Thus, the production of the long MgB₂ level sensor for the large liquid hydrogen tank is expected to be feasible.