The REBa₂Cu₃O_7-δ (REBCO) coated conductor has the best superconducting properties (high critical current at high temperatures and high magnetic fields) and high mechanical strength (tensile stress). Recently, a 1.2GHz (28.2T) NMR system has become commercially available. On the other hand, however, design and manufacturing technologies for REBCO coil systems that combine robustness, safety, reliability, economy, and convenience as industrial products while maximizing the excellent properties of REBCO coated conductor have not yet been established. Therefore, in this paper, with the aim of establishing stabilization and protection technologies suitable for REBCO coil systems, with comparisons to low-temperature superconducting coils, the author discusses the occurrence of normal state transition, especially degradation and defects, and normal zone propagation characteristics, quench detection and coil protection methods. The author will also attempt to summarize the current status of the methods for monitoring the health of coils.

We are developing a high-temperature superconducting (HTS) sextupole magnet for chromaticity correction of the electron and positron beams of the SuperKEKB accelerator. The HTS sextupole magnet consists of six two-layer rectangular REBa2Cu3Oy (REBCO) coils. Since quench protection is a critical issue for the HTS sextupole magnet, we have carried out a series of quench studies on the REBCO coil. At the same time, we have developed a numerical simulation code to perform quench protection designs for the HTS sextupole magnet. In this study, we performed experiments and numerical simulations on the quench characteristics of three epoxy-impregnated REBCO model coils, two double pancake and a two-layer rectangular REBCO coils. The two-layer rectangular coil is the same as the coil of the HTS sextupole magnet. The hotspot temperature was studied as a function of the transport current and quench detection threshold. As a result, we confirmed experimentally that the quench protection of the REBCO coil could be achieved by a conventional quench detection method using the balance voltage. The validity of the numerical simulation code was also confirmed by comparing it with the experiments. Using the relation of the hotspot temperature and quench detection threshold, we succeeded in limiting the maximum quench temperature of the HTS sextupole magnet below 150 K.

The no-insulation (NI) technique is promising as a protection method for high-field high-temperature superconducting (HTS) magnet. However, it is not suitable for layer-wound HTS coils because it has extremely long magnetic field delay due to short circuits inside winding. To solve this problem, a research group, including the current authors, proposed a novel winding method called intra-layer no-insulation (LNI) method. In this paper, we summarize the series of our studies on NI layer-wound coils and LNI coils as well as discuss issues affecting high-field magnet applications.

Local degradation of critical currents in REBa₂Cu₃O_y (REBCO, RE: rare-earth and Y) coated conductor is one of the serious issues especially for high field superconducting magnets. It may give rise to a localized hotspot, followed by a burn-out in the magnet. To mitigate such phenomena, a co-winding of bundled two REBCO coated conductors is considered. The bundled REBCO double pancake coil with a local damaged area was made and tested in LN₂ and conduction cooling conditions. The I_c of the damaged coil showed more than 90 % of that of the non-damaged coil. The comparison of the I_c distribution in the coil and the I_c value at the damaged area can explain the good Ic performance of the damaged coil. Even if the operation current exceeds the local I_c due to the local damage, the coil can be protected if we would detect the quench with a threshold of less than 18 mV for the practical 30 T cryogen-free superconducting magnet.

The no-insulation (NI) winding technique proposed by Hahn on 2011 drastically improves the thermal stability of REBCO pancake coils. Thereafter, several different kinds of NI winding techniques have been proposed, such as the partial-insulation, the metal-as-insulation (MI), the conductive-epoxy-resin-covering (CERC), and the intra-layer NI winding techniques. Recently, we have reported the thermal stability comparison of REBCO coils which these NI winding techniques are applied to. The reduction of contact resistance improves the thermal stability even more.

In addition, the multiple REBCO tape bundled (MB) technique was recently proposed. REBCO tapes may have a few defects before winding. When the MB technique applies to REBCO coils, the current can avoid the defect region to transfer into the neighboring REBCO tapes. That is why the technique can improve the charging delay. The MB winding technique is getting attention. We have also evaluated REBCO pancake coils with the MB winding technique comparing them with a conventional NI REBCO pancake coil.