In the high-temperature superconducting (HTS) magnets applications, fast quench detection is very important to protect the magnets from burning out. Usually, the method of voltage measurement is widely applied to detect quench. However, this method is not satisfied for large HTS magnets intrinsically. Besides, a novel quench detection, which is measured by Raman-scattering-based distributed temperature sensors (DTS) using flexible optical fibers, is suggested to be a potential candidate. To verify the feasibility of this method, we prepared a DTS system based on interrogating Raman-scattering signals and a no-insulation (NI) HTS coil mounted by a 2-m length optical fiber as a test sample. The main results show that critical current of the NI coil is 111 A. In the continued overcurrent-induced quench state under coil current of 160 A, the temperature rise was observed obviously at the position of 1.2 m in the optical fiber, and the temperature was up to 94 K in the near middle region of top surface in the NI coil.

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using solder. NI coils are widely studied and used in many situations. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. Epoxy impregnation is a common method to protect the HTS field from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability. However, traditional epoxy resins are electrical insulating materials which might cause the turn-to-turn contact resistance of NI coils to increase. Therefore, we proposed a new kind of impregnation method using the electrically conductive material, solder. Two coils were fabricated and tested for comparison. One is the NI coil without impregnation and the other is the NI coil impregnated with solder. The charge–discharge and sudden-discharge tests were performed in liquid nitrogen at 77 K. The experimental results were studied with equivalent circuit analyses. The results of this paper could provide useful data for the application of solder impregnated NI HTS coils.

High temperature superconducting (HTS) conductor on round core (CORC) cable is considered as a potential technology for power applications, because of its high current-carrying capacity, compactness, and strong mechanical properties. As the main component of CORC cable, center core is used to support the HTS conductors. However, for metal center core, eddy current induced in the core under alternating electromagnetic field is unneglectable, which will lead to extra AC loss in power applications. This paper focuses on the selection of center core of CORC cable. AC losses of CORC cable using different core materials are evaluated by both numerical and experimental methods, in which copper, stainless steel, fiberglass epoxy and nickel have been chosen as the core. Results illustrate that AC loss of CORC cable has a correspondingly increase with the conductivity of core material. Significant frequency dependence of AC loss indicates that for single layer CORC cable, proper core material should be chosen to minimize the eddy current loss. Besides, CORC cable with core of nickel shows a quite different AC performance due to its ferromagnetic characteristics. Conclusions obtained from this paper will provide essential data for future optimization on the design of CORC cables.

Conductor on round core (CORC) cables with the features of flexibility and a high engineering current density have been a potential candidate for high magnetic field magnets and accelerator magnets. In these applications, reducing the magnetization loss generated in superconductors is essential in the process of design. CORC cables fabricated by multifilament-coated conductor (CC) tapes have been proved to have a low magnetization loss. However, the degradation of the critical current (I _c ) and additional coupling losses significantly affects the performance of CORC cables. In this article, ultraviolet laser cutting technology is used to scribe both the superconducting layer and copper stabilization layers into narrow filaments. First, the magnetization loss of a single tape with different numbers of filaments was measured by a calibration-free method. Then, based on the testing results of a single tape, two typical CORC cables are wound by 10-mm-wide and 4-mm-wide CC tapes. The magnetization loss of cables made of striated tapes was discussed by both experimental and numerical methods. It is found that the magnetization loss significantly decreases with an increasing number of filaments under a high background magnetic field. The magnetization losses decrease in CORC cables made of striated CC tape. Due to the reduced coupling loss and attenuated shielding effect, there is not much difference between the two values of the magnetization loss reduction rates in CORC cables and single tapes. The conclusions obtained in this article will be instructive for the future design of CORC cables.

Mechanical stability of no-insulation (NI) high temperature su-perconducting (HTS) coils will be greatly enhanced after using solder impregnation (SIM) method. For superconducting high-field magnet applications, besides the mechanical stability, the thermal stability of SIM coils is also a crucial parameter which needs to be investigated. Due to its high insulation, convenient in-stallation, distributed temperature measurement, and insensitive to stress, Raman based optical distributed temperature sensor (RmDTS) system is an ideal option for monitoring the tempera-ture of SIM coils. In this work, an NI HTS coil and an optical fi-ber were successfully impregnated together by solder. The critical current of SIM coil determined by the central magnetic field is 69 A. To create quench process, a DC current of 240 A was injected into an SIM coil, and the temperature of the coil was monitored by the RmDTS simultaneously. The test results show that the temperature of the SIM coil was unchanged although a 4 times Ic was applied.

Quench detection and protection technologies are extremely important for high temperature superconducting (HTS) magnet applications. In a previous work, we have verified the feasibility of using a Raman-based distributed temperature sensor (RmDTS) system to measure a no insulation (NI) HTS coil temperature changes in an overcurrent induced quench event. However, rough and slow temperature measurement is unsuitable for actual application on quench detection and protection. Therefore, we designed a new RmDTS system consisting of an RmDTS device whose temporal resolution is 15 ms and a thinner polyimide-coated optical fiber with a diameter of 150 μm. Then the optimized RmDTS system and an original one were respectively applied to detect the NI coil temperature during quenches. The comparative results show that the highest heat was generated on the 22nd turn of the NI coil, and the temperature response time of the original DTS system is 2 s slower than that of the optimized one. By using an optimization method, the spatial and temporal resolutions of the RmDTS system have been enhanced triple and 66 times than before.