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.

Because of the small square dimension, a soldered-stacked-square (3S) high-temperature superconducting (HTS) wire with 1 mm width is considered as a candidate for cabling large current conductors through twist process. For studying the twist structure, in this work, we prepared several twisted 3S wire samples fabricated from the same piece of the 3S wire with 10 m length. For the 3S wire, the uniformity of critical current was evaluated. As well, we investigated basic electrical and mechanical characteristics for the twisted 3S wire. The main results show that the critical current distribution for the 3S wire is relatively uniform at the average value of 90 A; the allowable minimum twist pitch of the twisted 3S wire is about 60 mm; the critical current of the twisted 3S wire shows isotropy characteristics; through the test for frequency dependence of self-field loss, we can know the eddy-current loss in the twisted 3S wire can be ignored, and the measured losses are identical to the calculated one from the thin strip equation; the minimum bending diameter and allowable tensile force for the twisted 3S wire are approximately 80 mm and 400 N, respectively.

Magnetic field monitoring devices are essential for studying and operating high-temperature superconducting (HTS) magnets. Apart from conventional Hall sensor, passive optical fiber sensor is also an option for magnetic field monitoring. In this article, we proposed a novel access based on yttrium iron garnet magneto-optic (YM) sensor, which is one kind of optical fiber sensors, to measure magnetic fields generated by an HTS double pancake coil. Moreover, we used a magnetic field bypass tube made of silicon steel material to expand the measurement range of YM sensor. The test results of YM sensor in liquid nitrogen demonstrated that the YM sensor was benchmarked against Hall probe up to 24.7 mT at 77 K. Using a silicon steel tube to shield magnetic field could extend the field measurement range to 37.6 mT. YM sensor could be one of the promising methods for monitoring HTS magnet magnetic field.

Effective quench detection technologies are extremely important for protecting high temperature superconducting (HTS) applications. In recent years, an optical fiber encapsulated HTS tape (OF-tape) is proposed for quench detection by using distributed temperature sensing (DTS) technology. In this paper, an HTS coil (OF-coil) wound with a 39 m long OF-tape was prepared and energized by an AC overcurrent of 700 A to verify the feasibility of temperature observation during quench events. Furthermore, anti-stokes intensity was also measured to understand dependence of temporal resolutions on response times of coil quenches. The main results show that temperature variations of the OF-coil can be correctly measured after quench and the maximum temperature is shown at the middle turn of the OF-coil. Particularly, for the method of detecting anti-stokes intensity, quench response time of temporal resolution (TR.) with 30 ms is about 500 ms faster than that with 300 ms. However, when the TR. is 30 ms, more noise signal is observed. Accordingly, a trade-off should be considered between response time and noise signal for efficient quench detection.

In general, a high temperature superconductor (HTS) cable has a conductor and a shield encircling it. This structure causes electrical evaluation of AC loss in the HTS cable to be very difficult. Thus it is not clear yet. Using two voltage leads attached to the conductor and shield, respectively, we evaluated not only their AC loss but also the cable’s total loss. To experimentally elucidate our electrical evaluation presented in this work, first we measured the total loss of the 100 m/22.9 kV BSCCO cable by calorimetry, especially around 77 K. Besides the same 5 m cable sample as above, another cable model with a thin insulator was also tested electrically, and they were compared with two numerical models: a non-twisted polygon model and a twisted cylinder model.The results show that, according to the insulator thickness between a conductor and a shield, their magnetic interaction becomes very different. For the 100 and 5 m cables, their normalized total AC losses measured from calorimetry and electrical evaluation correspond well to each other, regardless of operating temperatures. Moreover, the numerical analysis shows that the total AC losses measured for the 5 m sample and cable model fit with the numerically calculated ones. Besides, the conductor’s AC losses measured for both the cables agree well with the numerical ones, regardless of their insulator thickness. This is because a conductor in an actual cable is inside a shield, and so does not receive any effect of the shield’s magnetic field. These experimental and theoretical results support the view that our electrical evaluation for the total and conductor losses is reliable. However, unlike the results for the total and conductor losses, the shield’s experimental loss is not in good agreement with any of the theoretical ones from either the non-twisted polygon model or the twisted cylinder model.