Honghai Song

High Field Superconducting Solenoids Via High Temperature Superconductors

High-field superconducting solenoids have proven themselves to be of great value to scientific research in a number of fields, including chemistry, physics and biology. Present-day magnets take advantage of the high-field properties of Nb3Sn, but the high-field limits of this conductor are nearly reached and so a new conductor and magnet technology is necessary for superconducting magnets beyond 25 T. Twenty years after the initial discovery of superconductivity at high temperatures in complex oxides, a number of high temperature superconductor (HTS) based conductors are available in sufficient lengths to develop high-field superconducting magnets. In this paper, present day HTS conductor and magnet technologies are discussed. HTS conductors have demonstrated the ability to carry very large critical current densities at magnetic fields of 45 T, and two insert coil demonstrations have surpassed the 25 T barrier. There are, however, many challenges to the implementation of HTS conductors in high-field magnets, including coil manufacturing, electromechanical behavior and quench protection. These issues are discussed and a view to the future is provided.

Quench behavior of conduction-cooled Y Ba2Cu3O7−δ coated conductor pancake coils stabilized with brass or copper

The quench behavior of conduction-cooled Y Ba2Cu3O7?? (YBCO) coated conductor pancake coils is reported. Two coils, one stabilized with copper and one with brass, are wound with 25?m of conductor and instrumented with a heater and a large number of voltage taps and thermocouples. The critical current, minimum quench energy (MQE) and two-dimensional normal zone propagation velocity (NZPV) are measured as a function of I/Ic, where I is the transport current and Ic is the critical current. Although the non-uniform temperature and self-field distributions within the coils result in a non-uniform Ic, the heater is able to induce quenches with energies above the MQE and both longitudinal and transverse propagation velocities are measured. In both coils, the longitudinal NZPV (10?40?mm?s ? 1) is about one order of magnitude larger than the transverse NZPV (1?2?mm?s ? 1). Moreover, a comparison between the Cu-stabilized coil and a short, straight Cu-stabilized sample shows that the one-dimensional longitudinal propagation in the short sample is significantly faster than the longitudinal propagation in the coil. This is due to transverse heat conduction (transverse propagation) which reduces the temperature gradients in the coil but also slows down the longitudinal propagation. Thus, designing a quench detection system based upon data from one-dimensional experiments may result in an unintended level of risk.

Stability and Quench Behavior of

YBa2Cu3O7- x (YBCO) coated conductors (CCs) are now capable of carrying very high transport critical current density Jc over a broad range of magnetic field and temperature space, and as a result, they are receiving significant interest for a wide range of applications. While many of these applications take advantage of the high-temperature performance of YBCO CCs, because the YBCO CC is typically produced on a high-strength substrate and carries very high Jc at very high magnetic field, there is now growing interest in using YBCO CCs at 4.2 K to generate very high magnetic fields. The transition from high-field conductor to high-field superconducting magnet, however, requires that some challenging issues be addressed. One of the most important challenges remaining is to better understand the stability and quench behavior at 4.2 K, so that an effective quench protection system can be developed. Here, we report on measurements of the stability and quench behavior of short YBCO CC at 4.2 K by inducing a quench via a heat pulse from a heater mounted on the conductor surface. Through gradually increasing heater pulse amplitude, the transition from stable to unstable (i.e., recovery to quench) is observed through voltage and temperature measurements. Using these data, the minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are determined. It is found that, for the same fraction of critical current (I/Ic), YBCO CCs have similar MQE and NZPV as Ag-alloy-clad Bi2Sr2CaCu2Ox wires and significantly higher MQE and lower NZPV than those of MgB2 round wires of similar Ic (4.2 K). Furthermore, the voltage and temperature versus time data are correlated to better understand the quench onset behavior at 4.2 K. It is determined that a normal temperature gradient exists from the CC surface to the YBCO layer within the conductor, as well as a directly measured longitudinal temperature gradient. After the heater pulse has ended but while the transport current continues, the temperature gradient along the length becomes dominant. Nevertheless, voltage and temperature measurements remain problematic for quench detection in large magnets because of the slow longitudinal propagation velocity. Thus, new approaches to quench detection and/or protection of high-field YBCO magnets are needed.

\hbox{YBa}_{2} \hbox{Cu}_{3}\hbox{O}_{7 - x}

We explored high magnetic field superconducting properties and stability at 4.2 K of Second Generation High Temperature Superconductors using both short conductors and small pancake coils. Short lengths of conductor were tested at 4.2 K and 0-25 T in parallel and perpendicular fields, demonstrating an overall critical current of 420 A/mm2 in a parallel field of 25 T. A 56 m length of wire was carefully characterized for performance along the length in self-field at 77 K, and in 10 m lengths at 75 K and a 0.52 T field oriented parallel and perpendicular to the face of the conductor. These characterized lengths were made into small pancake coils which were equipped with a central heater, voltage taps and taps for thermocouples. We report on the stability testing at 4.2 K of one of these coils.

Coated Conductor at 4.2 K, Self-Field

Tokamak fusion reactors require the development of magnets capable of generating large magnetic fields under stringent structural constraints. Magnets made with high temperature superconductors (HTS) are well suited to this application, but are vulnerable to quench occurrence during operation. Temperature and strain sensors based on fiber optics are being developed as a first step to counter this contingency. Optical fibers with Bragg gratings are amenable to embedding within superconducting magnets to monitor temperature, strain, irradiation, and to detect quench occurrence. Additionally, in the case of AgX/Ag/Bi2Sr2CaCu2Ox, (Bi2212) wire magnets, fiber optics can serve as a heat treatment process monitor for wind-and-react (W&R) manufacturing. Here we show that it is possible to detect quenches using fiber Bragg grating sensors and examine the effects of Bi2212/optical fiber co-sintering on Bi2212 performance and fiber survivability.