Slow Normal Zone Propagation Research Articles

Detecting Quench in HTS Magnets With LTS Wires—A Theoretical and Numerical Analysis

Protecting a high temperature superconducting (HTS) magnet from a quench event is a challenging task. Because of the slow normal zone propagation velocity, the long reliable quench detection method by directly monitoring coil voltage may not be timely for HTS anymore, leaving HTS magnets under danger of overheating. Using a NbTi low temperature superconducting (LTS) wire to detect quench in coils wound with ReBCO HTS tapes have recently been experimentally proved, yet a theoretical study is still needed to further develop this technique and make it prepared to be applied more generally in high field magnets. In this manuscript, we have demonstrated that it is the significant difference in the temperature dependence of critical current between LTS and HTS but not the normal zone propagation velocity (NZPV), that makes LTSs good quench detectors. Simulations show that LTS quench detectors should have low matrix fraction or high matrix resistivity. At last, at field up to 15 T or 20 T, Nb3Sn is proven to be a good quench detector.

Dipole Magnets Wound Using YBCO Cables-FEM Stability Issues Modeling

Quench and protection in magnets wound with coated conductors is of great interest for many applications, including high energy particle accelerators. The relatively large slow normal zone propagation and large MQE of coated conductors and the potential of burn out in well insulated coils has led to an interest in non-insulation coils. A stability of block type dipole magnets wound using non-insulated YBCO Roebel and CORC cables has been modeled by FEM. Critical currents of the magnets have been calculated using a self-consistent method based on anisotropic I_c(B) curves of the cables. Advantages and disadvantages of using the Roebel and CORC cables in dipole magnet windings have been discussed. Measured heat disturbances of different size, intensity and duration in a pancake coil wound using a non-insulated YBCO tape have been analyzed from the viewpoint of the magnet quench and stability. The effect of CORC cable magnetization on the field quality of a canted cosine theta dipole was evaluated.

Quench Detection and Protection for High-Temperature Superconductor Accelerator Magnets

High-temperature superconductors (HTS) are being increasingly used for magnet applications. One of the known challenges of practical conductors made with high-temperature superconductor materials is a slow normal zone propagation velocity resulting from a large superconducting temperature margin in combination with a higher heat capacity compared to conventional low-temperature superconductors (LTS). As a result, traditional voltage-based quench detection schemes may be ineffective for detecting normal zone formation in superconducting accelerator magnet windings. A developing hot spot may reach high temperatures and destroy the conductor before a practically measurable resistive voltage is detected. The present paper discusses various approaches to mitigating this problem, specifically focusing on recently developed non-voltage techniques for quench detection.

Fundamental Evaluations of Applicability of LTS Quench Detectors to REBCO Pancake Coil

Quench detection in a rare-earth barium copper oxide (REBCO) coil is difficult due to the slow normal zone propagation in it. To address this issue, we have proposed a quench detection method using a low temperature superconducting (LTS) wire as a quench detector. A demonstration experiment in our previous research using a REBCO tape showed that the quench detector was able to detect quench in the tape before it burned out. Therefore, to verify the applicability of this method to a real magnet system, we first numerically evaluated the quench detectability of Nb-Ti and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al detectors attached on stacked REBCO tapes at 4.2 K under magnetic fields up to 20 T assuming the coil excitation process. The results indicated that the detectors were not needed to be placed in each layer of the REBCO coil. Furthermore, we have conducted an experiment using a REBCO single pancake coil with the Nb-Ti quench detector at 4.2 K and self-field. As the results of the experiment, we demonstrated that one Nb-Ti detector was able to detect the quench in the REBCO coil even if there were at least three REBCO tapes and the polyimide tapes between the detector and the hot-spot in the REBCO coil.

Feasible voltage-tap based quench detection in a Ag/Bi-2212 coil enabled by fast 3D normal zone propagation

Small insert solenoids have been built using a commercial Ag/Bi-2212 multifilamentary round wire, insulated with a new thin TiO2–polymer coating insulation (thickness of ∼20 μm versus ∼100 μm for a commonly used mullite braided sleeve insulation), and characterized in a background magnetic field up to 14 T at 4.2 K to explore the high-field performance and quench detection of Bi-2212 magnets. The coil has no visible leakage and no electrical shorts after reaction, and it carries 280 A mm−2 in a background field of 14 T and generates an additional 1.7 T. A notable result is that, despite normal zones propagating slowly along the conductor, the hot spot temperature upon detection increases only from 40 K to 60 K when the resistive quench detection voltage threshold increases from 0.1 V to 1 V for all operating current density investigated, showing that quench detection using voltage taps is feasible for this coil. This is in strong contrast to a coil we have previously built to the same specifications but from wires insulated with mullite braided sleeve insulation, for which the hot spot temperature upon detection increases from ∼80 K to ∼140 K while increasing the detection voltage threshold from 0.1 V to 1 V, and thus for which quench detection using voltage taps presents significant risks, consistent with the common belief that the effectiveness of quench detection using voltage taps for superconducting magnets built using high-temperature superconductors is seriously compromised by their slow normal zone propagation. This striking difference is ascribed to the fast transverse quench propagation enabled by thin insulation and the improved thermal coupling between conductor turns. This work demonstrates that quench detection for high-temperature superconducting magnets highly depends on the design and construction of the coils such as the insulation materials used and this dependence should be factored into the overall magnet design.

High-field quench behavior and dependence of hot spot temperature on quench detection voltage threshold in a Bi2Sr2CaCu2Ox coil

Small insert solenoids have been built using a multifilamentary Ag/Bi2Sr2CaCu2Ox round wire insulated with a mullite sleeve (∼100 μm in thickness) and characterized in background fields to explore the quench behaviors and limits of Bi2Sr2CaCu2Ox superconducting magnets, with an emphasis on assessing the impact of slow normal zone propagation on quench detection. Using heaters of various lengths to initiate a small normal zone, a coil was quenched safely more than 70 times without degradation, with the maximum coil temperature reaching 280 K. Coils withstood a resistive voltage of tens of mV for seconds without quenching, showing the high stability of these coils and suggesting that the quench detection voltage should be greater than 50 mV in order not to falsely trigger protection. The hot spot temperature for the resistive voltage of the normal zone to reach 100 mV increased from ∼40–∼80 K while increasing the operating wire current density Jo from 89 A mm−2 to 354 A mm−2, whereas for the voltage to reach 1 V, it increased from ∼60–∼140 K. This shows the increasing negative impact of slow normal zone propagation on quench detection with increasing Jo and the need to limit the quench detection voltage to <1 V. These measurements, coupled with an analytical quench model, were used to assess the impact of the maximum allowable detection voltage and temperature upon quench detection on the quench protection, assuming a limit of the hot spot temperature to <300 K.

Hot Spot Temperature in an HTS Coil: Simulations With MIITs and Finite Element Method

MIITs, a zero-dimensional concept to study hot spot temperature, has been previously used to estimate hot spot temperatures and quench heater delays in NbTi and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnets. However, quench behavior is completely different in high-temperature superconducting (HTS) magnets due to the slow normal zone propagation velocity and the high temperature margin. Because the MIITs concept does not take into account thermal diffusion in the magnet, opposite to the finite-element method (FEM) analysis, the difference of these concepts is studied in this paper. Here, we have taken the approach to compute the hot spot temperatures for a future HTS magnet, designed to be built from REBCO Roebel cable, with MIITs and FEM simulations. The magnet protection is accomplished with a dump resistor, and the effect of quench detection threshold voltage on the hot spot temperature has been studied. Furthermore, the inductance of the magnet increases with the magnet length. Thus, there exists a maximum inductance of the magnet, which should not be exceeded to be able to protect the magnet only with a dump resistor. The hot spot temperatures with different values of inductance are also studied in this paper. Our simulations show that the hot spot temperatures computed with MIITs are from 60 to 150 K higher than those of FEM analysis. Thus, the MIITs concept seems unreliable when considering hot spot temperatures in HTS magnets protected with only dump resistors. However, the MIITs concept might be a usable tool when comparing different magnet designs. If 400 K is the upper limit for the hot spot temperature and the protection scheme includes only a dump resistor, the length of the investigated magnet can be increased to only such value that the magnet inductance is at most 50 mH.

Thermal conductivity and dielectric properties of a TiO2-based electrical insulator for use with high temperature superconductor-based magnets

Quench protection is a remaining challenge impeding the implementation of high temperature superconductor (HTS)-based magnet applications. This is due primarily to the slow normal zone propagation velocity (NZPV) observed in Bi2Sr2CaCu2OX (Bi2212) and (RE)Ba2Cu3O7 − x (REBCO) systems. Recent computational and experimental findings reveal significant improvements in turn-to-turn NZPV, resulting in a magnet that is more stable and easier to protect through three-dimensional normal zone growth (Phillips M 2009; Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). These improvements are achieved by replacing conventional insulation materials, such as Kapton and mullite braid, with a thin, thermally conducting, electrically-insulating ceramic oxide coating. This paper reports on the temperature-dependent thermal properties, electrical breakdown limits and microstructural characteristics of a titanium oxide (TiO2) insulation and a doped-TiO2-based proprietary insulation (doped-TiO2) shown previously to enhance quench behavior (Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). Breakdown voltages at 77 K ranging from ∼1.5 kV to over 5 kV are reported. At 4.2 K, the TiO2 increases the thermal conductivity of polyimide by about a factor of 10. With the addition of a dopant, thermal conductivity is increased by an additional 13%, and a high temperature heat treatment increases it by nearly an additional 100%. Similar increases are observed at 77 K and room temperature. These results are understood in the context of the various microstructures observed.

ITER CS Quench Detection System and Its Qualification by Numerical Modeling

The ITER Central Solenoid (CS) magnet needs to be protected against overheating of the conductor in the event of the occurrence of a normal zone (NZ). Due to a large amount of stored energy and slow NZ propagation, the NZ needs to be detected and the switchyard needs to open the breakers within 2 s after detection of the NZ. The CS will be discharged on a dump resistor with a time constant of 7.5 s. During operation of the CS and its interaction with the poloidal field (PF) coils and plasma current, the CS experiences large inductive voltages from multiple sources, including nonlinear signals from eddy currents in the vacuum vessel and plasma current variation, that makes the task of detecting the resistive signal even more difficult. This inductive voltage needs to be cancelled by quench detection (QD) hardware (e.g., bridges, converters, filters, processors) and appropriate processing of the QD signals to reliably detect NZ initiation and propagation. Two redundant schemes are proposed as the baseline for the CS QD System: 1) A scheme with Regular Voltage Taps (RVT) from triads of Double Pancakes (DP) supplemented by Central Difference Averaging (CDA) and by digital suppression of the inductive voltage from all active coils (the CS and PF coils). Voltage taps are taken from helium outlets at the CS outer diameter. 2) A scheme with Cowound Voltage Taps (CVTs) taken from cowound wires routed from the helium inlet at the CS inner diameter. Summary of results of the numerical modeling of the performance of both baseline CS QD systems is presented in this paper.

Enhanced Quench Propagation in $Bi_{2}Sr_{2}CaCu_{2}O_{x}$ and $YBa_{2}Cu_{3}O_{7-x}$ Coils via a Nanoscale Doped-Titania-Based Thermally Conducting Electrical Insulator

The significant amount of energy stored in a large high-field superconducting magnet can be sufficient to destroy the coil in the event of an unprotected quench. For magnets based on high-temperature superconductors (HTSs), such as Bi2Sr2CaCu2Ox (Bi2212) and YBa2Cu3O7-x (YBCO), quench protection is particularly challenging due to slow normal zone propagation. A previous computational study showed that the quench behavior of HTS magnets is significantly improved if the turn-to-turn electrical insulation is thermally conducting, enhancing 3-D normal zone propagation. Here, a new doped-titania electrical insulation with high thermal conductivity is evaluated. The thermal conductivity of the insulation is measured at cryogenic temperatures, and its chemical compatibility with Bi2212 round wires is determined. Thin layers of the insulation are deposited onto the surface of Bi2212 and YBCO wires, which are then wound into small coils to study the quench behavior. Results show that the critical current and homogeneity of Bi2212 coils are improved relative to coils reacted with mullite insulation. Relative to similar coils with conventional insulation (mullite for Bi2212 and Kapton for YBCO), the turn-to-turn quench propagation is increased by a factor of 2.8 in Bi2212 coils at 4.2 K and self-field and by a factor of 2.5 in YBCO coils at 4.2 K and 5 T. These results indicate that doped-titania insulation may significantly improve Bi2212 and YBCO coils. Increased normal zone propagation velocity enhances quench detection and quench protection, and the thinness of the insulation relative to the most common alternatives increases the magnet winding pack current density and reduces the coil specific heat.

Hybrid Model of Quench Propagation in Coated Conductors Applied to Fault Current Limiter Design

In order to help in designing more compact fault current limiters based on coated conductors, we recently developed a hybrid model of the quench propagation in such conductors. This model combines the finite element method to study the thermal propagation and analytical calculation of heat generation. The results obtained with this model are in very good agreement with the experiments, without the need for using free parameters for adjustment. Simulations demonstrate that even slight variations of the conductor characteristics have great influence on the temperature profile along the tape during the limitation phase, due to the slow normal zone propagation velocity. 2-D parametric studies are conducted with the aim of improving the temperature uniformity by varying some characteristics of the layers of the coated conductors. 3-D simulations on a wider scale are conducted to demonstrate how the temperature uniformity could be improved further in fault current limiters by using adapted meander geometries.

Effects of high magnetic field on the quench behavior of Bi2Sr2CaCu2Ox coils at 4.2 K

The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi2Sr2CaCu2Ox (Bi2212) coils addresses this question. The two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other.

A Hierarchical Three-Dimensional Multiscale Electro–Magneto–Thermal Model of Quenching in $\hbox{REBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}$ Coated-Conductor-Based Coils

Quench detection and protection in REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (REBCO) coated conductor (CC)-based superconducting magnets is difficult due to slow normal zone propagation velocity and the multilayer composite architecture of the conductor. To design effective quench detection and protection methods, it is essential to know the electrical, thermal, and structural behavior during the quench at multiple length scales ranging from the micrometer scale within the layers of the conductor to the macroscopic behavior of the coil. Here, a hierarchical multiscale approach is used to develop a modular 3-D electro-magneto-thermal coil quench model. The model uses an accurate experimentally validated micrometer-scale REBCO CC model as the basic building block. The CC model is embedded within a homogenized coil framework at one or more locations in the form of multilayer tape modules. This multiscale approach makes possible the studies of quench behavior at the micrometer scale within a tape at any location of interest within a coil without requiring a computationally extensive model of the entire coil. This approach also enables the building of more complicated models by hierarchically integrating smaller modular blocks with the same repeatable modeling techniques. Here, the development of the electro-magneto-thermal coil quench model is first presented, followed by its experimental validation. Simulation results and their implications for coil reliability and quench detection and protection are then discussed.

Quench detection method for 2G HTS wire

2G HTS conductors are increasingly used in various commercial applications and theirthermal and electrical stability is an important reliability factor. Detection and preventionof quenches in 2G wire-based cables and solenoids has proven to be a difficult engineeringtask. This is largely due to a very slow normal zone propagation in coated conductors thatleads to formation of localized hotspots while the rest of the conductor remains in thesuperconducting state. We propose an original method of quench and hotspot detection for2G wires and coils that is based upon local magnetic sensing and takes advantage of 2Gwire planar geometry. We demonstrate our technique experimentally and show that itssensitivity is superior to the known voltage detection scheme. A unique feature of themethod is its capability to remotely detect instant degradation of the wire criticalcurrent even before a normal zone is developed within the conductor. Variousmodifications of the method applicable to practical device configurations are discussed.

Quench in a conduction-cooled Nb3Sn SMES magnet

Due to the rapid development of cryocoolers, conduction-cooled Nb3Sn devices are nowadays enabled. A 0.2 MJ conduction-cooled Nb3Sn SMES system has been designed and constructed. The nominal current of the coil was 275 A at 10 K. The quench tests have been performed and in this paper the experimental data are compared to the computational one. Due to a slow normal zone propagation, Nb3Sn magnets are not necessarily self-protective. In conduction-cooled coils, a thermal interface provides a protection method known as a quench back. The temperature rise in the coil during a quench was measured with a sensor located on the inner radius of the coil. The current decay was also monitored. The measured temperature increased for approximately 15 s after the current had already decayed. This temperature rise is due to the heat conduction from the hot spot. Thus, the measured temperature does not represent the hot-spot temperature. A computational quench model which takes into account quench back and heat conduction after the current decay was developed in order to understand the measured temperatures. According to the results, a quench back due to the eddy current induced heating of the thermal interface of an LTS coil was an adequate protection method.

Protection of conduction cooled Nb/sub 3/Sn SMES coil

A conduction cooled Nb/sub 3/Sn SMES coil requires a protection analysis different from that of the liquid helium cooled NbTi magnets. In an SMES coil constructed with a bronze processed Nb/sub 3/Sn wire, hot spot temperature is high due to a slow normal zone propagation. In a conduction cooled system, the eddy current generated heating in the cryogenic interface enables a quench back. In order to design a proper protection scheme for a cryogen free Nb/sub 3/Sn SMES system operating at 10 K, a computer code has been developed. The code simulates the quench behaviour of the coil protected with any combination of a shunt resistor, subdivision and quench back. The modelling of the quench back requires the solution of the eddy current problem and the heat transfer between the coil and the interface. These are combined with the solution of the differential equations describing the protection with a shunt resistor or subdivision. Utilizing the developed code, the protection of a 0.2 MJ conduction cooled Nb/sub 3/Sn SMES coil has been designed.

Quench characteristics of Bi-2223 coil at liquid helium temperature

The quench characteristics of Bi-2223 single and four double pancake coils operating at liquid helium temperature were investigated based on experiments and numerical simulations. Broad resistive transition was observed in the double pancake coils. The Bi-2223 double pancake coil operating at liquid helium showed a slow normal zone propagation velocity. The stability margin of the coil was confirmed to be high because of the low operating temperature and a good cooling condition. The quench characteristics of a four double pancake coil system was simulated by a numerical method. It is clear that the protection resistance has a great influence on the current decay, transient voltage and hot-spot temperature.