@article{ma_geng_chan_schwartz_coombs_2020, title={A temperature-dependent multilayer model for direct current carrying HTS coated-conductors under perpendicular AC magnetic fields}, volume={33}, ISSN={["1361-6668"]}, DOI={10.1088/1361-6668/ab6fe9}, abstractNote={Abstract}, number={4}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Ma, Jun and Geng, Jianzhao and Chan, Wan Kan and Schwartz, Justin and Coombs, Tim}, year={2020}, month={Apr} }
 @article{ma_geng_chan_gawith_li_shen_ozturk_yang_hu_coombs_2020, title={Impact of Stabilizer Layers on the Thermal-Electromagnetic Characteristics of Direct Current Carrying HTS Coated Conductors under Perpendicular AC Magnetic Fields}, volume={30}, ISSN={["1558-2515"]}, DOI={10.1109/TASC.2020.2977004}, abstractNote={When a type-II high Tc superconductor carrying a direct current is subjected to a perpendicular AC magnetic field, a direct current voltage will appear. This phenomenon is called dynamic resistance effect. In general, the high temperature superconducting (HTS) coated conductor (CC) has two stabilizer layers. However, the impacts of two stabilizer layers on the dynamic resistance, DC electrical field, losses, and temperature rise haven't been studied yet. This paper presents the impacts of the stabilizer layers and their resistivity on the dynamic resistance effect and HTS CC tape's thermal-electromagnetic behaviors by using a temperature dependent FEM model. This work reveals that the stabilizer impacts significantly on the dynamic resistance, dc voltage, power loss, and temperature rise. It is will help design high-performance AC magnetic field-controlled PCS and switches based HTS devices.}, number={4}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Ma, Jun and Geng, Jianzhao and Chan, Wankan and Gawith, Jamie and Li, Chao and Shen, Boyang and Ozturk, Yavuz and Yang, Jiabin and Hu, Jintao and Coombs, Tim A.}, year={2020}, month={Jun} }
 @article{gao_chan_wang_zhou_schwartz_2020, title={Stress, strain and electromechanical analyses of (RE)Ba2Cu3Ox conductors using three-dimensional/two-dimensional mixed-dimensional modeling: fabrication, cooling and tensile behavior}, volume={33}, ISSN={["1361-6668"]}, DOI={10.1088/1361-6668/ab7778}, abstractNote={High temperature superconducting (HTS) conductors, represented by Rare Earth-Barium-Copper-Oxide (REBCO) conductors, are promising for high energy and high field superconducting applications. In practical applications, however, the HTS conductors experience different stresses and strains, including residual stresses due to thermal mismatch and tensile stresses due to Lorentz forces, resulting in some circumstances to a reduction in the load-carrying capacity as well as the risk of degradation in conductor critical current. In this study a mixed-dimensional high-aspect-ratio laminated composite finite element model for REBCO conductor is developed for stress and strain analyses in the processes of fabricating and cooling, as well as tensile testing. The model includes all the major constituent layers of a typical REBCO conductor and is experimentally validated. First, the thermal residual stresses and strains accumulated during the fabrication and cooling processes are analyzed by a multi-step modeling method that emulates the manufacturing process. Then, with the residual stresses and strains as initial stresses and strains, the mechanical behavior under a tensile load is studied. Lastly, a phenomenological critical current-strain model based on the Ekin power-law formula and the Weibull distribution function is combined with the mixed-dimensional conductor model to predict the strain dependence behavior of critical current in the reversible and irreversible degradation strain ranges. Simulation results show that the multi-step modeling is an effective method for stress and strain analyses of REBCO conductors during the fabrication and cooling processes and under and tensile loads. Compressive thermal residual stress generated on the REBCO layer during fabrication and cooling strongly affects the subsequent mechanical and current-carrying properties. Stress–strain curves generated by tensile loads are analyzed and experimentally validated at both the conductor and constituent-layer levels. Simulation results for the strain dependence of critical current are in good agreement with experiment data in both the reversible and irreversible degradation stages.}, number={4}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Gao, Peifeng and Chan, Wan-Kan and Wang, Xingzhe and Zhou, Youhe and Schwartz, Justin}, year={2020}, month={Apr} }
 @article{zhou_chan_schwartz_2019, title={Modeling of Quench Behavior of YBa2Cu3O7-delta Pancake Magnets and Distributed-Temperature-Sensing-Based Quench Detection for Operating Temperature 30-77 K}, volume={29}, ISSN={["1558-2515"]}, DOI={10.1109/TASC.2018.2874423}, abstractNote={A two-dimensional/three-dimensional (2-D/3-D) mixed electrothermal model is proposed for the simulation of quench behavior of high-temperature superconducting (HTS) pancake magnets, where a 2-D electrothermal model is proposed to simulate the YBa<sub>2</sub>Cu<sub>3</sub>O<inline-formula><tex-math notation="LaTeX">$_{7\text{-}\delta}$ </tex-math></inline-formula> (YBCO) subcoil and is coupled with the remaining parts of the YBCO magnet, which are treated as 3-D homogeneous coils. For operating temperature from 30 to 77 K, the quench behavior of four YBCO pancake coils (two Kapton-insulated coils and two TiO<sub>2</sub>-insulated coils) are simulated. Thermal equilibrium states are found for both Kapton- and TiO<sub>2</sub>-insulated coils. The thermal conductivity of insulating materials (Kapton, TiO<sub>2</sub>) significantly affects the equilibrium temperature profiles (ETPs) and the minimum quench energy (MQE), especially for relatively high operating temperature (e.g., 65–77 K). The distributed-temperature-sensing-based (DTS-based) quench detection criterion can be established on ETPs. The effect of the thickness of insulating materials on ETPs and MQEs is relatively weak, especially under relatively low operating temperature. The key parameters of ETP-based quench detection criterion, such as the reference temperature, the peak temperature, and the minimum normal zone size, are obtained for the operating temperature from 30 to 77 K.}, number={1}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Zhou, Jun and Chan, Wan Kan and Schwartz, Justin}, year={2019}, month={Jan} }
 @article{gao_chan_wang_schwartz_2018, title={Mixed-dimensional modeling of delamination in rare earth-barium-copperoxide coated conductors composed of laminated high-aspect-ratio thin films}, volume={31}, ISSN={["1361-6668"]}, DOI={10.1088/1361-6668/aac55c}, abstractNote={Rare earth-barium-copper-oxide (REBCO) coated conductors are promising conductors for high energy, high field and high temperature superconducting applications. In the case of epoxy-impregnated REBCO superconducting coils, however, excessive transverse stresses generated from winding, cooling, and Lorentz forces on the REBCO conductors can cause delamination, resulting in reduction in the load-carrying capacity as well as significant degradation in the coil’s critical current. In this study, the stresses and strains, and delamination in a REBCO conductor are analyzed via a mixed-dimensional finite element method (FEM) based on the cohesive zone model (CZM). The mixed-dimensional method models any number of laminated high-aspect-ratio thin layers in a composite as stacked two-dimensional (2D) surfaces, thus, resolving the thickness-dependent meshing and computational problems in modeling such composites with full three-dimensional (3D) FEM approaches. In the studied coated conductor, the major thin constituent layers, namely, the silver, REBCO and buffer layers, are modeled as 2D surfaces while the relatively thick stabilizer and substrate are in 3D layers. All the adjacent layers are coupled via spring equations under the CZM framework. The mixed-dimensional delamination model is validated by a full-3D FEM counterpart model. Simulation results show that the mixed-dimensional model performs simulations with much higher computational efficiency than the full-3D counterpart while maintaining sufficient accuracy. Effects of the anvil size and initial crack size on delamination behavior are discussed and compared to experimental phenomena. Furthermore, the stress distributions of the constituent layers of the conductor under different delamination initiation sites are predicted.}, number={7}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Gao, Peifeng and Chan, Wan-Kan and Wang, Xingzhe and Schwartz, Justin}, year={2018}, month={Jul} }
 @article{zhou_chan_schwartz_2018, title={Quench Detection Criteria for YBa2Cu3O7-delta Coils Monitored via a Distributed Temperature Sensor for 77 K Cases}, volume={28}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2018.2815920}, abstractNote={Distributed temperature sensing (DTS), such as Rayleigh-scattering interrogated optical fiber (RIOF) sensing, is a promising method for detecting quenches in high-temperature superconductor (HTS) magnets. One key for the successful implementation of RIOF-based DTS for quench detection is to identify effective quench detection criteria for the onset of a quench. In this paper, two DTS-based quench detection criteria, and their dependence on the operating current and heat disturbance characteristics, are investigated through numerical simulations of quench behavior in a YBa2 Cu3O7‐δ (YBCO) HTS helix coil cooled by a liquid nitrogen (LN2) bath and a YBCO HTS pancake coil cooled by conduction at 77 K. One is based on the minimum propagation zone (MPZ). The reference temperature to define the MPZ size is found for different operating currents. The other is based on the equilibrium temperature profile, in which the peak temperature and a characteristic normal zone length are found from a preselected reference temperature. The advantages and disadvantages of the two quench detection criterions are discussed and compared. Simulation results show that both criteria are independent of the nature of unpredictable heat disturbances. Similar to the helix coil, equilibrium temperature profiles independent of unpredictable disturbances are found for the pancake coil with different operating currents.}, number={5}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Zhou, Jun and Chan, Wan Kan and Schwartz, Justin}, year={2018}, month={Aug} }
 @article{chan_schwartz_2017, title={Improved stability, magnetic field preservation and recovery speed in (RE)Ba2Cu3Ox-based no-insulation magnets via a graded-resistance approach}, volume={30}, ISSN={["1361-6668"]}, DOI={10.1088/1361-6668/aa6eef}, abstractNote={The no-insulation (NI) approach to winding (RE)Ba2Cu3Ox (REBCO) high temperature superconductor solenoids has shown significant promise for maximizing the efficient usage of conductor while providing self-protecting operation. Self-protection in a NI coil, however, does not diminish the likelihood that a recoverable quench occurs. During a disturbance resulting in a recoverable quench, owing to the low turn-to-turn contact resistance, transport current bypasses the normal zone by flowing directly from the current input lead to the output lead, leading to a near total loss of the azimuthal current responsible for magnetic field generation. The consequences are twofold. First, a long recovery process is needed to recharge the coil to full operational functionality. Second, a fast magnetic field transient is created due to the sudden drop in magnetic field in the quenching coil. The latter could induce a global inductive quench propagation in other coils of a multi-coil NI magnet, increasing the likelihood of quenching and accelerating the depletion of useful current in other coils, lengthening the post-quench recovery process. Here a novel graded-resistance method is proposed to tackle the mentioned problems while maintaining the superior thermal stability and self-protecting capability of NI magnets. Through computational modeling and analysis on a hybrid multiphysics model, patterned resistive-conductive layers are inserted between selected turn-to-turn contacts to contain hot-spot heat propagation while maintaining the turn-wise current sharing required for self-protection, resulting in faster post-quench recovery and reduced magnetic field transient. Effectiveness of the method is studied at 4.2 and 77 K. Through the proposed method, REBCO magnets with high current density, high thermal stability, low likelihood of quenching, and rapid, passive recovery emerge with high operational reliability and availability.}, number={7}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Chan, Wan Kan and Schwartz, Justin}, year={2017}, month={Jul} }
 @article{rogers_chan_schwartz_2016, title={Effects of room-temperature tensile fatigue on critical current and n-value of IBAD-MOCVD YBa2Cu3O7-x/Hastelloy coated conductor}, volume={29}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/29/8/085013}, abstractNote={REBa2Cu3O7−x (REBCO) coated conductors potentially enable a multitude of superconducting applications, over a wide range of operating temperatures and magnetic fields, including high-field magnets, energy storage devices, motors, generators, and power transmission systems (Zhang et al 2013 IEEE Trans. Appl. Supercond. 23 5700704). Many of these are AC applications and thus the fatigue properties may be limiting (Vincent et al 2013 IEEE Trans. Appl. Supercond. 23 5700805). Previous electromechanical studies have determined the performance of REBCO conductors under single cycle loads (Barth et al 2015 Supercond. Sci. Technol. 28 045011), but an understanding of the fatigue properties is lacking. Here the fatigue behavior of commercial ion beam assisted deposition–metal organic chemical vapor deposition REBCO conductors on Hastelloy substrates is reported for axial tensile strains up to 0.5% and up to 100 000 cycles. Failure mechanisms are investigated via microstructural studies. Results show that REBCO conductors retained Ic(ε)/Ic0 = 0.9 for 10 000 cycles at ε = 0.35% and ε = 0.45% strain, and ε = 0.5% for 100 cycles. The main cause of fatigue degradation in REBCO conductors is crack propagation that initiates at the slitting defects that result from the manufacturing process.}, number={8}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Rogers, Samuel and Chan, Wan Kan and Schwartz, Justin}, year={2016}, month={Aug} }
 @article{wang_chan_schwartz_2016, title={Self-protection mechanisms in no-insulation (RE) Ba2Cu3Ox high temperature superconductor pancake coils}, volume={29}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/29/4/045007}, abstractNote={No-insulation (NI) high temperature superconducting (HTS) coils possess much higher thermal stability than similar traditionally insulated HTS coils. Some NI coils are self-protecting in the sense that they fully recover after a quench without any external protection mechanism to dissipate the stored energy. The underlying mechanisms that make NI coils highly stable or even self-protecting, however, remain unclear. To answer this question, a numerical multiphysics quench model for NI pancake coils is built to study the electrical, thermal and magnetic behavior of NI coils subjected to local heat disturbances. The multiphysics model is built from an electric network model, tightly coupled to a two-dimensional thermal coil model and a three-dimensional magnetic field coil model. The results show that when heat disturbance initiates a local normal region on a turn, the transport current is redistributed not only from the local normal region, but also along the entire turn. The redistributed current flows in the form of radial current across the turn-to-turn contact resistance along the entire turn to the neighboring turns which are still in the superconducting state, driving these turns to an overcurrent state. This full-turn current sharing and overcurrent operation accelerate the redistribution of current away from the hot-spot, reducing localized Joule heating that would otherwise cause a sustainable quench. The results also show that the magnetic field generated at the coil center drops rapidly and the coil voltage changes dynamically during the early stage of normal zone formation. These phenomena can be utilized as effective methods for quench detection in NI coils by monitoring the magnetic field and coil voltage.}, number={4}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Wang, Y. and Chan, Wan Kan and Schwartz, Justin}, year={2016}, month={Apr} }
 @article{phillips_chan_schwartz_2015, title={Enhanced Quench Protection in REBa2Cu3O delta-7-Based Coils by Enhancing Three-Dimensional Quench Propagation via Thermally Conducting Electrical Insulation}, volume={25}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2015.2452224}, abstractNote={This simulation explores the effects of insulation properties on quench propagation in ReBa2Cu3Oδ-7-based coils. At present, superconducting magnets primarily use insulators that are electrically and thermally insulating, for example, Kapton. Here, the impact of varying the thermal conductivity of the electrical insulation on quench behavior is reported. In particular, the behavior of a Kapton-insulated coil is compared with one insulated with doped TiO 2, one insulated with “ideal Al2O3”, and one noninsulated coil. The effects on minimum quench energy and normal zone propagation behavior are investigated. In addition, a new concept, the current sharing volume (CSV), which accounts for two- or three-dimensional normal zone propagation, is introduced. The CSV is defined as the volume of coil for which the temperature is above the current sharing temperature. The simulation results show that the transverse thermal conductivity and insulation thickness strongly influence the normal zone propagation velocity, thus impacting the quench detection time and hotspot temperature. As expected, the coils insulated with the higher thermal conductivity alternatives exhibited faster normal zone growth and lower hotspot temperatures relative to CSV growth. The impact of improved thermal conductivity of turn-to-turn insulation becomes even greater when distributed sensing replaces voltage-based sensing.}, number={5}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Phillips, Makita R. and Chan, Wan Kan and Schwartz, Justin}, year={2015}, month={Oct} }
 @article{le_chan_schwartz_2014, title={A two-dimensional ordinary, state-based peridynamic model for linearly elastic solids}, volume={98}, ISSN={["1097-0207"]}, DOI={10.1002/nme.4642}, abstractNote={SUMMARY}, number={8}, journal={INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING}, author={Le, Q. V. and Chan, W. K. and Schwartz, J.}, year={2014}, month={May}, pages={547–561} }
 @article{le_chan_schwartz_2014, title={Two-dimensional peridynamic simulation of the effect of defects on the mechanical behavior of Bi2Sr2CaCu2Ox round wires}, volume={27}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/27/11/115007}, abstractNote={Ag/AgX sheathed Bi2Sr2CaCu2Ox (Bi2212) is the only superconducting round wire (RW) with high critical current density (Jc) at high magnetic (>25 T) and is thus a strong candidate for high field magnets for nuclear magnetic resonance and high energy physics. A significant remaining challenge, however, is the relatively poor electromechanical behavior of Bi2212 RW, yet there is little understanding of the relationships between the internal Bi2212 microstructure and the mechanical behavior. This is in part due to the complex microstructures within the Bi2212 filaments and the uncertain role of interfilamentary bridges. Here, two-dimensional peridynamic simulations are used to study the stress distribution of the Bi2212 RWs under an axial tensile load. The simulations use scanning electron micrographs obtained from high Jc wires as a starting point to study the impact of various defects on the distribution of stress concentration within the Bi2212 microstructure and Ag. The flexibility of the peridynamic approach allows various defects, including those captured from SEM micrographs and artificially created defects, to be inserted into the microstructure for systematic study. Furthermore, this approach allows the mechanical properties of the defects to be varied, so the effects of porosity and both soft and hard secondary phases are evaluated. The results show significant stress concentration around defects, interfilamentary bridges and the rough Bi2212/Ag interface. In general, the stress concentration resulting from porosity is greater than that of solid-phase inclusions. A clear role of the defect geometry is observed. Results indicate that crack growth is likely to initiate at the Ag/Bi2212 interface or at voids, but that voids may also arrest crack growth in certain circumstances. These results are consistent with experimental studies of Bi2212 electromechanical behavior and magneto-optical imaging of crack growth.}, number={11}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Le, Q. V. and Chan, W. K. and Schwartz, J.}, year={2014}, month={Nov} }
 @article{chan_flanagan_schwartz_2013, title={Spatial and temporal resolution requirements for quench detection in (RE)Ba2Cu3Ox magnets using Rayleigh-scattering-based fiber optic distributed sensing}, volume={26}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/26/10/105015}, abstractNote={One of the key remaining challenges to safe and reliable operation of large, high temperature superconductor (HTS)-based magnet systems is quench detection and protection. Due to the slow quench propagation in HTS systems, the conventional discrete voltage-tap approach developed for NbTi and Nb3Sn magnets may not be sufficient. In contrast, a distributed temperature profile, generated by a distributed temperature sensor and facilitating continuous monitoring of the temperature at any monitored locations within a magnet with high spatial resolution, may be required. One such distributed temperature sensing option is the use of Rayleigh-based fiber optic sensors (FOS), which are immune to electromagnetic interference. The detection of a quench via Rayleigh-based FOS relies on converting the spectral shifts in the Rayleigh scattering spectra into temperature variations. As a result, the higher the spatial sampling resolution the larger the data processing volume, and thus the lower the temporal sampling resolution. So, for effective quench detection, which requires the quick and accurate identification of a hot spot, it is important to find a balance between the spatial and temporal resolutions executable on a given data acquisition and processing (DAQ) system. This paper discusses a method for finding an appropriate DAQ technology that matches the characteristic of a superconducting coil, and determining the acceptable resolutions for efficient and safe quench detection. A quench detection algorithm based on distributed temperature sensing is proposed and its implementation challenges are discussed.}, number={10}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Chan, W. K. and Flanagan, G. and Schwartz, J.}, year={2013}, month={Oct} }
 @article{chan_schwartz_2011, title={Three-Dimensional Micrometer-Scale Modeling of Quenching in High-Aspect-Ratio YBa2Cu3O7-delta Coated Conductor Tapes-Part II: Influence of Geometric and Material Properties and Implications for Conductor Engineering and Magnet Design}, volume={21}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2011.2169670}, abstractNote={YBa2Cu3O7-δ (YBCO) coated conductors (CCs) show great promise for applications, but due to a very slow normal-zone propagation velocity (NZPV), quench detection and protection in YBCO magnets may be difficult. Present YBCO CCs have been developed with a primary focus on maximizing the critical current density for elevated-temperature low-field or low-temperature high-field applications. As the market for magnet applications progresses, it becomes important to consider design parameters such as the thicknesses and properties of all YBCO CC components, with the intent of considering quench-related behaviors as an integral part of the conductor and magnet design processes. Thus, it is important to know the impacts of conductor parameters on quench behavior. Considering that the YBCO layer itself is on the order of a micrometer in thickness, quench behavior must also be considered at this scale length. Here, the highly accurate experimentally validated micrometer-scale 3-D tape model reported in Part I is used to study how variations in CC geometry and material properties affect quench behavior, including the NZPV, hot-spot temperature, and minimum quench energy. The parametric variations focus on quantities that can be most readily modified by CC manufacturers. Based on simulation results, the relative sensitivities of the quench quantities to the parametric variations are calculated to identify which CC design parameters are most impactful on quench behavior. The implications of these results for quench detection and protection are discussed.}, number={6}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Chan, Wan Kan and Schwartz, Justin}, year={2011}, month={Dec}, pages={3628–3634} }