Quench Current Research Articles

Effect of the dissipative states of non-ideal second-type superconductors on their current-carrying capacity

The influence of the dissipative states of non-ideal second-type superconductors on their current-carrying capacity is investigated. The study performed is based on the numerical solution of non-stationary Maxwell and Fourier equations with different features of the non-linear rise of their I-V characteristics. First, the power equation of the I-V characteristic with various n-values was used to examine the states of superconductors that occur when the I-V characteristic continuously increases. Second, the obtained results are compared with the results of computer experiments simulating the thermo-electrodynamic states of superconductors when the electric field is not present in their I-V characteristics in the subcritical current range. A piecewise linear I-V equation describes such idealized modes. The results of the simulations indicate that a superconductor's ability to carry the transport current drops as the n-value decreases. Accordingly, the maximum transport current (quench current) flowing stably through a superconductor with an idealized I-V characteristic are always higher than the corresponding value calculated for superconductor with the same critical current but with a continuously increasing I-V characteristic. Moreover, a deterioration in cooling conditions or an increase in the current charging rate will also lead to a reduction in the current-carrying capacity of non-ideal second-type superconductors. The non-trivial temperature change of superconductors during formation of stable modes should necessarily be taken into account in experiments to investigate the voltage-current characteristics of superconductors, their current-carrying capacity, and loss theory.

Training Curves of Nb3Sn Rutherford Cables With a Wide Range of Impregnation Materials Measured in the BOX Facility

Training of accelerator magnets is a costly and time consuming process. The number of training quenches must therefore be reduced to a minimum. We investigate training of impregnated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cable in a small-scale experiment named BOX (BOnding Experiment). The test involves a Rutherford cable impregnated in a meandering channel simulating the environment of a canted-cosine-theta (CCT) coil. The sample is powered using a transformer and the Lorentz force is generated by an externally applied magnetic field. The low material and helium consumption enable the test of a larger number of samples. In this article, we present training of samples impregnated with alumina-filled epoxy resins, a modified resin with paraffin-like mechanical properties, and a new tough resin in development at ETH Zürich. These new data are compared with previous results published earlier. Compared to samples with unfilled epoxy resin, those with alumina-filled epoxy show favorable training properties with higher initial quench currents and fewer training quenches before reaching 80% of the critical current.

Effect of Strand Damage in Nb3Sn Rutherford Cables on the Quench Propagation in Accelerator Magnets

Accelerator magnets employing Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables are more susceptible to conductor degradation than Nb-Ti magnets. Recent measurements on a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn accelerator magnet have revealed unexpected behaviour such as decaying voltages at constant current plateaus of V-I measurements, inverse ramp rate and temperature dependence of quench currents, and anomalous quench propagation measured by so-called quench antennas. Numerical modelling has shown that these anomalies can be explained by an inhomogeneous degradation in the Rutherford cable, in which a subset of strands is fully or partially degraded. In this paper, we study how this type of degradation can affect the early stages of quench propagation. With the aid of a network model, we show how quench antenna signals can be used to diagnose inhomogeneous conductor degradation in the Rutherford cable.

Design, fabrication, and characterization of a high-field high-temperature superconducting Bi-2212 accelerator dipole magnet

The use of high-field superconducting magnets has furthered the development of medical diagnosis, fusion research, accelerators, and particle physics. High-temperature superconductors enable magnets more powerful than those possible with Nb-Ti (superconducting transition temperature ${T}_{c}$ of 9.2 K) and ${\mathrm{Nb}}_{3}\mathrm{Sn}$ (${T}_{c}$ of 18.4 K) conductors due to their very high critical field ${B}_{c2}$ of greater than 100 T near 4.2 K. However, the development of high-field accelerator magnets using high-temperature superconductors is still at its early stage. We report the construction of the world's first high-temperature superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{x}$ (Bi-2212 with ${T}_{c}$ of $\ensuremath{\sim}82\text{ }\text{ }\mathrm{K}$) accelerator dipole magnet. The magnet is based on a canted-cosine-theta design with Bi-2212 Rutherford cables. A high critical current was achieved by an overpressure processing heat treatment. The magnet was constructed from a nine-strand Rutherford cable made from industrial 0.8 mm wires. At 4.2 K, it reached a quench current of 3600 A and a dipole field of 1.64 T in a bore of 31 mm. The magnet did not exhibit the undesirable quench training common in Nb-Ti and ${\mathrm{Nb}}_{3}\mathrm{Sn}$ accelerator magnets. It quenched a dozen times without degradation. The magnet exhibited low magnetic field hysteresis ($&lt;0.1%$) as measured by a cryogenic Hall sensor. It was fast cycled to 1.47 T at $0.54\text{ }\text{ }\mathrm{T}/\mathrm{s}$ without quenches. This work validates the canted-cosine-theta Bi-2212 dipole magnet design, illustrates the fabrication scheme, and establishes an initial performance benchmark.

Current quench and vessel currents characterisation at the COMPASS tokamak

The characterisation of plasma current quench and understanding of its underlying physical processes play a crucial role when designing large fusion devices such as ITER. For the first time, an extensive analysis of the COMPASS tokamak disruption database is presented. A unique set of magnetic diagnostics allows the investigation of local toroidal and poloidal vessel currents, including currents flowing along the open magnetic field lines from the plasma to the vacuum vessel (VV) (i.e., halo currents). Area-normalised current quench times are in agreement with the ITPA 1.67 ms m−2 lower limit. Extremely fast I p quench rates of up to 0.6 MA ms−1 are observed during runaway electron campaigns at COMPASS, which are under the ITPA lower limit. Vertical movement of the plasma column is accelerated by the I p quench during major disruptions. Toroidal vessel currents of around 2% – 4% of the predisruptive plasma current are observed during I p quench. Net poloidal eddy currents are obtained by Mirnov coils and diamagnetic loop, reaching 3% of . Using the Mirnov coils it is shown that the halo current magnitude grows and its poloidal profile broadens with increasing plasma current I p . Geometric features of the VV structure and in-vessel component positions on the poloidal vessel current measurements are discussed.

A general model for simulating the quench behavior of canted-cosine-theta (CCT) quadrupole magnets

Quench-back is a complex but fascinating effect that could occur in canted-cosine-theta (CCT) magnets: the fast discharge after quench of the superconducting coil induces significant eddy currents in the conductive coil formers and then the eddy current evenly heats up the whole coil, accelerating the current decay in the coil without increasing its terminal voltage. In this study, we introduce a multi-physics model, which involves an electrical circuit module and a heat transfer module as well as their coupling, to simulate the quench behavior in a CCT quadrupole magnet with special attention on the quench-back effect. The simulation is found to reproduce the measured current decay of a CCT quadrupole magnet well, with less than 5% deviation in the quench integrals at different quench currents. Under the nominal operating condition of the studied coil, the quench-back effect reduces the quench integral by more than two-thirds, strongly releasing the difficulty of quench protection. Accurate simulation of the quench-back effect would allow it to be fully utilized in magnet design, and eventually allows higher operating current density in CCT magnets.

Experimental Research of the New Developed High-Jc Nb3Sn Superconducting Strand for 14 T MRI Magnet

The Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) has announced a project to design and manufacture a high field MRI magnet with a 900 mm warm bore and comprised of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables in 2017. The maximum magnetic induction of magnet system is 14.3 T and magnetic field homogeneity in a spherical volume with 22 cm diameter is better than 0.5 ppm. The high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand is the potential options to use for such high field magnet system. ASIPP cooperates with Western Superconducting Technologies Company (WST) to develop high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand whose critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is higher than 2000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K, 12 T. To obtain the actual performance of WST high performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand, three short samples are prepared with different non-copper ratio or different heat treatment, the critical current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> or quench current of all sample is measured at different field by the voltage-current (V-I) method, the residual resistivity ratio (RRR) of all sample is also measured. To study the strand whether it will be affected by low field instability, a small coil is built with WST strand, and the experimental result is presented and discussed in this paper.

Dynamic Bolting Analysis of Centerstack Casing G10 Ring for NSTX-U

A new dynamic bolting analysis has been conducted to verify the design quality of a G10 ring for the National Spherical Torus Experiment Upgrade (NSTX-U). The G10 ring under the centerstack casing and lower skirt plays a critical role in NSTX-U with large electromagnetic forces during plasma drifts, quench, and halo currents. Inspections of the installed condition of the G10 ring showed a poorly shimmed interface between the ring and the toroidal field flags. This paper will describe how to define the bolt preload to resolve this underperformance issue with the G10 ring by transient bolting analysis. The simulation results of transient bolt reaction forces show that the loads imposed on the G10 ring are within the tested capacity of the Keenserts and G10 ring.

Fabrication and Test of Diameter 35 mm Iron-Based Superconductor Coils

Iron-based superconductor (IBS) is expected to be the next generation high temperature superconductor with their potential good properties for the high-field application. This study focus on exploration of the characteristics of IBS coils at high field and development of the IBS coil fabrication technology. A series of diameter 35 mm IBS coils including single pancake (SPC) and double pancakes (DPC) were designed and winded with the 7-filamentary Ba 1-x K x Fe 2 As 2 (Ba122) tape which was produced by the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE-CAS). The IBS coils were divided into 3 groups according to the different fabrication processes. The tests of the IBS coils were firstly carried out at 4.2 K and 10 T background field. The highest quench current of the coils at 10 T is 69.4 A, which is about 90% of the critical current of the short sample at 10 T. Then, two SPCs were selected and tested at 4.2 K at 24 T [1]. The highest quench current of the coils at 24 T is 25.6 A, which is about 39% of the quench current at self-field. The test results were presented and analyzed with respect to the different fabrication processes.

First Field Measurements of the 15 T Nb3Sn Dipole Demonstrator MDPCT1

Within the US Magnet Development Program (MDP), a 15 T Nb3Sn dipole was developed to demonstrate a possible magnet design for a post-LHC proton-proton Collider. The magnet design is based on 60 mm aperture 4-layer shell-type coils, graded between the inner and outer layers to maximize magnet performance. Magnet coils are surrounded by vertically split thick iron laminations, connected by aluminum I-clamps, and a thick stainless-steel skin. The magnet coils were preloaded to achieve 14 T in the first test. The magnet was tested at the Vertical Magnet Test Facility (VMTF) at Fermilab. This paper reports the first results of magnetic measurements of the 15 T Nb3Sn dipole demonstrator, including dipole strength, geometrical harmonics, coil magnetization and iron saturation effects. At the maximum quench current of 9742 A we obtained a dipole strength of 14.13 T.

Quench transient current and quench propagation limit in pancake wound REBCO coils as a function of contact resistance, critical current, and coil size

It is a general belief that no insulation (NI) coil technology is a path to very high field superconducting coils. Recent experience has shown that there are aspects of NI coil design that, if not addressed, can possibly lead to coil failures. One potential problem area is the large transient currents that are associated with quench propagation in NI coils. In an attempt to understand and possibly find ways to minimize the potential for damage from quench transients, a parameter study was undertaken to examine the factors that influence the magnitude of transient currents during quench in NI coils. The characteristics of the transient currents are first examined. A study is then made of a set of test coils, looking at quench propagation and the transient current magnitude as a function of contact resistance, critical current, and importantly coil size. For each coil size, it is found that as the contact resistance increases, the magnitude of quench transient currents is reduced until a condition where effective quench propagation ceases, called the quench propagation limit (QPL). As the QPL is approached, the amplitude of the transient current is decreased and may provide a regime where quench induced stress can be effectively contained in coil designs. As coil size increases, the value of contact resistance associated with the limit of quench propagation increases as well. At large coil sizes that will be characteristic of high field REBCO magnets, the QPL extends to truly large values of contact resistance compared to values observed between bare conductors. The use of methods such as resistive films on conductors and co-wind steel will be required to increase contact resistance. In recognition of this development, the use of high contact resistance achieved in this manner is appropriately called resistive insulation coil technology.

User defined elements in ANSYS for 2D multiphysics modeling of superconducting magnets

Dynamic simulation of superconducting magnets is critical for the design of quench protection systems to prevent potentially damaging temperatures and high voltage from developing after magnet quench. Modeling these scenarios is challenging due to the many multiscale phenomena which impact magnet behavior. These range from conductor scale effects of quench and interfilament coupling currents up to the behavior of the magnet in its powering and protection circuit. In addition, a strong coupling between electromagnetic and thermal domains is required to capture temperature and field dependent material properties and quench behavior. We present a finite element approach which integrates the various effects into the commercial software ANSYS by means of programming new element types. This is shown capable of simulating the strongly coupled transient electromagnetic, thermal, and circuit behavior of superconducting magnets required for quench protection studies. A benchmarking study is presented which shows close agreement between the new ANSYS elements and a COMSOL Multiphysics implementation developed at CERN for dump resistor and coupling loss induced quench based magnet protection of a Nb3Sn block dipole. Following this, the ANSYS implementation is shown reproducing strongly coupled quench back behavior observed during the test of a Nb3Sn superconducting undulator prototype at Lawrence Berkeley National Laboratory.

A New Quench Protection System Using Dual-Capacitor Switching for Fast Energy Extraction From HTS Magnets

The high-temperature superconducting magnets with high magnetic field can store a relatively large amount of energy because of their large inductance and operating current. The magnetic energy stored in the magnet is converted into heat energy when a quench occurs, and the heat generated by the quench resistance and magnet current is sufficient to damage the magnet. Therefore, the large amount of energy stored in a superconducting magnet must be removed quickly to protect the magnet. In this paper, a new quench protection method using a capacitor and dual-switches is proposed and compared to a conventional energy extraction system that uses only a dump resistor. The simulation and test results show that this new protection system is a feasible method for reducing the temperature increase by rapidly reducing the magnet current when a quench occurs.

Development of 2-m Model Magnet of the Beam Separation Dipole With New Iron Cross Section for the High-Luminosity LHC Upgrade

Large aperture beam separation dipoles (D1 magnet) must be developed for the interaction regions in the high-luminosity large hadron collider (HL-LHC). The most important specifications of this magnet are a coil aperture of 150 mm and field integral of 35 Tm, which will be realized by Nb-Ti superconducting magnets operated at 12 kA at 1.9 K. High Energy Accelerator Research Organization (KEK) is developing 2-m-long model magnets for D1. The first model magnet was re-assembled after increasing the coil pre-stress (MBXFS1b); subsequently, it showed good training performance, achieving maximum quench current, which was higher than the ultimate current. However, mechanical support of the coils was still insufficient in MBXFS1b. Especially at coil end, some coil end blocks deformed toward the coil bore due to Lorentz force. In the second model magnet (MBXFS2), the change of iron yoke cross section was requested so that the position of heat exchanger holes penetrating iron yoke is in agreement with those of the final focusing Nb 3 Sn quadrupole magnets. Because this has a large impact on field quality, magnetic design including the magnet's cross section and coil end shape was updated. Some countermeasures were also taken to enhance mechanical support of the coil such as wet-winding with radiation-resistant resin at coil end and increase in coil pre-stress. This paper reports design update, fabrication, and training performance of MBXFS2 with new iron cross section.

Mechanical Design, Assembly, and Test of LPF1: A 10.2 T Nb3Sn Common-Coil Dipole Magnet With Graded Coil Configuration

An Nb3Sn/NbTi subscale dipole magnet named LPF1 with graded coil design and common-coil configuration has been designed, assembled, and tested at the Institute of High Energy Physics, Beijing, China. Compared to the previously proposed FECD1, LPF1 is designed with a longer outer NbTi racetrack coil to reduce the peak field in the coil end. The computed bore field and peak coil field of LPF1 is 12.0 T and 12.1 T, if it works at 6100 A with a safety margin of 17% at 4.2 K. The horizontal and vertical directions of the coil packs are preloaded by using bladder and key technology while the axial direction of the coil packs is preloaded by pretightening four aluminum rods. Before assembly, we tested the electron-beam welded bladders in a subscale shell-support structure and verified the strain measurement system. During assembly, we preloaded the coil packs gradually by referring to the measured shell strain after inserting stainless steel keys and removing the bladders. After assembly, we started to test the subscale dipole magnet vertically at 4.2 K. LPF1 reached a bore field of 10.2 T after 13 training quenches and then it reached a stable quench current without detraining. This paper presents the updated design parameters, the stress analysis, the magnet assembly, and test of LPF1.

Tripled critical current in racetrack coils made of Bi-2212 Rutherford cables with overpressure processing and leakage control

We fabricated three racetrack coils (RC1, RC2, and RC3) from Bi-2212 Rutherford cables (17-strand, thickness × width = 1.44 mm × 7.8 mm, strand diameter = 0.8 mm) and applied overpressure processing heat treatment (OPHT). The quench currents of RC1 and RC2 reached 5268 A and 5781 A, respectively, despite them still, surprisingly, exhibiting some Bi-2212 leakage to the surface. After removing most of the leakages using a simple-to-implement insulation scheme, the quench current of RC3 improved to 6485 A, which is about three times the average quench current of a dozen racetrack coils that had been fabricated and reacted using the conventional 1 bar heat treatment. The results confirm the effectiveness of the OPHT technology and the new leakage control scheme for coils made from Bi-2212 Rutherford cables. Coils exhibited an increased quench current with increasing the current ramp rate from 5 to 200 A s−1; they were quite stable against point and transient disturbances, and were capable of adsorbing persistent Joule heating at ∼80 mW for >15 s before quenching. These behaviors are different from Nb–Ti and Nb3Sn accelerator magnets. Overall, our results provide a critical evaluation and verification of Bi-2212 wire and magnet technologies (wire, insulation, heat treatment, coil fabrication, and coil operation), reveal crucial new stability features of Bi-2212 magnets, and demonstrate technological options for it to become a practical high-field magnet technology.

Powering of an HTS dipole insert-magnet operated standalone in helium gas between 5 and 85 K

This paper describes the standalone magnet cold testing of the high temperature superconducting (HTS) magnet Feather-M2.1-2. This magnet was constructed within the European funded FP7-EUCARD2 collaboration to test a Roebel type HTS cable, and is one of the first high temperature superconducting dipole magnets in the world. The magnet was operated in forced flow helium gas with temperatures ranging between 5 and 85 K. During the tests a magnetic dipole field of 3.1 T was reached inside the aperture at a current of 6.5 kA and a temperature of 5.7 K. These values are in agreement with the self-field critical current of the used SuperOx cable assembled with Sunam tapes (low-performance batch), thereby confirming that no degradation occurred during winding, impregnation, assembly and cool-down of the magnet. The magnet was quenched many tens of times by ramping over the critical current and no degradation nor training was evident. During the tests the voltage over the coil was monitored in the microvolt range. An inductive cancellation wire was used to remove the inductive component, thereby significantly reducing noise levels. Close to the quench current, drift was detected both in temperature and voltage over the coil. This drifting happens in a time scale of minutes and is a clear indication that the magnet has reached its limit. All quenches happened approximately at the same average electric field and thus none of the quenches occurred unexpectedly.

Dynamics of edge currents in a linearly quenched Haldane model

In a finite time quantum quench of the Haldane model, the Chern number determining the topology of the bulk remains invariant, as long as the dynamics is unitary. Nonetheless, the corresponding boundary attribute, the edge current, displays interesting dynamics. For the case of sudden and adiabatic quenches the post quench edge current is solely determined by the initial and the final Hamiltonians, respectively. However for a finite time ($\tau$) linear quench in a Haldane nano ribbon, we show that the evolution of the edge current from the sudden to the adiabatic limit is not monotonic in $\tau$, and has a turning point at a characteristic time scale $\tau=\tau_c$. For small $\tau$, the excited states lead to a huge unidirectional surge in the edge current of both the edges. On the other hand, in the limit of large $\tau$, the edge current saturates to its expected equilibrium ground state value. This competition between the two limits lead to the observed non-monotonic behavior. Interestingly, $\tau_c$ seems to depend only on the Semenoff mass and the Haldane flux. A similar dynamics for the edge current is also expected in other systems with topological phases.

Feasibility Study of the Impregnation of a No-Insulation HTS Coil Using an Electrically Conductive Epoxy

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using an electrically conductive epoxy resin. Recently, several studies of HTS coils without turn-to-turn insulation have been reported for field coils used in rotating machines such as motors and generators. The NI winding technique enhances the thermal stability of the HTS coil without requiring complicated protection techniques because the quench current is automatically bypassed through the turn-to-turn contacts within the HTS coil. Nevertheless, there is still a question as to whether the NI technique can be applied to rotating machines. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. For HTS field coils intended for use in rotating machines, epoxy impregnation is generally necessary to protect the HTS field coil from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability [1] , [2] . However, the NI HTS coil cannot be fabricated by wet winding using epoxy resin because epoxy resins such as Stycast 2850 FT and CTD 521 are electrically insulating materials. This study examines the electrical stability of an NI HTS coil impregnated with an epoxy resin containing electrically conductive particles. The results are likely to present useful data for the application of electrically conductive epoxy impregnated NI HTS coils to rotating machines.

Degradation of the current-carrying capacity of low-temperature superconducting composites under the action of thermal perturbations

The stability of transport current introduced into a niobium titanium superconducting composite subjected to an external pulsed thermal perturbation has been studied. Stable states have been theoretically analyzed by solving Fourier and Maxwell equations that describe the thermoelectrodynamic states of lowtemperature superconductors with flux creep. It has been shown that, if the transport current is permanently introduced, subcritical thermal perturbations, i.e., perturbations that do not take the composite to a normal state provided that the current does not exceed the quench current, may result in the appearance of unstable current states. The higher the energy of the external thermal perturbation, the lower the instability onset current. It has been found that the degradation of the current-carrying capacity of the superconducting composite is due to intense heat release inside the superconductor, which is initiated by the thermal perturbations, and depends on the current input rate, the instant of time the current input is terminated, and cooling conditions.