Dump Resistor Research Articles

Design of a REBCO Insert Coil for a Cryogen-Free 25-T Superconducting Magnet

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T 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) insert coil and outer 14-T low-temperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single- and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford–McMahon/Joule–Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Ω.

Quench Simulation of the Hybrid Magnet Superconducting Outsert Being Built in China

Quench simulation of the hybrid magnet superconducting outsert being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences was performed. The hybrid magnet is composed of a superconducting outsert magnet and a resistive insert magnet. The superconducting outsert will produce a central magnetic field of 11.0 T at 13,410 A. An induced current of 1270 A in the superconducting outsert is generated during resistive insert magnet trip. In case of a quench occurs from an over-current condition, the superconducting outsert magnet will produce a high hot spot temperature. The positive temperature coefficient dump resistor was used for accelerating the current decay during a quench. In this paper, the quench propagation analysis of the superconducting outsert under an over-current condition was presented.

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.

Development of a Prototype Hybrid DC Circuit Breaker for Superconducting Magnets Quench Protection

A quench protection system for superconducting magnets mainly involves a mechanical dc circuit breaker (CB) and a dump resistor where the entire energy of the inductive magnet is dumped during quench. During the current commutation to the dump resistor, these mechanical CB experiences arcing at their contacts, which causes degradation in the performance as well as life expectancy of the CB, affecting the reliability of the entire quench protection system and appears as threat to the expensive and critical superconductor magnets also. A combination of mechanical and static breakers, which is termed a hybrid dc CB, presents a novel technique for arcless current commutation into the dump resistor. Furthermore, the current commutation time can also be reduced by adopting real-time switching schemes. A series of experiments has been done at the Magnet Technology Development Division Laboratory, Institute for Plasma Research, Gandhinagar, India, for testing the hybrid CB operation and operation at various current levels up to 800 A using a mechanical contactor as the mechanical CB and an insulated gate bipolar transistor as the static switch. A significant improvement in the voltage profile across the dump resistor has been observed using the hybrid CB exhibiting arcless current commutation into the dump resistor. Furthermore, an overall reduction in the current interruption time between a mechanical CB operation and a hybrid CB has been observed. The time for current commutation into the dump resistor with the conventional mechanical breaker is 48 ms, whereas that with the hybrid CB is observed to be 18 ms.

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.

AC Operation of Gd–Ba–Cu–O High ${\rm T}_{\rm C}$ Superconducting Magnet for Magnetic Refrigeration

Magnetic refrigeration requires a strong magnetic field and its alternation for the magnetization and demagnetization processes. In this paper, a conduction-cooled HTS magnet system was fabricated and tested for a magnetic refrigerator. It consists of a HTS magnet, a pair of metallic and HTS current leads, and a cryocooler. The Gd-Ba-Cu-O coated conductor with polyimide tape insulation was used for the HTS magnet and wound by standard layer-wound method. Stainless steel bore was assembled with OFHC copper supporting plates of the HTS magnet. The OFHC copper strips were inserted between interlayers of the conductors to enhance axial thermal conduction. Apiezon N grease with hexane solution was applied on every layer of the HTS magnet to minimize thermal contact resistance between the conductor and the OFHC copper strips. The HTS magnet was cooled to 17 K. The charging and discharging of the HTS magnet were achieved by the unipolar power supply and an external dump resistor. Stable magnetic field of 3.25 T was achieved with 150 A. Alternating magnetic field was stably generated with the peak of 2.16 T (0.11 T/s). In this paper, the fabrication method of AC magnet and the operational issues of the conduction-cooled HTS magnet are discussed.

Suppression of Catastrophic Thermal Runaway for a REBCO Innermost Coil of an LTS/REBCO NMR Magnet Operated at 400–600 MHz (9.4–14.1 T)

The use of REBCO coils is a promising approach to achieve super-high field NMR magnets. As the first step in this direction, we have started to develop an LTS/REBCO NMR magnet operated at 400-600 MHz (9.4-14.1 T). In this paper, based on numerical simulations, we describe a study of the thermal runaway of the REBCO innermost coil, which is one of the major problems of this type of magnet. Based on the simulation results, if an LTS coils quench occurs, the current in the REBCO coil continues to increase due to the electromagnetic induction and leads to a natural thermal runaway with a very high current density. The thermal runaway is limited to only the top and bottom ends of the REBCO coil, as the radial magnetic field is highest there and the quench propagation velocity is quite low. The current density for this type of thermal runaway is much higher than that of the operating current, resulting in the catastrophic overheating of the coil. This is the worst case scenario which must be prevented for the LTS/REBCO NMR magnet. An optimal size dump resistor for the REBCO coil can prevent the natural thermal runaway by suppressing the current increase due to electromagnetic induction.

The Study of Optimal Structure and Value of Dump Resistance in Direct-Drive Permanent Magnet Synchronous Generators

This paper studied the direct-drive permanent magnet synchronous machine (permanent magnet synchronous generator, PMSG) Chopper optimal topology and resistance value. Compared the different Chopper circuit low voltage ride-through capability in the same grid fault conditions in simulation. This paper computes the dump resistance ceiling according to the power electronic devices and over-current capability. Obtaining the dump resistance low limit according to the temperature resistance allows, and calculating the optimal value by drop voltage in the DC-Bus during the fault. The feasibility of the proposed algorithm is verified by simulation results.

Quench Analysis of the Hybrid-Magnet Superconducting Outsert

Quench propagation analysis of the superconducting outsert for the 45-T hybrid magnet being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences was performed. The superconducting outsert was wound with a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductor cabled in a 316LN jacket cooled with supercritical helium at 4.5 K. The superconducting outsert can produce 11.0-T central magnetic field at an operating current of 13410 A. An active protection circuit with a dump resistor of 0.272 Ω was adopted to transfer the stored magnetic energy during a dump operation. In this paper, the magnetic field and strain of the superconducting outsert were analyzed. During a quench, the hot spot temperature of the cable and the helium pressure inside a jacket for different quench delay times were introduced.

Cold Test Results of the LARP HQ $\hbox{Nb}_{3} \hbox{Sn}$ Quadrupole Magnet at 1.9 K

The high gradient quadrupole magnet is a 120-mm-aperture, 1-m-long Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn quadrupole developed by the LHC Accelerator Research Program collaboration in support of the High-Luminosity LHC project. Several tests were performed at Lawrence Berkeley National Laboratory in 2010-2011 achieving a maximum gradient of 170 T/m at 4.4 K. As a next step in the program, the latest model (HQ01e) was sent to CERN for testing at 1.9 K. As part of this test campaign, the magnet training has been done up to a maximum current of 16.2 kA corresponding to 85% of the short sample limit. The ramp rate dependence of the quench current is also identified. The efficiency of the quench heaters is then studied at 4.2 K and at 1.9 K. The analyses of the magnet resistance evolution during fast current discharge showed evidence of quench whereas high energy quenches have been successfully achieved and sustained with no dump resistor.

Conceptual Design of the JLab Hall D Replacement Solenoid

The 12 GeV upgrade at Jefferson Lab includes plans for a new solenoid that will replace the existing solenoid made with the Large Aperture Superconducting Solenoid (LASS) coils from Stanford Linear Accelerator Center (SLAC). The conceptual design for the replacement solenoid presented here includes the magnetic design, winding arrangement, conductor selection, quench detection, and protection and cooling scheme. The magnetic design implements three separate coils to provide a 3.8 T field parallel to the beam direction and addresses the fringe field requirements of the facility while integrating into the existing iron yoke. The conductor consists of Superconducting Super Collider (SSC) cable (Nb-Ti) soldered into a copper channel stabilizer. The conductor is layer wound the hard way onto individual internal mandrels. The quench protection system implements a dump resistor and switch. Results from a 3-D quench code are provided for quench initiating at different locations. The magnet is conduction cooled by natural circulation of two-phase helium through cooling tubes mounted at the Outside Diameter (OD) of the cold mass.

高温超電導コイルの安定性評価基準とクエンチ検出・保護

Since high-temperature superconducting (HTS) conductors have a greater volumetric heat capacity at operating temperatures envisaged for practical applications, the possibility of quench caused by a mechanical disturbance such as friction due to wire movement is much lower than that for low-temperature superconducting (LTS) coils. In real applications such as superconducting magnetic energy storage (SMES) systems, however, electrical charging and discharging are repeated and the HTS conductor may deteriorate locally due to cyclic mechanical strain. We also have to consider the possibility of quench caused by a failure in the power supply, cooling system or similar problems. Therefore, a protection scheme assuming quench is also required for HTS coils. In this report, we describe a method of determining the appropriate stabilizer thickness of the YBCOcoated conductors from the perspective of the quench protection of the YBCO coil, which dumps the stored magnetic energy in the coils via external dump resistance. We have been developing a quench detection method for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of an electrically insulated YBCO-coated conductor. Since the normal-zone propagation velocity is quite slow in HTS coils, the detection of a non-recovering normal zone using a voltage signal is quite difficult. Therefore, quench must be detected using some other method. We numerically show that quench in the coil can be detected by observing the transposition of current in the bundle conductor caused by local normal transition. Furthermore, we show the results of experiments on a small model coil wound with a four-YBCO-strand bundle conductor to examine the possibility of a quench detection method by observing the transposition of current in the bundle conductor. This work was mainly supported by the New Energy and Industrial Technology Development Organization (NEDO) as part of the technological development of yttrium-based superconducting power equipment.

Thermal-Electromagnetic Analysis for 14 kA Fast Discharge of a Model Coil

A model coil for 40-T hybrid magnet superconducting outsert magnet has been constructed and tested at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The model coil was wound with Nb3Sn cable-in-conduit conductor (CICC) cabled in a 316LN jacket cooled with supercritical helium. The model coil alone can produce about 4 T maximum magnetic field with an operating current of 14 kA. The model coil, in combination with 7.57-T NbTi background coil, can produce 11.5 T central field at 14 kA. During the test campaigns, a fast discharge was triggered by a dump resistor of 3.6 mΩ to evaluate the thermal-electromagnetic behavior of the model coil. In order to avoid a quench of the background coil, no current was exerted on the background coil through a power supply during the fast discharge of the model coil. The test results show that the central magnetic field is not scaled proportionally to the current decay of the model coil. The circuit model gives excellent results compared with the measured ones for the central magnetic field evolution as a function of time in this paper. For the thermal-hydraulic behavior during the fast discharge, the maximum temperature at the inlet simulated by the 1-D Gandalf code gives excellent agreement results compared with the measured ones with the conductor coupling time constant of 63 ms.

Integrated analysis of quench propagation in a system of magnetically coupled superconducting coils

We demonstrate in this paper how to use direct modeling of heat transfer and circuital equations based on “packaged” simulation tools to produce a model suitable for the study of quench propagation in a system of magnetically coupled solenoids protected without an external dump resistor. Our application example is a system of three, layer-wound and bath-cooled coils, all using a NbTi conductor. The same model can be easily adapted to study coils with different topologies, geometry and conductors. We demonstrate the use of the model by parametric analysis of physical parameters in the system, showing under which conditions a quench-back due to inductive coupling is triggered. The results, and in particular longitudinal and transverse propagation velocities, are consistent with expected analytical scalings. The advantage of the model is that it provides self-consistent results, i.e. based on first principles rather than assumptions on quench propagation, and it gives access to local details that are not intuitive, nor easily measurable.

Quench Protection Design of an 8-T Magnet Built With Low- and High-Temperature Superconducting Coils

A quench protection system has been proposed for an 8-T split superconducting magnet consisting of low-temperature superconducting (LTS) NbTi coils and high-temperature superconducting (HTS) Bi2223 coils. For the LTS coils, a code has been developed to adjust the circuit parameters and the heater geometry. The calculation method of the internal electric potential in the LTS coil is introduced. For the HTS coils, the temperature and current margins are studied. The voltage signal for quench detection is chosen, and quench simulations on the HTS coil with different heating fluxes and heating zones are performed. The simulation results show that the employment of only the dump resistors can bring down the current and the temperature rise rapidly in the event of a quench. The coupling effect of the LTS and HTS coils is simulated and analyzed.

Design of a Quench Protection System for a Coated Conductor Insert Coil

Coated conductors are promising material for the construction of high field magnets, but the slow propagation velocity of the normal zone makes the protection against quenches a critical issue. The quench protection system for a small insert coil under development at CRPP has been studied. The voltage drop and the hot spot temperature during the quench have been estimated using analytical expressions; the cross section of the copper stabilizer, the voltage threshold and the time constant were set as parameters. Detailed calculations have been carried out using the Cryosoft code THEA. The copper cross section available from the tape manufacturer is enough to ensure that the hot spot temperature remains below 200 K, but only in conjunction with a sensitive quench detection system (voltage threshold lower than 100 mV) and a fast dumping time (time constant shorter than 100 ms). These requirements can be fulfilled in the small insert coil under construction, because the inductance is about 50 mH and because the coil can be quickly discharged independently from the main magnet. The passive protection circuit consists of dump resistor and diodes. In case of longer time constants (few seconds), a larger copper cross section (comparable to the one used in low temperature superconducting magnets) is required in order to maintain the temperature within safe values.

Quench Protection System for the KSTAR Toroidal Field Superconducting Coil

The design of the integrated quench protection (QP) system for the high current superconducting magnet (SCM) has been fabricated and tested for the toroidal field (TF) coil system of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The QP system is capable of protecting the TF SCM, which consists of 16 identical coils serially connected with a stored energy of 495 MJ at the design operation level at 35.2 kA per turn. Given that the power supply for the TF coils can only ramp up and maintain the coil current, the design of the QP system includes two features. The first is a basic fast discharge function to protect the TF SCM by a dump resistor circuit with a 7 s time constant in case of coil quench event. The second is a slow discharge function with a time constant of 360 s for a daily TF discharge or for a stop demand from the tokamak control system. The QP system has been successfully tested up to 40 kA with a short circuit and up to 34 kA with TF SCM in the second campaign of KSTAR. This paper describes the characteristics of the TF QP systems and test results of the plasma experiment of KSTAR in 2009.

Protection System for the Superconducting Outsert Coils of the HFML 45 T Hybrid Magnet

The High Field Magnet Laboratory of the Radboud University Nijmegen is developing a 45 T hybrid magnet consisting of a 33 T resistive insert magnet and a 12 T, all Nb3Sn-Cu Cable-In-Conduit Conductor (CICC) based superconducting outsert magnet. Standard quench protection of the outsert coils is achieved with a permanently connected external dump resistor parallel to the coils. A major challenge is to prevent a quench of the outsert magnet during spurious trips of the insert circuit that may occur. Such an event is characterized by a fast decay of the insert current generating field changes at the windings of the outsert coils of a few T/s. Without precautions, the resulting AC losses in the superconducting strands would quench the inner windings. Here we present a model to analyse the effectiveness of an adapted protection scheme in terms of system and component characteristics and the required AC loss properties of the proposed CICC design.

Finite-Element Analysis of Dump Resistor for Prototype Superconducting Magnet Carrying 3.60 MA-t

Stability margin, transient ac responses, and protection of the magnet system in the case of abnormal quench are some of the important design drivers of superconducting magnets for fusion reactors. A prototype magnet has been designed using a cable-in-conduit conductor with a nominal operation current of 30 kA at 12 T and 5 K at the Institute for Plasma Research, Gandhinagar, India. Protection of the magnet system during off-normal scenarios leading to quench makes the operation of the magnet system more challenging. An external dump resistor is required to extract the stored magnetic energy of the magnet in case of quench. Such a dump resistor has been designed for this 3.60 MA-turns current-carrying prototype superconducting magnet. Finite-element thermal, structural, and electromagnetic analyses have been done for validating the dump resistor parameters using commercially available Ansys software. The maximum temperature rise of the dump resistor system is found to be 540 K, considering cool down by natural convection only. In addition, the maximum stress between the parallel plates of the is found to be less than 1.0 GPa, which is in the acceptable range.

Coupled Transient Finite Element Simulation of Quench in Jefferson Lab's 11 GeV Super High Momentum Spectrometer Superconducting Magnets

This paper presents coupled transient thermal and electromagnetic finite element analysis of quench in the Q2, Q3, and dipole superconducting magnets using Vector Fields Quench code. Detailed temperature distribution within coils and aluminum force collars were computed at each time step. Both normal (quench with dump resistor) and worst-case (quench without dump resistor) scenarios were simulated to investigate the maximum temperatures. Two simulation methods were utilized, and their algorithms, implementation, advantages, and disadvantages are discussed. The first method simulated the coil using nonlinear transient thermal analysis directly linked with the transient circuit analysis. It was faster because only the coil was meshed and no eddy current was modeled. The second method simulated the whole magnet including the coil, the force collar, and the iron yoke. It coupled thermal analysis with transient electromagnetic field analysis which modeled electromagnetic fields including eddy currents within the force collar. Since eddy currents and temperature in the force collars were calculated in various configurations, segmentation of the force collars was optimized under the condition of fast discharge.