First experience with the new Coupling Loss Induced Quench system

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating.A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack.The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted.Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage.     

Coupled transient thermal and electromagnetic finite element analysis of quench in MICE coupling magnet

The superconducting coupling magnet used for the international Muon Ionization Cooling Experiment (MICE) will be passively protected through coil subdivision and quench back simultaneously. The design of such type quench protection system requires detailedly understanding of the heat transfer and electromagnetic process in the magnet during quench process. A coupled transient thermal and electromagnetic finite element model was developed to study the quench process of the coupling magnet. This model sequentially solves two different physics environments, one is thermal physics environment and the other one is coupled-electromagnetic-circuit physics environment. The two environments are coupled by applying results from one environment as loads in another one. The results such as current, hot spot temperature, resistance and over voltage during quench process are presented. The results of this model were compared with that of a semi-empirical model, and the respective advantages of both models were pointed out. The quench propagation process in the coupling magnet and the effect of the quench back on the speeding up the quench process were analyzed. The goal of such work is to predict the quench evolution of the coupling magnet in detail and guide its protection scheme.     

Analysis of voltage signals from superconducting accelerator magnets

We present two techniques used in the analysis of voltage tap data collected during recent tests of superconducting magnets developed by the Superconducting Magnet Program at Lawrence Berkeley National Laboratory. The first technique was used on a quadrupole to provide information about quench origins that could not be obtained using the time-of-flight method. The second technique illustrates the use of data from transient flux imbalances occurring during magnet ramping to diagnose changes in the current-temperature margin of a superconducting cable. In both cases, the results of this analysis contributed to make improvements on subsequent magnets.     

Magnetic Field and Strain Measurements of a Superconducting Solenoid Magnet for C-ADS Injector-II During Excitation and Quench Test

A superconducting solenoid magnet has been fabricated and tested for the Accelerator Driven System (ADS), which is a dedicated burner reactor in a double strata fuel cycle to incinerate nuclear waste, at the Institute of Modern Physics of Chinese Academy of Science (IMPCAS). The magnetic field of the superconducting solenoid was measured with hall probes during the magnet excitation and spontaneous quench test. The strains in the superconducting solenoid magnet and its cryogenic support structures were measured by using a compensation bridge of low-temperature strain gauges and a wireless strain acquisition system. The transient strains and their abrupt changes were recorded and compared with the collected signals of the transport current and temperature of the superconducting solenoid during a spontaneous quench test. It indicates that, not only the mechanical behaviors of the superconducting solenoid and the support structures during the excitation can be captured effectively, but also the quench feature can be detected with observations of abrupt strain variations as a quench occurs. The predictions of the magnetic and strain field by means of a coupled finite element analysis were also performed and compared with the experimental measurements resulting in a good agreement.     

Numerical analysis of heat-induced transients in forced flow helium cooling systems

This work investigates theoretically the consequences of a space and time dependent energy input into the cooling system of a forced flow cooled superconducting magnet. Based on a proved computer code for calculation of instationary fluid flows, a new program was developed which is able quantitatively to calculate pressure and temperature transients caused by heat pulses. The program was checked by comparing the results with those of a simulation experiment. A further test was made by comparison with stationary flow results calculated from a well know program. In combination with computer programs, which are able to calculate the quench propagation in a current loaded superconductor, the numerical procedure allows for the investigation of the influence of transient helium flow processes on the stability of the superconductor. Our program is proposed as a good tool for the design of forced flow cooling systems of large superconducting magnets.     

Quench propagation and stability analysis of Rutherford resistive core cables

The complex phenomena occurring in superconducting cables involve thermal, electromagnetic, fluid-dynamic and mechanical problems that require multiphysics analysis codes. In this paper varied methods of analysis are studied, comparing a detailed model of a resistive core Rutherford cable with simplified models, in order to show at what extent the simplified models can be applied. The detailed model considers every single strand and its core as thermal and electric independent elements, while in the simplified models equivalent electric and thermal parameters are calculated to group several strands into equivalent superstrands. The minimum quench energy and quench propagation velocities are taken as parameters for the comparison, evaluating the scaling of the numerical results with the number of electric and thermal elements considered. Computation times for the different models are compared, indicating possible ways to reduce the numerical burden and making desired accuracies.     

Design of epoxy-free superconducting dipole magnets and performance in both Helium I and pressurized Helium II

Three model superconducting dipole magnet 1m long, without iron, having a bore diameter of 76 mm have been built without epoxy resins or other adhesives and tested in He I and He II. The conductor is the 23-strand Rutherford-type cable used in the Fermilab Doubler Saver magnets, and is insulated with Mylar and Kapton. The two-layer winding is highly compressed by a system of structural support rings and tapered collets. Little "training" was required to reach quench currents greater than 95 percent of "short sample" in Helium I. The maximum quench current in He II is increased 20 to 30 percent, compared with He I operation at 4.4 K. Test results are given on cyclic losses, heater-induced quenches, and charge-rate effects.     

Quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet by an active power method

When a quench occurs in a superconducting magnet, excessive joule heating may damage the magnet. We have presented the quench protection system based on an active power method. Our previous quench protection tests have been carried out for small superconducting magnets whose self inductances are less than several hundred mH to verify principles of our proposed system. In this paper, we present experimental results of quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet (self inductance 15.5 H), which is a commercial size magnet made by Tamakawa Co., Ltd. We confirmed that our proposed system could inhibit the maximum temperature of the superconducting magnet (initial temperature 4.3 K) after the quench to less than about 44 K at operation magnetic field 5.5 T. Experimental results suggest that our proposed system is useful for practical used superconducting magnets.     

Finite element analysis of the localised thermal quench of an HTS tape

In this work, we perform a finite element method (FEM) analysis of the localised thermal quench of a high temperature superconducting (HTS) tape. One 3D thermo-electric coupling FEM model, which is constructed to address the actual development of the localised thermal quench occurred in the HTS tape, has been proposed. One quench experiment is performed to validate this model. It is shown that the mode can quantitatively reflect the dynamic and static quench characteristics when comparing the results of the experiment with the model. The FEM model generates an estimate of the location of the highest temperature and visualisation of the quench dynamics.     

A physical model and numerical method for losses investigation in superconducting cable-in-conduit conductors (CICC)

A reliable prediction of losses in superconducting cable-in-conduit conductors (CICCs) is important for a successful application of fusion and SMES coils, given that every Watt dissipated at 4-5 K requires 400-800 W of electrical power to remove this heat load. There are also losses caused by field transients (like plasma disruption, plasma initiation and step discharge) that may influence the temperature margin in the conductor design. It is commonly supposed that the CICC losses are associated with the so-called coupling losses characterized by the nτ parameter. However, we would like to highlight the importance of the eddy current losses occurring in the external layers of the bundle: in some cases they are greater than the coupling losses. Therefore, the nτ parameter is a simplification and does not accurately describe the real physical processes taking place in a CICC. Also, we believe that the coupling currents (additional currents) generated in loops can either be added to or subtracted from the transport current flowing in superconducting strands, depending on mutual orientation of the varying magnetic flux density and transport current vectors. This can affect the stability margin and the ramp rate limitation of a conductor. The physical model and numerical method for estimating different kinds of losses in CICCs are proposed. The suggested physical model, albeit somewhat incomplete, allows an explanation of some experimental results obtained earlier. The purpose of this work is to develop a method for an independent numerical calculation of the eddy current and coupling current loss components and thereby avoid the costly experiments at the initial stage of the design work.     

Analysis of the quench propagation along Nb3Sn Rutherford cables with the THELMA code. Part II: Model predictions and comparison with experimental results

To improve the technology of the new generation of accelerator magnets, prototypes are being manufactured and tested in several laboratories. In parallel, many numerical analyses are being carried out to predict the magnets behaviour and interpret the experimental results. This paper focuses on the quench propagation velocity, which is a crucial parameter as regards the energy dissipation along the magnet conductor. The THELMA code, originally developed for cable-in-conduit conductors for fusion magnets, has been used to study such quench propagation. To this purpose, new code modules have been added to describe the Rutherford cable geometry, the material non-linear thermal properties and to describe the thermal conduction problem in transient regime. THELMA can describe the Rutherford cable at the strand level, modelling both the electrical and thermal contact resistances between strands and enabling the analysis of the effects of local hot spots and quench heaters. This paper describes the model application to a sample of Short Model Coil tested at CERN: a comparison is made between the experimental results and the model prediction, showing a good agreement. A comparison is also made with the prediction of the most common analytical models, which give large inaccuracies when dealing with low n-index cables like Nb₃Sn cables.     

Modelization of the thermal coupling between the ITER TF coil conductor and the structure cooling circuit

The ITER Toroidal Field (TF) coils are required not to quench during the most demanding event: a plasma disruption followed by a fast discharge of the Central Solenoid (CS), the Poloidal Field (PF) coils and the Correction Coils (CC). This event creates large heat deposition in the ITER magnet stainless steel structures in addition to the conductor AC losses. In order to prevent quench occurring in the TF conductor, cooling channels, implemented in the TF coil structure (TFCS), have to remove a large fraction of the heat deposited. The first integrated TF and structure mock-up has been manufactured and then tested in the HELIOS cryogenic test facility (CEA Grenoble) to determine the thermal coupling between the TFCS and the TF conductor, both actively cooled by supercritical helium at 4.4 K and 5 bar. It consists in a stainless steel casing, a cooling pipe glued with resin in the casing groove, winding pack (WP) ground insulation, a radial plate and a copper dummy cable-in-conduit-conductor (CICC). Steady state as well as transient thermal characterizations have been completed in May 2015. Simulation results by thermal hydraulic codes (VENECIA/SuperMagnet) and some of the experimental data are presented and discussed. The thermal coupling between the helium in the cooling tube and the TF coil structure is then modelled as an equivalent heat transfer coefficient in order to simplify the thermal hydraulic (TH) models. Comparison between simplified coupling and detailed coupling is presented.     

Experimental simulation of helium pressure rise during a quench of a superconducting coil cooled by a superfluid helium bath

Experimental and numerical studies have been conducted with the aim of modeling pressure rises which occur in the helium, during quenches of the 11.7-T superconducting magnet named Iseult. Iseult is based on a double-pancake winding internally cooled by superfluid helium channels opening into a pressurized He II bath at 1.8 K. A scale mock-up has been built of 10 copper equivalent pancake slices and 7 helium channels per pancake. The heat produced by a quench of the Iseult magnet is simulated by electrical heaters put inside each copper plate. Cryogenic pressure and temperature sensors have been fitted in the helium channels and in the bath. Bath pressure measurements are given for various heating powers, various numbers of heated plates and various bath volumes. Comparisons with a simple numerical model permit to identify the main physical mechanisms which drive the pressure rise during a quench.     

Guidelines for LTS magnet design based on transient stability

Stabilities of low critical temperature superconducting (LTS) magnets and their designs are studied and discussed. There are two contradictory necessities; those are low cost and high performance, in the other words, high magnetic field and large current density. Especially, the maximum magnetic fields of the latest high performance Nb₃Sn magnets are around 20 T. Mentioned necessities result in the small stability margins. Needless to say, the superconducting magnet must produce its nominal field reliably. Therefore, maintaining adequate stability margin, the magnet design to draw out the high potential of the superconductor is required. The transient stability of the superconducting magnet is determined by the relationship between mechanical disturbance energy and stability margin. The minimum quench energy (MQE) is one of the index of stability margin and it is defined as the minimum energy to trigger quenching of a superconductor. MQE should be beyond any possible disturbance energy during the operation. It is difficult to identify the mechanical disturbance energy quantitatively. On the contrary, MQE had been evaluated precisely by means of our developed resistive carbon paste heater (CPH). At the same time, we can predict MQE by numerical simulations. Because the magnet comes to quench if the mechanical disturbance exceeds the MQE, the disturbance energies are suspected to be equivalent to MQEs during the magnet-training. When we achieved somewhat larger MQE, we may exclude numbers of training quenches.In this paper, we discuss the guidelines of LTS magnet design from the standpoint of MQE. We represent some case studies for various superconducting magnets and/or some different winding methods.     

Superconducting magnets

Although small superconducting magnets have been in regular use in many laboratories for several years, it is only recently that the technical difficulties preventing the use of superconductors in large magnet systems have been overcome. A better understanding of the transient thermal and magnetic behavior of such devices has led to various methods of reducing or eliminating the degradation effects present in earlier magnets. The use of large amounts of very high conductivity normal metal in conjunction with the superconductor has resulted in "stabilized" magnets whose behavior is completely predictable and which will not quench except under unusual conditions. Other techniques have been developed to improve "unstable" magnets so that very high field coils can be constructed. These developments are dramatically altering the equipment used in high-energy physics and controlled thermonuclear experiments, and their effect on other areas of technology is already beginning to be felt.     

中小型超导磁体用低温恒温器的安全

从工作压力角度分析了中小型超导体磁作用低温恒温器的安全问题，并给出了超导磁体猝灭引起低温恒温器压力变化的实验观察实例，以及超导Ｗｉｇｇｌｅｒ磁体低温恒温器的安全设计。     

Analysis of observations during operation of the NHMFL 45-T hybrid magnet system

The NHMFL hybrid magnet system, which was designed to produce steady field in excess of 45 T in a 32-mm, room-temperature bore, was first tested in December 1999. Since then, the system has served users of the NHMFL at the full design currents in both the superconducting outsert (10 kA) and the resistive insert (67 kA), reaching a combined field of 45.2 T. This magnet system combines both superconducting and resistive magnet technologies, which, whether taken together or separately, define new states of the art. Operating alone, the superconducting outsert has been charged repeatedly to 10 kA, corresponding to a maximum field of nearly 16 T at its 710-mm winding i.d. More recently, operation of the outsert has been limited to 8 kA as a consequence of degradation suffered during an “unprotected” quench, but insert upgrades and higher-current operation (up to 74 kA) have allowed the system to provide 45 T to users still. Because the system was designed from the outset as a facility rather than an experiment, there is a minimum of instrumentation––in fact there is none internal to the steel vessel housing the superconducting magnet. Therefore, projections of internal conditions in the superconducting magnet are deduced from detailed analysis of observations from coil voltage taps and various external temperature sensors and pressure transducers. We present these with comments regarding their value in future magnet design as well as an introduction for more complete analysis by complex computer codes.     

Hybrid finite difference/finite element solution method development for non-linear superconducting magnet and electrical circuit breakdown transient analysis

The problem of non-linear superconducting magnet and electrical protection circuit system transients is formulated. To enable studying the effects of coil normalization transients, coil distortion (due to imbalanced magnetic forces), internal coil arcs and shorts, and other normal and off-normal circuit element responses, the following capabilities are included: temporal, voltage and current-dependent voltage sources, current sources, resistors, capacitors and inductors. The concept of self-mutual inductance, and the form of the associated inductance matrix, is discussed for internally shorted coils. This is a Kirchhoff's voltage loop law and Kirchhoff's current node law formulation. The non-linear integrodifferential equation set is solved via a unique hybrid finite difference/integral finite element technique.     

Influence of losses on the stability of a high current density superconducting magnet winding during the energy removal process

The influence of losses on the stability of a high current density, superconducting, magnet winding during energy removal, are presented. The magnet is wound from intrinsically stable superconductors. The possibility of magnet quench due to losses during the energy removal is demonstrated. Under certain conditions during the energy removal process, the quench magnetic field level is independent of the magnetic field decay velocity.