M. Maciejewski

Protecting a full-scale Nb3Sn magnet with CLIQ, the new coupling-loss-induced quench system

A new protection system for superconducting magnets called coupling-loss induced quench system (CLIQ) has been recently developed at CERN. Recent tests on Nb-Ti coils have shown that CLIQ is a valid, efficient, and promising method for the protection of high-magnetic-field superconducting magnets. However, the protection of new-generation Nb3Sn accelerator magnets is even more challenging due to the much higher stored energy per unit volume and to the significantly larger enthalpy needed to initiate and propagate a normal zone in such coils. Now, the CLIQ system is tested for the first time on a Nb3Sn magnet in the CERN magnet test facility in order to investigate its performance in practice, thereby validating the method for this type of superconducting magnets as well. Furthermore, we successfully reproduced the electrothermal transients during a CLIQ discharge. Finally, the implementation of various CLIQ-based protection schemes for the full-scale Nb3Sn quadrupole magnet for the LHC high luminosity upgrade is discussed. The impact of key system parameters on CLIQ performance and the advantages and drawbacks of using multiple CLIQ units on a single magnet are discussed.

First Implementation of the CLIQ Quench Protection System on a 14-m-Long Full-Scale LHC Dipole Magnet

The coupling-loss-induced quench (CLIQ) is an innovative system for the protection of superconducting magnets. Its energy-deposition mechanism, based on coupling loss generated directly in the superconductor, is fundamentally faster than heat diffusion, upon which traditional quench-heater-based systems rely. CLIQ electrical design relies on simple and robust components, i.e., easy to install and be replaced in case of damage. After being successfully tested on model magnets of different geometries and types of superconductor, CLIQ is now applied for the first time for the protection of a full-scale dipole magnet. For this purpose, a 14-m-long LHC twin-aperture dipole magnet is equipped with CLIQ terminals and two 80-mF 500-V CLIQ units are connected to its windings. Experimental results obtained under various operating conditions convincingly show that a CLIQ-based quench protection system can effectively protect large-scale magnets by quickly and homogeneously transferring to the normal-state voluminous regions of the winding packs. A developed dedicated simulation code correctly reproduces the complex electrothermal transient occurring during a CLIQ discharge. The successful test completes the development program of CLIQ quench protection systems, which has convincingly demonstrated the maturity and readiness of the system for application in large-scale magnet systems.

First Implementation of the CLIQ Quench Protection System on a Full-Scale Accelerator Quadrupole Magnet

The coupling-loss induced quench system (CLIQ) is an innovative method for the protection of high-field superconducting magnets. With respect to the conventional method based on quench heaters, it offers significant advantages in terms of electrical robustness and energy-deposition velocity. Its effective intrawire heating mechanism targets a fast and homogeneous transition to the normal state of the winding pack, hence assuring a quick magnet discharge and avoiding overheating of the coils hot spot. Furthermore, it is possible to implement CLIQ as a time- and cost-effective repair solution for the protection of existing magnets with broken quench heaters. After being successfully tested on model magnets of different geometries and made of different types of superconductor, CLIQ is now applied for the first time for the protection of a full-scale quadrupole magnet at the CERN magnet test facility. One aperture of a 3.4-m-long LHC matching quadrupole magnet is equipped with dedicated terminals to allow the connection of a CLIQ system. Experimental results convincingly show that CLIQ can protect this coil over the entire range of operating conditions. The complex electrothermal transients during a CLIQ discharge are successfully reproduced by means of a 2-D model. The test is part of the R&D program of CLIQ quench protection systems, which has convincingly demonstrated the maturity of this technology and its effectiveness also for large-scale magnet systems. The proposed CLIQ-based solution for the quench protection of the LHC matching quadrupole magnet is now ready to be implemented in the LHC machine if needed.

Automated lumped-element simulation framework for modelling of transient effects in superconducting magnets

The paper describes a flexible, extensible, and user-friendly framework to model electrothermal transients occurring in superconducting magnets. Simulations are a fundamental tool for assessing the performance of a magnet and its protection system against the effects of a sudden transition from the superconducting to the normal state (also known as a quench). The application has a scalable and modular architecture based on the object-oriented programming paradigm, which opens an easy way for future extensions. Models are composed of thousands of lumped-element blocks automatically created in MATLAB&Simulink. Additionally, it is possible to run sets of simulations with varying parameters and model structure. Due to its flexibility the framework has been used to simulate various protection and magnet configurations. The experimental results were in a very good agreement with simulations.

submitter : Quench Protection of a 16-T Block-Coil Dipole Magnet for a 100-TeV Hadron Collider Using CLIQ

Protection against the effects of a quench is a crucial challenge for 16-T-class superconducting dipole magnets for a future 100-TeV Hadron collider. To avoid damage due to overheating of the coils hot spot, heat generated during the quench has to be homogeneously distributed in the winding pack by quickly and uniformly transferring to the normal-state voluminous coil sections. Conventional protection systems rely on quench heaters placed on the outer surfaces of the coils. However, this technique has to confront significant challenges in order to achieve the fast transitions required by high magnetic field magnets. The recently developed coupling-loss-induced quench (CLIQ) utilizes interfilament coupling loss as an effective intrawire heat deposition mechanism, which, in principle, is faster than thermal diffusion. Furthermore, the CLIQ technology is based on simple and robust electrical components in contact with the coil only in a limited number of easily accessible and well-insulated points. Hence, expected occurrence of failure and electrical breakdown is significantly reduced. As a case study, the design of a CLIQ-based protection system for a 14-m-long 16-T Nb 3Sn block-coil dipole magnet is demonstrated here. Various magnet design features can be adjusted to improve CLIQ performance and optimize its integration in the magnet system. CLIQ provides future magnet designers with a solution for a very effective, yet electrically robust, quench protection system, resulting in better magnet performance and lower cost than would be possible with a traditional approach to magnet protection.

A Consistent Simulation of Electrothermal Transients in Accelerator Circuits

Transient effects occurring in a superconducting accelerator circuit can be correctly simulated only if the models consistently account for the electrothermodynamic coupling between the magnets, the protection systems, and the remaining network. We present a framework based on the idea of cosimulation. The core component is a coupling interface exchanging information between the independent models. Within the framework, we simulate selected parts of a magnet and the electrical network, combining appropriately different commercial tools. This modularity gives the possibility of integrating new tools in the framework, to provide further insights on different physical domains as mechanics or fluid dynamics. The workflow is applied to the field-circuit coupling of an LHC main dipole magnet.

Advanced Quench Protection for the Nb 3 Sn Quadrupoles for the High Luminosity LHC

The goal of the High Luminosity LHC project is upgrading the LHC in order to increase its luminosity by a factor of five. To achieve this, 24 150-mm-aperture 12-T Nb3Sn quadrupole magnets are to be installed close to the two interaction regions at ATLAS and CMS. This new generation of high-field magnets poses a significant challenge concerning the protection of the coils in the case of a quench. The very high stored energy per unit volume requires a fast and effective quench heating system in order to limit the hot-spot temperature and hence avoid damage due to overheating. Conventional protection systems based on quench heaters have a limited response time due to the thermal insulation between the heater and the coil. An advanced solution for the protection of high-field magnets is the coupling-loss induced quench (CLIQ) system, recently developed at CERN. Due to its fast intrawire energy-deposition mechanism, CLIQ is a very effective, yet electrically robust, quench protection system. Various protection scenarios, including quench heaters, CLIQ, or combinations of the two methods, are analyzed and discussed, with the aim of minimizing the coils hot-spot temperature and thermal gradients during the discharge. The proposed design assures a fully redundant system.

Coupling of Magnetothermal and Mechanical Superconducting Magnet Models by Means of Mesh-Based Interpolation

In this paper, we present an algorithm for the coupling of magnetothermal and mechanical finite element models representing superconducting accelerator magnets. The mechanical models are used during the design of the mechanical structure as well as the optimization of the magnetic field quality under nominal conditions. The magnetothermal models allow for the analysis of transient phenomena occurring during quench initiation, propagation, and protection. Mechanical analysis of quenching magnets is of high importance considering the design of new protection systems and the study of new superconductor types. We use field/circuit coupling to determine temperature and electromagnetic force evolution during the magnet discharge. These quantities are provided as a load to existing mechanical models. The models are discretized with different meshes and, therefore, we employ a mesh-based interpolation method to exchange coupled quantities. The coupling algorithm is illustrated with a simulation of a mechanical response of a standalone high-field dipole magnet protected with Coupling-Loss Induced Quench Technology.

Modeling of interfilament coupling currents and their effect on magnet quench protection

Variations in the transport current of a superconducting magnet cause several types of transitory losses. Due to its relatively short time constant, usually of the order of a few tens of milliseconds, interfilament coupling loss can have a significant effect on the coil protection against overheating after a quench. This loss is deposited in the strands and can facilitate a more homogeneous transition to the normal state of the coil turns. Furthermore, the presence of local interfilament coupling currents reduces the magnets differential inductance, which in turn provokes a faster discharge of the transport current. The lumped-element dynamic electrothermal model of a superconducting magnet has been developed to reproduce these effects. Simulations are compared to experimental electrical transients and found in good agreement. After its validation, the model can be used for predicting the performance of quench protection systems based on energy extraction, quench heaters, the newly developed coupling-loss-induced quench protection system, or combinations of those. The impact of interfilament coupling loss on each protection system is discussed.

CLIQ-Based Quench Protection of a Chain of High-Field Superconducting Magnets

Conventional quench protection systems for high-magnetic-field superconducting magnets are based on external heaters composed of resistive strips in close contact with the coil and rely on thermal diffusion across insulation layers on the order of tens of micrometers. The large contact areas between the coil and the heater strips, and the thin insulation between them required for an effective protection constitute a significant risk of electrical breakdown and one of the most common causes of magnet damage. Coupling-loss-induced quench (CLIQ) technology offers a valid option for a time- and cost-effective repair of magnets with failing heater-based protection systems. In fact, its effective heating mechanism utilizing coupling loss, its robust electrical design, and its fast implementation, as compared to alternative repair options, constitute definite advantages over the conventional technology. In the past years, CLIQ was successfully implemented on various coils in a single-magnet configuration. Now the design of a CLIQ-based protection system integrated in a chain of series-connected magnets is presented. The protection of a chain of superconducting magnets usually is considerably more challenging than the protection of stand-alone magnets due to the increased energy stored in the circuit and the presence of transitory effects. The effectiveness of this new method is demonstrated by means of electrothermal simulations modeling the transition to the normal state and the temperature evolution in one quenched magnet, and the electrodynamics of the entire magnet chain.