Toroidal Field Magnet Research Articles

Performance analysis of the toroidal field ITER production conductors

The production of the superconducting cables for the toroidal field (TF) magnets of the ITER machine has recently been completed at the manufacturing companies selected during the previous qualification phase. The quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers include performance tests of several conductor samples from selected unit lengths. The short full-size samples (4 m long) were subjected to DC and AC tests in the SULTAN facility at CRPP in Villigen, Switzerland. In a previous work the results of the tests of the conductor performance qualification samples were reported. This work reports the analyses of the results of the tests of the production conductor samples. The results reported here concern the values of current sharing temperature, critical current, effective strain and n-value from the DC tests and the energy dissipated per cycle from the AC loss tests. A detailed comparison is also presented between the performance of the conductors and that of their constituting strands.

Numerical analysis of the static performance of an annular aerostatic gas thrust bearing applied in the cryogenic turbo-expander of the EAST subsystem

The EAST superconducting tokamak, an advanced steady-state plasma physics experimental device, has been built at the Institute of Plasma Physics, Chinese Academy of Sciences. All the toroidal field magnets and poloidal field magnets, made of NbTi/Cu cable-in-conduit conductor, are cooled with forced flow supercritical helium at 3.8 K. The cryogenic system of EAST consists of a 2 kW/4 K helium refrigerator and a helium distribution system for the cooling of coils, structures, thermal shields, bus-lines, etc. The high-speed turbo-expander is an important refrigerating component of the EAST cryogenic system. In the turbo-expander, the axial supporting technology is critical for the smooth operation of the rotor bearing system. In this paper, hydrostatic thrust bearings are designed based on the axial load of the turbo-expander. Thereafter, a computational fluid dynamics-based numerical model of the aerostatic thrust bearing is set up to evaluate the bearing performance. Tilting effect on the pressure distribution and bearing load is analyzed for the thrust bearing. Bearing load and stiffness are compared with different static supply pressures. The net force from the thrust bearings can be calculated for different combinations of bearing clearance and supply pressure.

Russia's Contribution to the ITER TF Magnets

The ITER toroidal field (TF) magnet system consists of 18 D-shaped coils, each of which is formed by five regular double pancakes (rDP) and two side double pancakes (sDP). Manufacturing of conductors for the TF coils, which are Nb 3 Sn-based cable-in-conduit conductors, has been shared among six ITER Domestic Agencies, including the Russian Federation (RF). The RF share includes manufacturing of eighteen 760-m conductors for rDP and eight 415-m conductors for sDP, i.e., almost 4 out of 18 TF coils will be produced from the conductors supplied by RF. In order to fulfil project obligations, facilities for Nb 3 Sn strand, cable, and conductor production had been established, and after successful qualifications, the manufacturing phase began. The manufacturing activities went on for six years, starting with the first strand production in 2009 and finishing with a successful wrap-up of conductor manufacturing and delivery in 2015. In this paper, highlights of the TF conductor production from the superconducting strand manufacturing up to the results of conductor full-size sample tests in the SULTAN facility are presented.

European Nb3Sn Superconducting Strand Production and Characterization for ITER TF Coil Conductor

The European Union contributes around 20% of the cable-in-conduit conductor lengths needed for the ITER toroidal field (TF) magnet coils. For that purpose, 97 tons of Nb 3Sn superconducting strand have been fabricated over five years, the production being completed in 2014. This superconducting strand has been manufactured by two companies, namely, Bruker EAS (Germany) and OST (USA), through the bronze route and the internal tin diffusion, respectively. This paper reports the outcomes of this strand mass production and of the strand characterization as performed by the suppliers and cross-checked on a regular basis by Durham University.

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system RD and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

Overview of Performance Test of Korean CICCs for ITER TF Magnet

The Korean Domestic Agency (KODA) for the International Thermonuclear Experimental Reactor (ITER) project has taken the responsibility of the procurement of cable-in-conduit conductors for the ITER toroidal field magnet. While procuring 27 conductor unit lengths, which are the Korean sharing, 10 conductors have gone through the superconducting performance test, under low temperature and high magnetic field, as one of the final acceptance tests. The sample assembly and instrumentation of various voltage taps and temperature sensors have been done according to the specified procedure, which had been agreed between domestic agencies and the ITER International Organization. The performance test program, which has also been agreed, was carried out. For the assessment of the current sharing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> ), the standard analysis procedure has been adopted. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> of all samples, at 68 kA and a background field of 10.78 T, after 1000 cyclic current loads are well above the acceptance criterion. By analyzing the electric field as a function of temperature, the effective strain (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) and transition index (n-value) were reduced, and the behavior of ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> will be discussed in terms of the electromagnetic load, i.e., I × B. The effective strain was compared with the physical strain and results with a numerical simulation.

Investigation of the Inter-Coil Structures Influence on the Mechanical Behavior of the TF Magnet in a Tokamak Machine

The tokamak machine is an effective solution to achieve fusion magnetic confinement and is suitable to be adapted to a wide range of plasma performances and scenarios. Due to the high current led and to the relevant magnetic field obtained in the most demanding devices, the toroidal field (TF) coils are among the key components in such machines. For this reason, they have to be carefully designed to withstand the huge loads due to the Lorentz forces during operation. This paper deals with the effects that the inter-coil structures have on the distribution of loads and on the stress and displacement peak on the TF coil. A conceptual magnet system, which is characterized by a symmetric D-shaped TF coil plus a set of six poloidal field (PF) coils, and a reference plasma scenario have been chosen as a base for a parametric investigation by varying the position and the stiffness of the upper, lower, and intermediate inter-coil structures. The proposed approach is suitable to support the engineers in optimizing the force distribution between the coils and in improving the structural behavior.

Magnet Design Considerations for Fusion Nuclear Science Facility

The Fusion Nuclear Science Facility (FNSF) is a nuclear confinement facility that provides a fusion environment with components of the reactor integrated together to bridge the technical gaps of burning plasma and nuclear science between the International Thermonuclear Experimental Reactor (ITER) and the demonstration power plant (DEMO). Compared with ITER, the FNSF is smaller in size but generates much higher magnetic field, i.e., 30 times higher neutron fluence with three orders of magnitude longer plasma operation at higher operating temperatures for structures surrounding the plasma. Input parameters to the magnet design from system code analysis include magnetic field of 7.5 T at the plasma center with a plasma major radius of 4.8 m and a minor radius of 1.2 m and a peak field of 15.5 T on the toroidal field (TF) coils for the FNSF. Both low-temperature superconductors (LTS) and high-temperature superconductors (HTS) are considered for the FNSF magnet design based on the state-of-the-art fusion magnet technology. The higher magnetic field can be achieved by using the high-performance ternary restacked-rod process Nb 3 Sn strands for TF magnets. The circular cable-in-conduit conductor (CICC) design similar to ITER magnets and a high-aspect-ratio rectangular CICC design are evaluated for FNSF magnets, but low-activation-jacket materials may need to be selected. The conductor design concept and TF coil winding pack composition and dimension based on the horizontal maintenance schemes are discussed. Neutron radiation limits for the LTS and HTS superconductors and electrical insulation materials are also reviewed based on the available materials previously tested. The material radiation limits for FNSF magnets are defined as part of the conceptual design studies for FNSF magnets.

Design and optimization of Artificial Neural Networks for the modelling of superconducting magnets operation in tokamak fusion reactors

In superconducting tokamaks, the cryoplant provides the helium needed to cool different clients, among which by far the most important one is the superconducting magnet system. The evaluation of the transient heat load from the magnets to the cryoplant is fundamental for the design of the latter and the assessment of suitable strategies to smooth the heat load pulses, induced by the intrinsically pulsed plasma scenarios characteristic of today's tokamaks, is crucial for both suitable sizing and stable operation of the cryoplant. For that evaluation, accurate but expensive system-level models, as implemented in e.g. the validated state-of-the-art 4C code, were developed in the past, including both the magnets and the respective external cryogenic cooling circuits. Here we show how these models can be successfully substituted with cheaper ones, where the magnets are described by suitably trained Artificial Neural Networks (ANNs) for the evaluation of the heat load to the cryoplant. First, two simplified thermal-hydraulic models for an ITER Toroidal Field (TF) magnet and for the ITER Central Solenoid (CS) are developed, based on ANNs, and a detailed analysis of the chosen networks' topology and parameters is presented and discussed. The ANNs are then inserted into the 4C model of the ITER TF and CS cooling circuits, which also includes active controls to achieve a smoothing of the variation of the heat load to the cryoplant. The training of the ANNs is achieved using the results of full 4C simulations (including detailed models of the magnets) for conventional sigmoid-like waveforms of the drivers and the predictive capabilities of the ANN-based models in the case of actual ITER operating scenarios are demonstrated by comparison with the results of full 4C runs, both with and without active smoothing, in terms of both accuracy and computational time. Exploiting the low computational effort requested by the ANN-based models, a demonstrative optimization study has been finally carried out, with the aim of choosing among different smoothing strategies for the standard ITER plasma operation.

Thermo-hydraulic analyses associated with a CEA design proposal for a DEMO TF conductor

The future DEMO Toroidal Field (TF) magnets are likely to feature cable-in-conduit conductors (CICC) cooled by forced flow of supercritical helium. Design activities were carried out at CEA to provide a winding pack compatible with DEMO plant requirements. The CEA proposal comprises, for each of the 16 D-shaped windings, 10 double-pancakes (2×392m long) wound in 10 turns. The conductor is a square-shaped Nb3Sn double channel conductor with a central spiral, carrying a nominal current of 95.5kA. We present a thermo-hydraulic analyses focused on the central, most critical pancake, where the maximum field is reached, aiming at evaluating the integrity of the proposed conductor design. Both normal and off-normal simulations were performed using detailed electromagnetic and neutron heating load maps as input, and evaluating operational quantities such as the temperature margin in burn conditions, and the hot spot temperature in quench conditions. We assessed the sensitivity of these quantities to some driving parameters, notably mass flow rate and the choice of friction factor correlation for the temperature margin, and quench initiation features for the hot spot temperature. Furthermore, the influence of the casing cooling on the temperature margin is analyzed. The study is carried out using two thermohydraulic models.

Development of a Thermal-Hydraulic Model for the European DEMO TF Coil

In the framework of the “roadmap to fusion electricity by 2050,” the design of the European DEMO machine strongly relies on available technologies. The superconducting toroidal field (TF) magnets will be built using available low-temperature superconducting strands; the ENEA winding pack (WP) proposal will exploit graded layer-wound rectangular conductors, whose design is ongoing. The WP will be encapsulated in a stainless steel casing, whose cooling has not yet been designed. In this paper, we present the thermal-hydraulic (TH) model of an entire TF coil for the European demonstration power plant DEMO. Two cooling options are proposed and investigated for the casing, whereas for the WP, the ENEA design, with multiple low-impedance hydraulic channels, is considered. The thermal coupling between WP and casing is parametrically included in the model. The TH behavior of a TF coil (WP + casing) during a plasma burn is presented and discussed, comparing the two cooling options of the casing.

Analysis of the ITER computational shielding benchmark with the Monte Carlo TRIPOLI-4® neutron gamma coupled calculations

With the growing interest in using the continuous-energy TRIPOLI-4® Monte Carlo radiation transport code for ITER applications, a key issue that arises is whether or not the released TRIPOLI-4 code and its associated nuclear data libraries are verified and validated for the D–T fusion neutronics calculations. Previous published benchmark results of TRIPOLI-4 code on the ITER related activities have concentrated on the first wall loading, the reactor dosimetry, the nuclear heating, and the tritium breeding ratio. To enhance the TRIPOLI-4 verification and validation on neutron-gamma coupled calculations for fusion device application, the computational ITER shielding benchmark of M. E. Sawan was performed in this work by using the 2013 released TRIPOLI-4.9S code and the associated CEA-V5.1.1 data library. First wall, blanket, vacuum vessel and toroidal field magnet of the inboard and outboard components were fully modelled in this 1-D toroidal cylindrical benchmark. The 14.1MeV source neutrons were sampled from a uniform isotropic distribution in the plasma zone. Nuclear responses including neutron and gamma fluxes, nuclear heating, and material damage indicator were benchmarked against previous published results. The capabilities of the TRIPOLI-4 code on the evaluation of above physics parameters were presented. The nuclear data library from the new FENDL-3.0 evaluation was also benchmarked against the CEA-V5.1.1 results for the neutron transport calculations. The results show that both data libraries can be run with TRIPOLI-4 for the fusion neutronics study. This work also demonstrates that the “safety factors” concept is necessary in the nuclear analyses of ITER.

ITER TF Magnet system analysis in faulted conditions

The ITER toroidal field (TF) magnet system must be designed, built, and operated, so that failures, which could occur under off-normal conditions, cannot cause damage to the confinement barriers. In fact, even if the magnets are not part of the barrier, the coils store a relevant amount of energy, which, in case of failure, may lead to large displacement of the coils, with the risk of a crash against confinement structures, which cannot be damaged. In this paper, the outcome of a structural analysis of the ITER TF coil magnet system, simulating possible faulted condition scenarios, is described. Two possible electrical failures of the TF coil system are considered: a short, resulting in a relevant current peak in one coil, and a quench without the discharge of the other ones. Since in both scenarios the assumption of cyclic symmetry condition is not valid anymore, a complete 18-coil finite-element (FE) model of the whole ITER TF magnet system has been developed for this purpose. By means of FE models, both electromagnetic and mechanical behaviors have been simulated, determining the magnetic field distribution; the forces acting on the coils and, consequently, stress; the displacement; and the forces on the interfaces. In particular, the mechanical model features a detailed description of all the subsystems which play a structural role in the TF magnet system, such as the intercoil structures, the poloidal shear keys, the TF wedged vault, the TF gravity support, the precompression ring system, and the poloidal field support structures. The model also includes all the bolted connection and all the nonlinearities (pretension, contact, etc.) characterizing the mechanical system. The results show that the deformations of the structure and the loads on the subsystems are within the allowable limits at off-normal conditions, thus demonstrating the soundness of the design, even in the failure scenarios.

Engineering, Manufacture and Preliminary Testing of the ITER Toroidal Field (TF) Magnet Helium Cold Circulator

The ITER cryodistribution system provides the supercritical Helium (SHe) forced flow cooling to the magnet system using cold circulators. The cold circulators are located in each of five separate auxiliary cold boxes planned for use in the facility. Barber-Nichols Inc. has been awarded a contract from ITER-India for engineering, manufacture and testing of the Toroidal Field (TF) Magnet Helium Cold Circulator. The cold circulator will be extensively tested at Barber-Nichols’ facility prior to delivery for qualification testing at the Japan Atomic Energy Agency's (JAEA) test facility at Naka, Japan. The TF Cold Circulator integrates features and technical requirements which Barber-Nichols has utilized when supplying helium cold circulators worldwide over a period of 35 years. Features include a vacuum-jacketed hermetically sealed design with a very low helium leak rate, a heat shield for use with both nitrogen & helium cold sources, a broad operating range with a guaranteed isentropic efficiency over 70%, and impeller design features for high efficiency. The cold circulator will be designed to meet MTBM of 17,500 hours and MTBF of 36,000 hours. Vibration and speed monitoring are integrated into a compact package on the rotating assembly with operation and health monitoring in a multi-drop PROFIBUS communication environment using an electrical cabinet with critical features and full local and network PLC interface and control. For the testing in Japan and eventual installation in Europe, the cold circulator must be certified to the Japanese High Pressure Gas Safety Act (JHPGSA) and CE marked in compliance with the European Pressure Equipment Directive (PED) including Essential Safety Requirements (ESR). The test methodology utilized at Barber-Nichols’ facility and the resulting test data, validating the high efficiency of the TF Cold Circulator across a broad operating range, are important features of this paper.

Artificial Neural Networks: a viable tool to design heat load smoothing strategies for the ITER Toroidal Field coils

In superconducting tokamaks, cryoplants provide the helium needed to cool the superconducting magnet systems. The evaluation of the heat load from the magnets to the cryoplant is fundamental for the design of the latter and the assessment of suitable strategies to smooth the heat load pulses induced by the pulsed plasma scenarios is crucial for the operation. Here, a simplified thermal-hydraulic model of an ITER Toroidal Field (TF) magnet, based on Artificial Neural Networks (ANNs), is developed and inserted into a detailed model of the ITER TF winding and casing cooling circuits based on the state-of-the-art 4C code, which also includes active controls. The low computational effort requested by such a model allows performing a fast parametric study, to identify the best smoothing strategy during standard plasma operation. The ANNs are trained using 4C simulations, and the predictive capabilities of the simplified model are assessed against 4C simulations, both with and without active smoothing, in terms of accuracy and computational time.

Electromagnetic and mechanical analysis of CFETR toroidal field coils

The toroidal field (TF) magnets of the China Fusion Engineering Test Reactor (CFETR) provide a steady magnetic field with peak value of 11.5T. Enormous electromagnetic forces are generated on TF structures due to the high magnetic field and large current. Structural analysis of TF coils is required to investigate the feasibility of the current design scheme and to provide guidance for optimization. The design status is described and electromagnetic analyses of different current scenarios are reported. The results of the mechanical analysis show that TF structures can safely withstand the reference operational scenarios, and the fatigue life can be assumed to be infinite based on the results of this fatigue assessment.

Artificial Neural Network Model for the Thermal-Hydraulic Response of a TF Superconducting Magnet in ITER

In a tokamak with superconducting magnets, the operation of the cryoplant requires the knowledge of the heat load coming from the cryogenic loops that cool the different magnet systems.Artificial Neural Networks (ANNs) are applied for the first time to the ITER Toroidal Field (TF) magnets. Two different models are developed: 1) a simpler one, aiming at checking the effects of the different operating scenarios on the cryoplant; 2) a more complex one, aiming at helping in the design of suitable control strategies for the magnet operation, to reduce the variation of the heat load to the cryoplant.The developed ANNs are suitably trained based on results obtained with the state-of-the-art thermal-hydraulic code 4C, that simulates the TF magnet response when subject to a broad spectrum of heat load variations. The predictive capability of the resulting ANN models is tested in different operating scenarios.

EU ITER TF coil: Dimensional metrology, a key player in the Double Pancake integration

The ITER Toroidal Field (TF) magnet system consists of 18 “D” shaped coils. Fusion for Energy (F4E), the European Domestic Agency for ITER, is responsible for the supply of 10 out the 19 TF coils (18 installed plus one spare coil). Each TF coil, about 300 t in weight, is made of a stainless steel case containing a Winding Pack (WP).The European manufacturing of the Radial Plates (RPs) and WPs has been awarded to two different industrial partners, whose activities are strongly linked with each other. In order to manufacture a Double Pancake (DP), first, the conductor has to be bent onto a D-shaped double spiral trajectory, then heat treated and inserted in the grooves of the RP. This represents the most challenging manufacturing step: in order to fit inside the groove, the double spiral trajectory of the conductor must match almost perfectly the trajectory of the groove, over a length above 700m. In order to achieve this, the conductor trajectory length must be controlled with an accuracy of 1mm over a length of 350m while the radial plate groove has to be machined with tolerances of ±0.2mm over dimensions of more than 10m. In order to succeed, it has been essential to develop a metrology process capable to control with high accuracy both the DP conductor and the RP groove trajectories.This paper reports on the work carried out on the development and qualification of the dimensional metrology to monitor the manufacturing of the conductor. Reference is made to the final dimensional check of the RP focusing on the groove centreline length. In addition the results obtained on the one to one scaled prototype DP are described. Finally, the strategy and foreseen improvements for the production of DPs are discussed.

Optimal design of a toroidal field magnet system and cost of electricity implications for a tokamak using high temperature superconductors

The potential for reducing the Cost of Electricity (CoE) by using High Temperature Superconductors (HTS) in the Toroidal Field (TF) coils of a fusion tokamak power plant has been investigated using a new HTS module in the PROCESS systems code. We report the CoE and the design of HTS tokamaks that have been optimised by minimising the major radius of the plasma. Potential future improvements in both the superconducting properties and the structural materials for TF coils operating at 4.8K and 30K are considered. Increasing the critical current density by a factor of 10 (with a commensurate reduction in costskA−1m−1) results in a CoE 4.4% less than equivalent tokamaks using current low temperature superconductors (LTS). If the yield strength of the TF casing material is increased by 40% to 1400MPa, the CoE is further reduced by 3.4%. Implementing both improvements and operating the TF coils at 4.8K leads to CoE of 19.1 (10.1) €centkW−1h−1 for a 500MW (1.5GW) HTS reactor compared to 20.7 (11.1) €centkW−1h−1 for an LTS reactor (2013 costs). Operating the HTS TF coils at 30K with both improvements, gives a similar CoE for HTS and LTS tokamaks.

Progress of the engineering analyses for the JT-60SA toroidal field coils structures

JT-60SA is a fully superconducting tokamak presently being built at the JAEA Naka site, in the framework of the JA-EU Satellite Tokamak Project under the Broader Approach (BA) Programme and JAEA's national Programme. The mission of the JT-60SA project is to contribute to the early realization of fusion energy, supporting the exploitation of ITER and research towards DEMO by addressing key physics issues.The whole magnet system of JT-60SA has a cold mass of more than 670tonnes and rests entirely on the toroidal field (TF) magnet which is made of 18 D-shaped NbTi superconducting coils, each enclosed in a stainless steel casing, which are supported toroidally by a fully bolted outer intercoil structure (OIS), and which are connected to one another by complex bolted and pinned inner intercoil structures (IIS).Due to the demanding electromagnetic loads experienced by the tokamak magnet system during operation, together with the tight space requirements around the coils, the intercoil structures have to transfer massive shear and normal forces, in excess of several MN, while providing enough flexibility to avoid unnecessary internal reactions to the immense centripetal and wedging forces at the magnet centre vault.This paper illustrates the engineering analyses carried out to define the requirements for the coil casings and intercoil structures, to assist the development of the technological solutions in view of the manufacturing of the components, and to predict the behaviour of the components from their assembly through to off-normal and fault conditions during operation.The latest results obtained from the global electromagnetic and structural analyses, briefly presented in the paper, show that the magnet system is suitable to perform the intended number of cycles in the operating conditions. More emphasis is given to the results of the multiple detailed analyses of the TF intercoil structures, which show that each individual component can withstand the operating loads with a comfortable safety margin.