Mechanical Analysis and Optimization of ITER Upper ELM Coil & Feeder

International thermonuclear experimental reactor （ITER） edge localized mode （ELM） coils are used to mitigate or suppress ELMs. The location of the coils in the vacuum vessel and behind the blankets exposes them to high radiation levels and high temperatures. The feeders provide the power and cooling water for ELM coils. They are located in the chinmey ports and experience lower radiation and temperature levels. These coils and feeders work in a high magnetic field environment and are subjected to alternating electromagnetic force due to the interaction between high magnetic field and alternating current （AC） current in the coils. They are also subjected to thermal stresses due to thermal expansion. Using the ITER upper ELM coil and feeder as an example, mechanical analyses are performed to verify and optimize the updated design to enhance their structural performance. The results show that the conductor, jacket and bracket can meet the static, fatigue and crack threshold criteria. The optimization indicates that adding chamfers to the bracket can reduce the high stress of the bracket, and removing two rails can reduce the peak reaction force on the two rails arising from thermal expansion.     

Design of Tokamak ELM Coil Support in High Nuclear Heat Environment

In Tokomak, the support of the ELM coil, which is close to the plasma and subject to high radiation level, high temperature and high magnetic field, is used to transport and bear the thermal load due to thermal expansion and the alternating electromagnetic force generated by high magnetic field and AC current in the coil. According to the feature of ITER ELM coil, the mechanical performance of rigid and flexible supports under different high nuclear heat levels is studied. Results show that flexible supports have more excellent performance in high nuclear heat condition than rigid supports. Concerning thermal and electromagnetic （EM） loads, optimized results further prove that flexible supports have better mechanical performance than rigid ones. Through these studies, reasonable support design can be provided for the ELM coils or similar coils in Tokamak based on the nuclear heat level.     

Electromagnetic–thermal–structural coupling analysis of the ITER edge localized mode coil with fiexible supports

In a fusion reactor, the edge localized mode (ELM) coil has a mitigating effect on the ELMs of the plasma. The coil is placed close to the plasma between the vacuum vessel and the blanket to reduce its design power and improve its mitigating ability. The coil works in a high-temperature, high-nuclear-heat and high-magnetic-field environment. Due to the existence of outer superconducting coils, the coil is subjected to an alternating electromagnetic force induced by its own alternating current and the outer magnetic field. The design goal for the ELM coil is to maintain its structural integrity in the multi-physical field. Taking as an example the middle ELM coil (with flexible supports) of ITER (the International Thermonuclear Fusion Reactor),an electromagnetic–thermal–structural coupling analysis is carried out using ANSYS. The results show that the flexible supports help the three-layer casing meet the static and fatigue design requirements. The structural design of the middle ELM coil is reasonable and feasible. The work described in this paper provides the theoretical basis and method for ELM coil design.     

Structural Design Study for ITER Upper ELM Coils

ITER ELM coils are important parts of in-vessel coils and they are mounted on the vacuum vessel and behind the blanket module. They consist of three sets of coils, referred to as the upper, mid, and lower coils. In order to verify the structural design feasibility and find the better structure for upper edge localized modes (ELM) coil, two different variants of coil support structures are studied under the electromagnetic load, thermal and other loads. Results show that besides the bracket of variant 2 does not meet the fatigue criteria, the conductor, jacket and bracket of the two structures can meet the static, fatigue and crack threshold criteria and both of them are valid and feasible. In addition, the better structure is chosen for upper ELM coil.     

Multiaxial fatigue analysis for IMIC of ITER upper ELM coil

Inconel Jacketed Mineral Insulated Conductor (IMIC) is a very important component of International Thermonuclear Experimental Reactor (ITER) Edge Localized Modes (ELM) coils, which are located between the vacuum vessel (VV) and blanket shield modules and subject to high radiation levels, high temperature and high magnetic field. These coils will experience thermal pulsed, cyclic electromagnetic (EM) load during operation. They are designed to sustain at 1.5e8 total stress cycles and shall have sufficient strength and excellent fatigue to transport and bear the high cyclic load. For IMIC, multiaxial fatigue analysis is used to evaluate failure. Two methods based on the alternating stress and mean stress in American Society of Mechanical Engineers (ASME) code provide the design codes for multiaxial fatigue evaluation: constant principal stress direction and variation of principal stress direction. Results show that using the two methods obtains basically the same equivalent alternating stress. Both of them can be recommended for the ELM coils and IMIC can meet the fatigue criteria.     

Electromagnetic–thermal–structural coupling analysis of the ITER edge localized mode coil with flexible supports

In a fusion reactor, the edge localized mode(ELM) coil has a mitigating effect on the ELMs of the plasma. The coil is placed close to the plasma between the vacuum vessel and the blanket to reduce its design power and improve its mitigating ability. The coil works in a high-temperature,high-nuclear-heat and high-magnetic-field environment. Due to the existence of outer superconducting coils, the coil is subjected to an alternating electromagnetic force induced by its own alternating current and the outer magnetic field. The design goal for the ELM coil is to maintain its structural integrity in the multi-physical field. Taking as an example the middle ELM coil(with flexible supports) of ITER(the International Thermonuclear Fusion Reactor), an electromagnetic–thermal–structural coupling analysis is carried out using ANSYS. The results show that the flexible supports help the three-layer casing meet the static and fatigue design requirements. The structural design of the middle ELM coil is reasonable and feasible. The work described in this paper provides the theoretical basis and method for ELM coil design.     

Physical design of a new set of high poloidal mode number coils in the EAST tokamak

An external resonant magnetic perturbation (RMP) field, which is an effective method to mitigate or suppress the edge localized mode (ELM), has been planned to be applied on the ELM control issue in ITER. A new set of magnetic perturbation coils, named as high m coils, has been developed for the EAST tokamak. The magnetic perturbation field of the high m coils is localized in the midplane of the low field side, with the spectral characteristic of high m and wide n, where m and n are the poloidal and toroidal mode numbers, respectively. The high m coils generate a strong localized perturbation field. Edge magnetic topology under the application of high m coils should have either a small or no stochastic region. With the combination of the high m coils and the current RMP coils in the EAST, flexible working scenarios of the magnetic perturbation field are available, which is beneficial for ELM control exploration on EAST. Numerical simulations have been carried out to characterize the high m coil system, including the magnetic spectrum and magnetic topology, which shows a great flexibility of magnetic perturbation variation as a tool to investigate the interaction between ELM and external magnetic perturbation.     

Detailed nuclear analysis of ITER ELM coils

The ELM coils in ITER are intended to provide control of Edge Localized Modes (ELMs). These coils are located on the outboard side of ITER between the shield modules and vacuum vessel (VV) and are subject to high radiation levels. Detailed three-dimensional (3-D) models of the toroidal and poloidal legs of the ELM coil and surrounding region for the MCNP code were updated to reflect the latest design changes. Neutronics calculations were performed to determine a variety of radiation damage parameters for the ELM coils as well as the VV located behind them. Additionally, detailed CAD based models for the upper ELM coil region were used to perform a CAD based analysis using the DAG-MCNP5 code. The results show that the ELM coil will meet the specified material radiation limits. However, the nuclear heating on the vacuum vessel behind the poloidal multi-pipe manifolds will exceed the specified limit.     

Design Study of Support for ITER ELM Coils

The support is an important part of ITER ELM coils. It should withstand the alternating electromagnetic (EM) force and thermal stresses. Based on the finite element method, 2D and 3D structures of the rigid and flexible support of ITER upper ELM coil in different loads are studied. Results show that the flexible support can reduce the stresses of the conductor and jacket. In the lower level of nuclear heat, two types of supports can be used in the quarter model. In the high level of nuclear heat, the flexible support is needed and 50 mm support is proposed for the quarter model. Considering the EM load, the rigid support has better performance than the flexible support. Therefore, reasonable support can be provided for ELM coil or similar coil according to the thermal expansion and EM load.     

Investigation and analysis on ITER in-vessel coils’ raw-materials

The ITER in-vessel coils (IVCs) consist of 27 coils edge localized modes (ELM) and 2 coils vertical stabilization (VS) which are all mounted on the vacuum vessel wall behind the shield modules. The IVCs design and manufacturing work is being conducted in between Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) and Princeton Plasma Physics Laboratory (PPPL). Because the position of ELM and VS coils is close and face to the plasma, the IVCs must undergo a severe environment, such as the high dose of radiation and high operation temperature, thus the conventional electrical insulation materials cannot be used. And the technology of “Stainless Steel Jacketed Mineral Insulated Conductor” (SSMIC) is deemed as the best choice to provide the necessary radiation resistance and compatibility strength in ITER's vacuum vessel. While mineral insulated conductor technology is not new, and is similar to the mineral insulated cable used in industrial. Some difficulties still need to be solved, such as searching for the proper raw-materials to make sure that the conductor have the properties of high current carrying capability, the necessary radiation resistance, the proper strength, at the same time, it must be come true in manufacture technology. This paper described the analysis of the materials for VS and ELM coil conductor.     

Stress and Thermal Analysis of the In-Vessel RMP Coils in HL-2M

A set of in-vessel resonant magnetic perturbation(RMP) coils for MHD instability suppression is proposed for the design of a HL-2M tokamak.Each coil is to be fed with a current of up to 5 kA,operated in a frequency range from DC to about 1 kHz.Stainless steel(SS) jacketed mineral insulated cables are proposed for the conductor of the coils.In-vessel coils must withstand large electromagnetic(EM) and thermal loads.The support,insulation and vacuum sealing in a very limited space are crucial issues for engineering design.Hence finite element calculations are performed to verify the design,optimize the support by minimizing stress caused by EM forces on the coil conductors and work out the temperature rise occurring on the coil in diferent working conditions,the corresponding thermal stress caused by the thermal expansion of materials is evaluated to be allowable.The techniques to develop the in-vessel RMP coils,such as support,insulation and cooling,are discussed.     

Electromagnetic Analyses of ITER Lower ELM Right Bottom Corner

ELM(edge localized mode) coils are key components of ITER that suppress the edge localized mode phenomenon. A giant electromagnetic force is generated during normal operations by the current flowing in the ELM coils interacting with the external background field. The Lorentz force will induce Tresca stress in the ELM coils. If the load goes beyond the allowable threshold, the coils can hardly satisfy the safety requirements. The right-hand bottom corner was chosen to perform our electromagnetic analyses. Based on the Maxwell equation, the detailed magnetic field and Lorentz force were calculated. By use of the finite element software ANSYS,the Tresca stress was extracted and evaluated based on our analytical design. The present analysis aims to verify the feasibility of the current design. It can also serve as guidance for fabrication and structural optimization.     

Structural considerations for a Tokamak fusion reactor

Several preliminary structural analyses are presented which validate a design for the experimental power reactor. Three components are singled out as requiring special attention: the magnetic coils, the blanket support structure, and the blanket modules. Repeated loading of a coil structure by magnetic forces should produce only linear elastic deformation. An analysis for minimum preload necessary to ensure this is presented. Using axisymmetric thin shell theory, a stress analysis of the blanket support structure is described. To account for the welded ring structure, a perforated plate analysis is used to compute the structural displacements and the ligament stresses. Temperature distributions and thermal stresses in the blanket module are determined using both finite element and analytical analysis. The stresses are all acceptable, including the effects produced by creep and fatigue. Thermal stress in the liner produced by a nonlinear temperature gradient is also shown to be acceptable.     

Thermal anchoring of wires in large scale superconducting coil test experiment

Effective and precise thermal anchoring of wires in cryogenic experiment is mandatory to measure temperature in milikelvin accuracy and to avoid unnecessary cooling power due to additional heat conduction from room temperature (RT) to operating temperature (OT) through potential, field, displacement and stress measurement instrumentation wires. Instrumentation wires used in large scale superconducting coil test experiments are different compare to cryogenic apparatus in terms of unique construction and overall diameter/area due to errorless measurement in large time-varying magnetic field compare to small cryogenic apparatus, often shielded wires are used. Hence, along with other variables, anchoring techniques and required thermal anchoring length are entirely different in this experiment compare to cryogenic apparatus. In present paper, estimation of thermal anchoring length of five different types of instrumentation wires used in coils test campaign at Institute for Plasma Research (IPR), India has been discussed and some temperature measurement results of coils test campaign have been presented.     

Quench detection of fast plasma events for the JT-60SA central solenoid

The JT-60 is planned to be modified to a full-superconducting tokamak referred to as the JT-60 Super Advanced (JT-60SA). The maximum temperature of the magnet during its quench might reach the temperature of higher than several hundreds Kelvin that will damage the superconducting magnet itself. The high precision quench detection system, therefore, is one of the key technologies in the superconducting magnet protection system.The pick-up coil method, which is using voltage taps to detect the normal voltage, is used for the quench detection of the JT-60SA superconducting magnet system. The disk-shaped pick-up coils are inserted in the central solenoid (CS) module to compensate the inductive voltage. In the previous study, the quench detection system requires a large number of pick-up coils. The reliability of quench detection system would be higher by simplifying the detection system such as reducing the number of pick-up coils. Simplifying the quench detection system is also important to reduce the total cost of the protection system. Hence the design method is improved by increasing optimizing parameters. The improved design method can reduce the number of pick-up coils without reducing the sensitivity of detection; consequently the protection system can be designed with higher reliability and lower cost. The applicability of the disk-shaped pick-up coil for quench detection system is evaluated by the two dimensional analysis. In the previous study, however, the analysis model only took into account the CS, EF (equilibrium field) coils and plasma. Therefore, applicability of the disk-shaped pick-up coil for the quench detection system remains open question because the fast plasma events, such as disruption, mini collapse and ELM (edge localized mode), directly influences on the voltage of pick-up coil making the quench signal undetectable. Consequently, a new analysis model proposed in the present paper was designed to avoid this difficulty by introducing the passive coil series such as vacuum vessel and stabilizer. The influence of fast plasma events is absorbed by passive coil series like real system, and the evaluation of applicability can be examined in detail. The analysis results show that the disk-shaped pick-up coil is applicable whenever the standard operation, disruption, mini collapse and ELM.     

Thermo-mechanical analysis of RMP coil system for EAST tokamak

Resonant magnetic perturbation (RMP) has been proved to be an efficient approach on edge localized modes (ELMs) control, resistive wall mode (RWM) control, and error field correction (EFC), RMP coil system under design in EAST tokamak will realize the above-mentioned multi-functions. This paper focuses on the thermo-mechanical analysis of EAST RMP coil system on the basis of sensitivity analysis, both normal and off-normal working conditions are considered. The most characteristic set of coil system is chosen with a complete modelling by means of three-dimensional (3D) finite element method, thermo-hydraulic and thermal-structural performances are investigated adequately, both locally and globally. The compromise is made between thermal performance and structural design requirements, and the results indicate that the optimized design is feasible and reasonable.     

Magnet design of toroidal field coils for the UWMAK-Tokamak systems

The design of toroidal field coils for the UWMAK series of Tokamak reactor designs is described. These are cryogenically stable coils cooled in liquid helium to 4.2 K. Each individual turn of composite conductor of TiNb plus matrix conductor is epoxied into a groove in a thin disk structure. The magnet is divided into 12, 18 or 24 sectors; each sector is comprised of 15–20 thin disks which are spaced and bolted together to form a rigid structure with all disk surfaces exposed to cooling. The overall shape of each ‘D’ magnet sector is chosen so that only constant tension forces are present. Bending forces do occur but only near transition sections from the D to the central straight section of each coil. This method of rigid mounting should be compared with loose ‘jelly-roll’ windings on a central coil form, a more typical magnet fabrication technique. The design procedure is for the composite conductor TiNb plus copper (or aluminum) to be mounted in stainless steel (or aluminum alloy) disks. Full stability is obtained for strains less than 0.2% for steel support and less than 0.4% for aluminum supports based on stress-strain resistivity experiments in progress. The use of high purity aluminum conductor and high strength aluminum alloy structure reduces costs significantly dependent only on the orderly development of new aluminum TiNb composite conductors. The amount of TiNb is conservatively chosen to carry full current at 5.2 K although operation at 4.2 K is planned and full recovery to the superconducting state could be obtained with full current wire quantities selected at 4.3 K. This conservative choice doubles the amount of TiNb used at 8 tesla but provides an extra temperature rise of ΔT = 0.9 K above expected usual temperature excursions.Magnet safety and protection is based on the natural mutual coupling of many coils which are closely coupled to each other. If one coil loses current, the other coils increase their currents to keep the flux as constant as possible. The uncoupled flux and companion field energy would be discharged by a high voltage power supply temporarily set to discharge the one bad coil. Such sub-division and partial energy removal requires that there be substantial subdivision of coils into many separate dewars, so that problems can be isolated. An expression for the magnetic forces on sectioned toroidal field coils is given in closed form and is used to compute the shape of a specific coil. Data obtained here are shown to be in good agreement with those given by more complex procedures. The most severe structural design requirement is based on simultaneous loss of current in two adjacent sectors. The remaining sectors attempt to straighten out into a solenoid which compresses the structure between coils except beside the bad coil or coils where tension might exist. Such current loss in two adjacent sectors is considered an extremely unlikely occurrence since the discharge procedure mentioned above takes place in less than 1 min so that simultaneous refers to a 1 min overlap. Because of such rapid amelioration of the causes of current change and flux motion, no temperatures can exceed room temperature during the orderly shutdown of one or two coils. In general, the study illustrates that fully stable magnets using composite conductors should be engineered without major uncertainties according to straightforward scientific concepts. While subsequent designs will undoubtedly include improvements there is no reason to expect that superconductivity implies venturesome unknown TF coil performance.     

Structural and Fracture Mechanics Analysis for the Bracket of ITER Upper ELM Coil

The brackets are the important components of ITER edge localized modes (ELM) coils to connect the coils and rails. In order to assure the structural integrity and security of the bracket, the maximum tresca stress and stress intensity factor are examined from the viewpoints of structural and fracture mechanics. Based on the finite element method, the global upper ELM coils with simplified and detailed bracket are investigated. Since it is difficult to perform in-service inspection due to inaccessibility of in-vessel coils, it is important to estimate the allowable initial defect. Assuming an initial crack in the maximum first principal stress region on the bracket, the fracture mechanics analyses under different loads are performed. Results show that the bracket design is valid and feasible and the calculation method of finite element for stress intensity factor is feasible and reliable. Assuming the initial crack of 7 mm depth, the bracket can meet the crack growth criteria. The stress intensity factor of the bracket is mainly caused by electromagnetic (EM) load and the thermal load can reduce the stress intensity factor under EM load. The thermal load can make the crack grow on the surface of the bracket and the EM load can cause the crack to extend in the inner of the bracket.     

Seismic analysis of ITER fourth PF (Poloidal Field Coil) feeder

The ITER feeder systems connect the ITER magnet systems located inside the main cryostat to the cryo-plant, power-supply and control system interfaces outside the cryostat. The main purpose of the feeders is to convey the cryogenic supply and electrical power to the coils as well as house the instrumentation wiring. The PF busbar which carries 52 kA current will suffer from high Lorentz force due to the background magnetic field inspired by the coils and the self-field between every pair of busbars. Except their mechanical strength and thermal insulation performance must be achieved, the dynamic mechanism on PF structure should be assessed. This paper presents the simulation and seismic analysis on ITER 4th PF feeder including the Coil Terminal Box and S-bend Box (CTB and SBB), the Cryostat Feed-through (CFT), the In-Cryostat-Feeder (ICF), especially for the ground supports and main outer-tube firstly. This analysis aims to study seismic resistance on system design under local seismograms with floor response spectrum, the structural response vibration mode and response duration results of displacement, membrane stress, and bending stress on structure under different directions actuating signals were obtained by using the single-seismic spectrum analysis and Dead Weight analysis respectively. Based on the simulative and analytical results, the system seismic resistance and the integrity of the support structure in the 4th PF feeder have been studied and the detail design confirmed.     

Validation of welding technology for ITER TF coil structures

Japan Atomic Energy Agency (JAEA), acting as the Japan Domestic Agency (JADA) in the ITER project is responsible for the procurement of 9 Toroidal Field (TF) coil winding packs (WPs), structures for 19 TF coils (including one spare), and assembly of the WPs and the coil structures for 9 TF coils [1]. The TF Coil structures which support large electromagnetic force generated in TF coils under the cryogenic temperature (about 4 K), are very large welded structures composed of a coil case and structural attachments made of high strength and high toughness stainless steel. JAEA has been performing welding trials for heavy thickness materials since 2008 and is planning fabrication of full-scale mock-ups of main sub-components (1 set for the inboard side and 1 set for the outboard side) in 2011 in order to investigate the technical issues for manufacturing the TF coil structures. This paper presents the results of welding trials and the status of full scale mock-ups fabrication to confirm the validity of welding technology and manufacturing design before fabricating the actual products.