On a scenario assessment tool for the operation of the JT-60SA superconducting tokamak

JT-60SA is a superconducting tokamak to be assembled and operated at the JAEA laboratories in Naka (Japan). The tokamak is designed, manufactured and operated under the funding of the Broader Approach Agreement (between the government of Japan and the European Commission) and of the Japan Fusion National Programme; JT-60SA aims to prepare, support and complement the ITER experimental programme. The European contribution to the JT-60SA is, for a large fraction, procured by France, Germany, Italy, Spain and Belgium.This paper summarizes the activities carried out at F4E to develop a user-friendly software tool able to assess in real-time if an operational scenario could be structurally withstood by the magnet system of JT-60SA. Such tool is based on a theoretical formulation which is supported by a series of dedicated finite element method (FEM) calculations, and is able to provide a comparative assessment of any candidate scenario with respect to the baseline scenarios, and a quantitative assessment of all electro magnetic (EM) forces acting on the magnet system at any time during the candidate scenario. The tool as it is presented is specifically designed to be used for the JT-60SA tokamak, though it is designed so to that its usage could be extended easily to any other tokamak.     

Structural analysis of the JT-60SA cryostat base

Design and manufacturing of the JT-60SA cryostat is being performed by CIEMAT, according to the Broader Approach Agreement between Japan and the European Commission. Taking into account both the limitations of transport and the assembly sequence of JT60-SA, the cryostat is divided in two main parts, namely the cryostat base and the cryostat vessel body.The paper is focused on the structural analyses carried out by CIEMAT to evaluate the mechanical behavior of the JT-60SA cryostat base final design, since the cryostat vessel body will be designed and manufactured in a subsequent stage.The overall structural integrity of the cryostat base has been verified and confirmed utilizing the ‘limit analysis’ procedure defined in ASME code 2007 Section VIII, Div. 2. The study has been complemented by further finite element analyses that include the detail of the bolted fastenings, aimed to evaluate the mechanical behavior of the bolted joints themselves, as well as the stresses and deformations in the overall cryostat base structure.     

A coil test facility for the cryogenic tests of the JT-60SA TF coils

In the framework of the Broader Approach Activities, the EU will deliver to Japan the 18 superconducting coils, which constitute the JT-60SA Toroidal field magnet. These 18 coils, manufactured by France and Italy, will be cold tested before shipping to Japan. For this purpose, the European Joint Undertaking for ITER, the Development of Fusion Energy (“Fusion for Energy”, F4E) and the European Voluntary Contributors are collaborating to design and set-up a coil test facility (CTF) and to perform the acceptance test of the 18 JT-60SA Toroidal Field (TF) coils. The test facility is designed to test one coil at a time at nominal current and cryogenic temperature. The test of the first coil of each manufacturer includes a quench triggered by increasing the temperature.The project is presently in the detailed design phase.     

Present Status of JT-60SA Project and Development of Heating Systems for JT-60SA

Present status of the JT-60SA (JT-60 Super Advanced) project, implemented jointly by Europe and Japan since 2007, is described. The design of the main tokamak components was completed in late 2008, and all the scientific missions are preserved to contribute to ITER and DEMO reactors. The construction of the JT-60SA has begun with procurement activities for the superconducting magnet systems, vacuum vessel, in-vessel components and other components under the relevant procurement arrangements between the implementing agencies of JAEA (Japan Atomic Energy Agency) in Japan and Fusion for Energy in Europe. Designs and developments of the auxiliary heating systems for JT-60SA have been progressing at JAEA so as to provide the total injection power of 41 MW for 100 s.     

Overview of engineering design,manufacturing and assembly of JT-60SA machine

The JT-60SA experiment is one of the three projects to be undertaken in Japan as part of the Broader Approach Agreement, conducted jointly by Europe and Japan, and complementing the construction of ITER in Europe. The JT-60SA device is a fully superconducting tokamak capable of confining break-even equivalent deuterium plasmas with equilibria covering high plasma shaping with a low aspect ratio at a maximum plasma current of I_p = 5.5 MA. This makes JT-60SA capable to support and complement ITER in all the major areas of fusion plasma development necessary to decide DEMO reactor construction. After a complex start-up phase due to the necessity to carry out a re-baselining effort with the purpose to fit in the original budget while aiming to retain the machine mission, performance, and experimental flexibility, in 2009 detailed design could start. With the majority of time-critical industrial contracts in place, in 2012, it was possible to establish a credible time plan, and now, the project is progressing on schedule towards the first plasma in March 2019. After careful and focused R&D and qualification tests, the procurement of the major components and plant is now well advanced in manufacturing design and/or fabrication. In the meantime the disassembly of the JT-60U machine has been completed and the engineering of the JT-60SA assembly process has been developed. The actual assembly of JT-60SA started in January 2013 with the installation of the cryostat base. The paper gives an overview of the present status of the engineering design, manufacturing and assembly of the JT-60SA machine.     

Design Study of Top Lid with Clamp Structure in JT-60SA Cryostat

JT-60SA is a fully superconducting coil tokamak upgraded from the JT-60U. This paper focused on the integrity of the top lid of cryostat in JT-60SA. The design requirement for the cryostat in normal operations is to achieve vacuum insulation of 10 3 Pa, and the top flange of the top lid is lightly welded onto its body flange. The weld is tensile-loaded by bending deformation of the top lid due to vacuum pressure of external 0.1 MPa. This weld integrity is evaluated with tensile-load reduction, which results in clamp reinforcement. The structural integrity of the top lid is validated.     

Design and manufacturing of JT-60SA vacuum vessel

JT-60 is planned to be upgraded to JT-60SA tokamak machine with fully superconducting coils, which is a project of the JA-EU satellite tokamak program under both Broader Approach program and Japanese domestic program. The JT-60SA vacuum vessel (VV) has a D-shape poloidal cross section and a toroidal configuration with 10° facet segmented in toroidal direction. The material of the VV is 316L stainless steel with low cobalt content of <0.05 wt%. A double wall structure is adopted for the VV to ensure high rigidity and high toroidal one-turn resistance simultaneously.Fundamental welding R&D and a trial manufacturing of the 20° upper half of the VV have been performed to study the manufacturing procedure. After the confirmation of the quality of the mock-up, manufacturing of the actual VV started in November 2009.     

Broader Approach to fusion energy

The Broader Approach activities aim at complementing the ITER project and at an acceleration of fusion energy in the framework of collaboration between Japan and EURATOM. Three research projects are to be undertaken: (1) Satellite Tokamak Programme, (2) Engineering Validation and Engineering Design Activities for the International Fusion Materials Irradiation Facility (IFMIF/EVEDA), and (3) International Fusion Energy Research Centre (IFERC). While the Satellite Tokamak Programme is to be conducted at the site of the existing JT-60 tokamak, the other two projects are to be undertaken at a new research site in Rokkasho, Japan.     

Plan of ITER remote experimentation center

Plan of ITER remote experimentation center (REC) based on the broader approach (BA) activity of the joint program of Japan and Europe (EU) is described. Objectives of REC activity are (1) to identify the functions and solve the technical issues for the construction of the REC for ITER at Rokkasho, (2) to develop the remote experiment system and verify the functions required for the remote experiment by using the Satellite Tokamak (JT-60SA) facilities in order to make the future experiments of ITER and JT-60SA effectively and efficiently implemented, and (3) to test the functions of REC and demonstrate the total system by using JT-60SA and existing other facilities in EU. Preliminary identified items to be developed are (1) Functions of the remote experiment system, such as setting of experiment parameters, shot scheduling, real time data streaming, communication by video-conference between the remote-site and on-site, (2) Effective data transfer system that is capable of fast transfer of the huge amount of data between on-site and off-site and the network connecting the REC system, (3) Storage system that can store/access the huge amount of data, including database management, (4) Data analysis software for the data viewing of the diagnostic data on the storage system, (5) Numerical simulation for preparation and estimation of the shot performance and the analysis of the plasma shot. Detailed specifications of the above items will be discussed and the system will be made in these four years in collaboration with tokamak facilities of JT-60SA and EU tokamak, experts of informatics, activities of plasma simulation and ITER. Finally, the function of REC will be tested and the total system will be demonstrated by the middle of 2017.     

Assembly study for JT-60SA tokamak

The assembly scenarios and assembly tools of the major tokamak components for JT-60SA are studied in the following. (1) The assembly frame (with a dedicated 30-tonne crane), which is located around the JT-60SA tokamak, is adopted for effective assembly works in the torus hall and the temporary support of the components during assembly. (2) Metrology for precise positioning of the components is also studied by defining the metrology points on the components. (3) The sector segmentation for weld joints and positioning of the vacuum vessel (VV), the assembly scenario and tools for VV thermal shield (TS), the connection of the outer intercoil structure (OIS) and the installation of the final toroidal field coil (TFC) are studied, as typical examples of the assembly scenarios and tools for JT-60SA.     

JT-60SA Toroidal Field Coils test cryostat development

Within the Broader Approach Agreement, Fusion for Energy will deliver to the Japanese Atomic Energy Association, amongst other components, the 18 Toroidal Field Coils (TFCs) for the superconducting Tokamak JT-60SA [1]. These coils will be individually tested at cryogenic temperatures and at the nominal current in a test cryostat. This cryostat is provided as an in-kind contribution by Belgium and is being developed jointly with CEA-Saclay/France.The vessel is large, oval shaped with an overall length of 11 m, a width of 7.2 m and a height of 6.5 m. To reduce the heat load to the coils the cryostat is covered by LN₂ cooled thermal shields. In addition to the cryostat, three test frames for the coils, the valve box vessel and the insulation vacuum system are also provided by Belgium. The Belgian contribution is design, manufacturing, assembly and test of the vacuum chamber, thermal shield and test frames by the Belgian company Ateliers de la Meuse (ALM), with the support of Centre Spatial de Liège (CSL). The TF coil test facility is assembled and the coil tests are performed by CEA/Saclay.The Belgian contribution, namely the design, manufacturing, assembly and test of the vacuum vessel, the thermal shields, and the test frames as well as of the vacuum pumping system are described in the presentation.     

Final design of the Switching Network Units for the JT-60SA Central Solenoid

This paper describes the approved detailed design of the four Switching Network Units (SNUs) of the superconducting Central Solenoid of JT-60SA, the satellite tokamak that will be built in Naka, Japan, in the framework of the “Broader Approach” cooperation agreement between Europe and Japan.The SNUs can interrupt a current of 20 kA DC in less than 1 ms in order to produce a voltage of 5 kV. Such performance is obtained by inserting an electronic static circuit breaker in parallel to an electromechanical contactor and by matching and coordinating their operations. Any undesired transient overvoltage is limited by an advanced snubber circuit optimized for this application. The SNU resistance values can be adapted to the specific operation scenario. In particular, after successful plasma breakdown, the SNU resistance can be reduced by a making switch.The design choices of the main SNU elements are justified by showing and discussing the performed calculations and simulations. In most cases, the developed design is expected to exceed the performances required by the JT-60SA project.     

Electromagnetic analysis, structural integrity and progress on mechanical design of the ITER ferromagnetic insert

Ferromagnetic material is used to reduce the toroidal field ripple in JFT-2M [H. Kawashima, et al., Demonstration of ripple reduction by ferritic steel board insertion in JFT-2M, Nucl. Fusion, 41 (2001) 257-263] and JT-60U [H. Takenaga, the JT-60 Team, Overview of JT-60U results for development of steady-state advanced Tokamak scenario, Proceedings of the 21st IAEA Fusion Energy Conference, Chengdu, China, 2006]. In ITER, since the ferromagnetic material is inserted in the space between the double walls of ITER Vacuum Vessel (VV), it is called “ferromagnetic inserts”. Suitable material is selected to satisfy the design requirements of ITER. The proper location and amount of the ferromagnetic inserts are optimized with the goal of reduction of the toroidal field ripple. The ferromagnetic inserts are designed to minimize electromagnetic forces acting on them. The electromagnetic forces have been calculated with the latest disruption scenarios. Magnetization forces due to magnetic fields have also been calculated. Structural integrity has been validated by a structural analysis.     

Structural analysis of the JT-60SA cryostat vessel body

The JT-60SA cryostat is a stainless steel vacuum vessel (14 m diameter, 16 m height) which encloses the Tokamak providing the vacuum environment (10^?3 Pa) necessary to limit the transmission of thermal loads to the components at cryogenic temperature. It must withstand both external atmospheric pressure during normal operation and internal overpressure in case of an accident.The paper summarizes the structural analyses performed in order to validate the JT-60SA cryostat vessel body design. It comprises several analyses: a buckling analysis to demonstrate stability under the external pressure; an elastic and an elastic–plastic stress analysis according to ASME VIII rules, to evaluate resistance to plastic collapse including localized stress concentrations; and, finally, a detailed analysis with bolted fasteners in order to evaluate the behavior of the flanges, assuring the integrity of the vacuum sealing welds of the cryostat vessel body.     

Parametric thermo-hydraulic analysis of the TF system of JT-60SA during fast discharge

The evolution of the conductor temperature and of the helium pressure of the central pancake of the TF superconducting magnet of the JT-60SA tokamak in a quench scenario are here discussed. The quench is triggered by a heat disturbance applied at the end of burning and followed by a fast safety discharge. A parametric study aimed at assessing the robustness of the calculation is also addressed with special regard to the voltage threshold, used to define the occurrence of the quench, and to the time delay, that cover all the possible delays in the fast discharge after quench detection. Finally, due to sensitivity analyses the influences of different parameters were assessed: the material properties of the strands (RRR, copper fraction), the magnitude and the spatial length of the triggering disturbance and the magnetic field distribution. The numerical evaluations were performed in the framework of the Broader Approach Agreement in collaboration with CEA, ENEA and the JT-60SA European Home Team using the 1D code Gandalf [1].     

Status of design and procurement activities in JT-60SA

The JT-60SA experiment is one of the three projects to be undertaken in Japan as part of the Broader Approach Agreement, conducted jointly by Europe and Japan, and complementing the construction of ITER in Europe. It is a fully superconducting tokamak capable of confining break-even equivalent deuterium plasmas with equilibria covering high plasma shaping with a low aspect ratio at a maximum plasma current of I_p = 5.5 MA. In late 2007 the BA Parties, prompted by cost concerns, asked the JT-60SA Team to carry out a re-baselining effort with the purpose to fit in the original budget while aiming to retain the machine mission, performance, and experimental flexibility. Subsequently the Integrated Project Team has undertaken a machine re-optimization followed by engineering design activities aimed to reduce costs while maintaining the machine radius and plasma current. This effort led the Parties to the approval of the new design in late 2008 and hence final design and procurement activities have commenced. The paper will describe the process leading to the re-baselining, the resulting final design and technical solutions and the present status of procurement activities.     

Upgrading the NIFS superconductor test facility for JT-60SA cable-in-conduit conductors

The superconductor test facility at the National Institute for Fusion Science (NIFS) was upgraded to test cable-in-conduit (CIC) conductors for the JT-60SA equilibrium field (EF) coil. Supercritical helium (SHe) lines were assembled with transfer tubes and a heat exchanger. The CIC conductor was covered with a thermal insulation vessel, filled with gas helium at atmospheric pressure. The temperature of the conductor was varied using a film heater attached to an inlet pipe. Critical current (Ic) and current sharing temperature (Tcs) measurements of the prototype CIC conductor were carried out successfully in the upgraded test facility. During the measurements, the conductor temperature was precisely controlled.     

Fabrication and installation of equilibrium field coils for the JT-60SA

Recently, fabrication of the first superconducting coil in JT-60SA tokamak (EF4 coil) was finished. EF4 coil has ten double pancake (DP) coils. All DP coils were stacked up to form the winding pack. In order to check the manufacturing error of DP coils, their circularities (in-plane ellipticities) were evaluated for all DP coils. Positions of conductors for each DP coil were measured before curing process. Error bars of the current centers, which were used for the index for DP coil's circularity, ranged between 1.1 and 2.5 mm. During stacking the DP coils, the positions of these coils were optimized in order to cancel the error of circulation of the winding pack. As the result, the manufacturing error of the radial current center was achieved 0.6 mm for the winding pack. This value was an order of magnitude smaller than the required manufacturing error of EF4 coil.     

Progress in JT-60 Experiments

The Japan Atomic Energy Research Institute completed in April 1985 the construction of the large tokamak JT-60, which constitutes a focus of the Second Phase of the Japanese fusion development program under the Atomic Energy Commission started in 1975. Initial experiments of JT-60 were carried out in hydrogen plasma after the completion of the device. Full installation and testing of the heating devices was completed in July 1986 and subsequently the heating experiment was initiated. The target parameters were achieved in deuterium-equivalent values in September 1987 by high power heating of high density hydrogen plasmas at high plasma current. JT-60 has entered the phase of advanced experiments in 1988.