K. Kizu

Conceptual Design of Superconducting Magnet System for JT-60SA

The upgrade of JT-60U magnet system to superconducting coils (JT-60SA) has been decided by both parties of Japanese government (JA) and European commission (EU) in the framework of the Broader Approach (BA) agreement. The magnet system for JT-60SA consists of 18 toroidal field (TF) coils, a Central Solenoid (CS) with four modules, seven Equilibrium Field (EF) coils. The TF case encloses the winding pack and is the main structural component of the magnet system. The CS consists of independent winding pack modules, which is hung from the top of the TF coils through its pre-load structure. The seven EF coils are attached to the TF coil cases through supports which include flexible plates allowing radial displacements. The CS modules operate at high field and use Nb3 Sn type superconductor. The TF coils and EF coils use NbTi superconductor. The magnet system has a large heat load from nuclear heating from DD fusion and large AC loss. This paper describes the technical requirements, the operational interface and the outline of conceptual design of the superconducting magnet system for JT-60SA.

The Manufacturing of the Superconducting Magnet System for the JT-60SA

JT-60SA is the satellite tokamak for ITER in the Broader Approach agreement. The JT-60SA uses 18 toroidal field coils, a central solenoid with 4 modules, and 6 equilibrium field coils, they are all superconducting coils with forced flow cooled conductors. All detailed designs of these superconducting coils have been completed. The manufacturing of conductors and coils are progressing in Japan and EU. This paper shows the latest manufacturing activities and final design adjusting of its magnet system and their utilities.

Conductor Design of CS and EF Coils for JT-60SA

The conductor for central solenoid (CS) and equilibrium field (EF) coils of JT-60 Super Advanced (JT-60SA) were designed. The conductor for CS is Nb3Sn Cable-In-Conduit (CIC) conductor with JK2LB jacket. EF coil conductors are NbTi CIC conductor with SS316LN jacket. The field change rate (3.9 T/s), faster than ITER generates the large AC loss in conductor. The analyses of current sharing temperature (Tcs)margins for these coils were performed by the one-dimensional fluid analysis code with transient heat loads. The margins of these coils are 1 K for the plasma standard and disruption scenarios. The minimum Tcs margin of CS conductor is 1.2 K at plasma break down (BD). The margin is increased by decreasing the rate of initial magnetization. It is found that the disruption mainly impacts the outer low field EF coil. The disruption decreases the Tcs margin of the coil by >1 K. A coupling time constant of <100 ms, Ni plating, and a central spiral are required for NbTi conductor.

JT-60SA Toroidal Field Magnet System

The broader approach agreement between Europe and Japan includes the construction of a fully superconducting tokamak, the JT-60 Super Advanced (JT-60SA), as a satellite experiment to ITER. In particular, the whole Toroidal Field magnet system, described in this paper, will be provided to Japan by the EU. All the TF coil main constituents, i.e. conductor, winding pack, joints, casing, current leads, are here presented and discussed as well as the design criteria adopted to fulfil the machine requirements. The results of the analyses performed by the EU and JA to define and assess the TF magnet system conceptual design are reported and commented. Future work plan is also discussed.

Development and Test of JT-60SA Central Solenoid Model Coil

A central solenoid (CS) model coil (CSMC) was manufactured by using real manufacturing jigs and procedure to validate the CS manufacturing processes for JT-60SA. The winding accuracy and the temperature control precision during the heat treatment met the requirements. The vacuum pressure impregnation process was also successfully finished. The cold test of the CSMC was performed as a final check of the manufacturing process. The joint resistance, the Ic, and the pressure drop measurements were conducted as the verification test. The results of verification test satisfied the design requirements. These results indicate that the manufacturing processes of the JT-60SA CS has been established. The manufacturing of real CS pancakes just started after finishing the CSMC test.

Development of Central Solenoid for JT-60SA

Several components for central solenoid (CS) of JT-60 Super Advanced (JT-60SA) were newly developed and tested. The butt-type joint, the electrical resistance of which is about 2 nΩ, was developed to increase the winding diameter. The insulation system, which consists of Glass/Kapton/Glass tape and Bisphenol A diglycidyl ether (DGEBA) epoxy, showed sufficient tensile strength after the irradiation of 100 kGy. Insulation characteristics of 4 × 4 winding stack sample after the compression of 705 kN 36 000 times was able to withstand voltages larger than 21 kV. The heat treatment and transfer of the CS model coil with superconductor were conducted. The pancake temperature during flat top was maintained at 923 ± 4 K. The maximum temperature difference in the pancake was 30 K. All manufacturing processes were confirmed so that the mass production of CS will be started in 2013.

Mass Production of Superconducting Magnet Components for JT-60SA

JT-60SA is foreseen in the Broader Approach Agreement as the satellite tokamak for ITER. It uses 18 toroidal field coils, a central solenoid with four modules, and six equilibrium field coils. The coils are all superconducting with forced-flow cooled conductors. Series production of conductors and coils is progressing in Japan and Europe. This paper shows the latest manufacturing activities of the magnet system and its utilities.

Recent Progress of the Design Activity for the Poloidal Field Coil System in JT-60SA

PA (procurement arrangement) for poloidal field (PF) coil system, which consists of the central solenoid (CS) and the equilibrium field (EF) coils, was agreed between Japan and EU. During this activity, design of PF coils system was continued to be modified. For CS, material for the jacket of this conductor was changed into stainless steel (316LN) to make providing easier. In the modified material, maximum stress at the jacket was kept within the allowable limit. Accompanying this modification, the amount of pre-compress had to be re-estimated. Therefore, it was clarified that designs of pre-compression and tie plates need not to be major modification. For EF coils, positions and the number of turns were modified since the progress of the research for the plasma operations required in JT-60SA. Due to this optimization, total amount of superconducting material was reduced. The detail designs of PF coils were also performed to reduce the materials of supports and to evaluate the mechanical strength considering the various events. Thickness of clamp plate of the EF coil which received relatively small electromagnetic force was able to be reduced. Regarding the design of support legs with flexible plate, deformation of toroidal field (TF) coil was considered that should be included the evaluation of stress at this parts because this parts are directly attached on the TF coil case. Therefore, the revised designs of supports with sufficient mechanical strength were obtained for EF1 and EF4.

Manufacture of the Winding Pack and Development of Key Parts for the JT-60SA Poloidal Field Coils

Manufacture of poloidal field (PF) coil system in JT-60SA is progressing. Recently, fabrication of the winding machines for equilibrium field (EF) coils and central solenoid (CS) were completed and the winding with the superconducting conductor was started for an EF coil (EF4). A few double pancake (DP) coils for EF4 were fabricated, and it was realized that the error of circularity for DP coils became less than the designed value. Design of the conductor end structure was also progressed. This part had the role of restraining the conductor end to prevent its separation from the winding pack. It was confirmed by the structural analysis that the conductor end structure for CS, which receives the large electromagnetic (EM) force, had sufficient mechanical strength. It was also checked by the cold test that this structure satisfied the designed performance for the electrical isolation in addition to the mechanical performance. Regarding the design of the inlet for CS, mechanical reinforcement was considered for its structure. Final design of CS inlet was determined by structural analysis to confirm the sufficient mechanical strength against the vertical compression induced by EM forces.

Design of JT-60SA Magnets and Associated Experimental Validations

In the framework of the JT-60SA project, aiming at upgrading the present JT-60U tokamak toward a fully superconducting configuration, the detailed design phase led to adopt for the three main magnet systems a brand new design. Europe (EU) is expected to provide to Japan (JA) the totality of the toroidal field (TF) magnet system, while JA will provide both Equilibrium field (EF) and Central Solenoid (CS) systems. All magnet designs were optimized trough the past years and entered in parallel into extensive experimentally-based phases of concept validation, which came to maturation in the years 2009 and 2010. For this, all magnet systems were investigated by mean of dedicated samples, e.g. conductor and joint samples designed, manufactured and tested at full scale in ad hoc facilities either in EU or in JA. The present paper, after an overall description of magnet systems layouts, presents in a general approach the different experimental campaigns dedicated to qualification design and manufacture processes of either coils, conductors and electrical joints. The main results with the associated analyses are shown and the main conclusions presented, especially regarding their contribution to consolidate the triggering of magnet mass production. The status of respective manufacturing stages in EU and in JA are also evoked.