Makoto Matsukawa

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.

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.

Design optimization for plasma performance and assessment of operation regimes in JT-60SA

The design of the modification of JT-60U, JT-60SA has been optimized from the viewpoint of plasma performance, and operation regimes have been evaluated with the latest design. Upper and lower divertors with different geometries will be prepared for flexibility of the plasma shape, which will enable both low aspect ratio (A ~ 2.65) and ITER shape (A = 3.1) configurations. The beam lines of negative-ion neutral beam injection will be shifted downwards by ~0.6 m for the off-axis current drive (CD), in order to obtain a weak/reversed shear plasma, as well as having the capability of heating the central region. The feedback control coils along the openings in the stabilizing plate are found effective in suppressing the resistive wall mode and sustaining high βN close to the ideal wall limit. Sustainment of plasma current of 3–3.5 MA for 100 s will be possible in ELMy H-mode plasmas with moderate heating power, βN, and density within an available flux swing. It is also expected that higher βN, high-density ELMy H-mode plasmas will be maintained for 100 s with higher heating power. The expected regime of full CD operation has been extended with upgraded heating and CD power. Full CD operation for 100 s with reactor-relevant high values of normalized beta and bootstrap current fraction (Ip = 2.4 MA, βN = 4.3, fBS = 0.69, , HH98y2 = 1.3) is expected in a highly-shaped low-aspect-ratio configuration (A = 2.65).

Final Design of the Quench Protection Circuits for the JT-60SA Superconducting Magnets

This paper describes the detailed design of the Quench Protection Circuits (QPC) for the superconducting Toroidal Field (TF) and Poloidal Field (PF) magnets of the Satellite Tokamak JT-60SA, which will be installed in Naka, Japan [1]. The nominal currents to be interrupted and the maximum reapplied voltages are 25.7 kA and 2.8 kV for the TF QPCs and 20 kA and 5 kV for PF QPCs. The innovative solution proposed in the QPC design is based on a Hybrid Circuit Breaker (CB) composed of a mechanical Bypass Switch for conducting the continuous current, in parallel to a static CB for current interruption. The main choices of the final design are presented and discussed, either to confirm or to update and complete the study performed at the conceptual design level.

Overview of the National Centralized Tokamak programme

An overview is given of the National Centralized Tokamak (NCT) programme as a research programme for advanced tokamak research to succeed JT-60U. The mission of NCT is to establish high beta steady-state operation for DEMO and to contribute to ITER. The machine flexibility is pursued in aspect ratio and shape controllability for the demonstration of the high-β steady-state, feedback control of resistive wall modes, wide current and pressure profile control capability and also very long pulse steady-state operation. Existing JT-60 infrastructure such as the heating and current drive system, power supplies and cooling systems will be best utilized for this modification.

Development of a vacuum switch carrying a continuous current of 36 kA DC

This paper describes the development of a vacuum switch carrying a continuous current of 36 kA DC. This switch consists of three vacuum interrupters connected in parallel. Generally, it is required to reduce the resistive loss and to increase the heat removal capability for increasing the current carrying capability of VCB. Then, maximizing the cross section of the conductor, and shortening the current path are principally important. However, a coil structure which produces an axial magnetic field to extinguish the arc stably, makes it difficult due to the geometric complexity. Then, the authors adopted unique design of the coil structure to solve this difficulty. The newly developed vacuum interrupter is possible to carry a current of 8 kA without any cooling, which is twice that of the largest VCB available at present in the world. Moreover, introducing the forced-air cooling enhanced the performance up to 12 kA by improving the cooling efficiency. They are verified through a heat running test.

Experimental Qualification of the Hybrid Circuit Breaker Developed for JT-60SA Quench Protection Circuit

This paper deals with the qualification process of the full scale prototypes of the Hybrid mechanical-static dc Circuit Breaker (HCB) for the Quench Protection Circuits (QPC) of the Toroidal Field (TF) and Poloidal Field (PF) superconducting coils of the Satellite Tokamak JT-60SA. The HCB developed for JT-60SA QPC is the first dc circuit breaker based on hybrid mechanical-static design at this level of power (25.7 kA-1.93 kV, ±20 kA- ±3.8 kV). Moreover, the JT-60SA QPC represents the first application of protection for superconducting magnets based on this hybrid technology. Special type tests have been designed to verify the performance of the device up to the nominal ratings and beyond, thus proving the suitability of the technology, the design margins, and the reliability; the results of the most significant tests are presented and discussed. The qualification program also includes the validation of the electrical models developed during the design phase, which are described in the paper, too.

Mechanical Design of JT-60SA Magnet System

Latest design of superconducting magnet system in JT-60SA is presented. This magnet system consists of the TF coils and PF coils (CS and EF coils). For this magnet system, stress analyses are systematically carried out to optimize the structural design. In the analysis of TF coil, it is clarified that jacket of conductor has significant strength under the designed electromagnetic (EM) load. In addition, coil case has margin of mechanical strength. For the PF coils, maximum stress appears at jackets of CS and EF conductor are less than 500 MPa, which is within fatigue limit. The latest design of CS support structure is very strong, because the no gap is made between CS modules even though total repulsive force of CS is maximum with no pre-compression. Therefore, optimization or simplification of structure design is possible for all magnet systems from the latest design in the detail design phase.

Advancement on the procurement of Power Supply systems for JT-60SA

JT-60SA will be provided with a set of power supply systems procured by Europe and Japan under the framework of Broader Approach Agreement. The toroidal circuit is supplied by an ac/dc thyristors converter rated for 25.7 kA in steady state, and the toroidal superconducting coils are protected by three Quench Protection Circuits (QPC) assuring fast dissipation of the stored magnetic energy of about 1 GJ in case of fault. The poloidal circuits are supplied by ten ac/dc thyristor converters, almost all rated for ±20 kA and ±1 kV; ten QPC rated for the same nominal current and ±3.8 kV assure the protection of the poloidal superconducting coils. The high voltage required for the plasma breakdown is generated in the poloidal circuits by four Booster ac/dc converters and by six Switching Network Units (SNU). The Booster converters are rated +4/-14.5 kA and ±5 kV for short time, thus are inserted in the circuits only when needed and then bypassed. The SNU are operated in order to insert in the circuits settable resistors, producing up to 5 kV at the nominal current of 20 kA, and then to by-pass them after plasma initiation phase. Two in-vessel coils for fast control of plasma position are supplied by two independent ac/dc thyristor converters, rated ±5 kA and ±1 kV, and the in-vessel coils for Resistive Wall Mode control are supplied by 18 fast inverters rated for 300 A and 240 V. After a preliminary definition of the reference schemes and main requirements of the components, the procurement of the Power Supply systems has been started by means of contracts awarded to industrial suppliers including detailed design, manufacturing and test. Besides highlighting the main characteristics of Power Supply systems of JT-60SA as resulting after the detailed design, the paper describes the present status of their procurement: the QPC units have been already installed in Japan for the final acceptance test; the manufacture and factory tests of some SNU have been successfully completed; the detailed design of ac/dc converters for toroidal and poloidal circuits has been finalized, and the first units have been manufactured and are ready for factory testing.

A Conceptual Design Study for the Error Field Correction Coil Power Supply in JT-60SA ⁄

This paper describes a conceptual design study for the circuit configuration of the Error Field Correction Coil (EFCC) power supply (PS) to maximize the expected performance with reasonable cost in JT-60SA. The EFCC consists of eighteen sector coils installed inside the vacuum vessel, six in the toroidal direction and three in the poloidal direction, each one rated for 30 kA-turn. As a result, star point connection is proposed for each group of six EFCC coils installed cyclically in the toroidal direction for decoupling with poloidal field coils. In addition, a six phase inverter which is capable of controlling each phase current was chosen as PS topology to ensure higher flexibility of operation with reasonable cost.