Katsuhiro Shimada

Application of parallel connected power-MOSFET elements to high current d.c. power supply

The low aspect ratio spherical torus (ST), which has single turn toroidal field coil, requires the extremely high d.c. current like as 20 MA to energize the coil. Considering the ratings of such extremely high current and low voltage, power-MOSFET element is employed as the switching device for the a.c./d.c. converter of power supply. One of the advantages of power-MOSFET element is low on-state resistance, which is to meet the high current and low voltage operation. Recently, the capacity of power-MOSFET element has been increased and its on-state resistance has been decreased, so that the possibility of construction of high current and low voltage a.c./d.c. converter with parallel connected power-MOSFET elements has been growing. With the aim of developing the high current d.c. power supply using power-MOSFET, the basic characteristics of parallel operation with power-MOSFET elements are experimentally investigated. And, the synchronous rectifier type and the bi-directional self commutated type a.c./d.c. converters using parallel connected power-MOSFET elements are proposed.

Low voltage, high current DC power supply with self commutated converter using power-MOSFET

The DC power supply for a nuclear fusion experimental system based on the low aspect ratio spherical torus requires a low voltage and high current output to power a toroidal field coil consisting of a single turn center conductor. To meet such requirements, the self-commutated converter using a power-MOSFET, which is more efficient than the conventional thyristor or diode converter, is proposed. The converter has the advantage of fast switching speed, low on-state resistance and voltage gate signal. The operating characteristics of a power-MOSFET and the switching unit using four MOSFET elements are also discussed.

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.

Detailed Analysis of the Transient Voltage in a JT-60SA PF Coil Circuit

A superconducting coil system is actually complicated by the distributed parameters, e.g. the distributed mutual inductance among turns and the distributed capacitance between adjacent conductors. In this paper, such a complicated system was modeled with a reasonably simplified circuit network with lumped parameters. Then, a detailed circuit analysis was conducted to evaluate the possible voltage transient in the coil circuit. As a result, an appropriate (minimum) snubber capacitance for the Switching Network Unit, which is a fast high voltage generation circuit in JT-60SA, was obtained.

Measurement of high frequency impedance of ITER CS Model Coil and the simulation analysis of internal resonance phenomenon

This paper describes the internal resonance phenomenon of a large scale superconducting coil. The frequency dependence of electric impedance defined at the coil terminal and the voltage distribution of the layer winding were measured at the ITER CS model coil under the superconducting state in advance of the pulse operation test. As a result, large change of the impedance and unbalance of the layer voltage were observed. The AC circuit analysis was carried out in order to evaluate the measurement results and to calculate the voltage of layer insulator. It was concluded that the higher voltage, which may appear on the layer insulation, is about a half of the coil terminal voltage in high frequency.

Advanced application of power-MOSFET to large capacity converter

Advanced application of power-MOSFET to a large capacity novel converter system is studied. Parallel operation of many power-MOSFET elements is investigated in room and liquid nitrogen temperatures. The total on-state resistance shows lowered value in parallel connected and cooled down conditions, and the reduced resistance is effective for operational loss reduction of the converter circuit. The bi-directional switching unit with four power-MOSFET elements is proposed, which can be applied to the self commutated type AC/DC converter. Another type of AC/DC converter using power-MOSFET consists of a three phase synchronous rectifier circuit, which is suitable to output one directional high DC current.

Design and analysis of plasma position and shape control in superconducting tokamak JT-60SC

Abstract The analyses of the plasma position and shape control in the superconducting tokamak JT-60SC in JAERI are presented. The vacuum vessel and stabilizing plates located closely to the plasma are modeled in 3 dimension, and we can take into account the large ports in the vacuum vessel. The linear numerical model used in the design for the plasma feedback control system is based on Grad–Shafranov equation, which allows the plasma surface deformation. For a slower control of the plasma shape, the superconducting equilibrium field (EF) coils outside toroidal field coils are used, while for a fast control of the plasma position, in-vessel normal conducting coils (IV coil) are used. It is shown that the available loop voltages of the EF and IV coils are very limited, but there are sufficient accuracy and acceptable response time of plasma position and shape control.

Resonance Characteristics and Maximum Turn Voltage of JT-60SA EF Coil

The withstand voltage of the turn insulation is one of the most essential issues for the superconducting magnet system. There is a possibility that the actual differential voltage between each turns, called turn voltage in this paper, is larger than the maximum turn voltage under the ideal condition because of the voltage fluctuations of the power supply and the inhomogeneous voltage distribution in the magnet induced by the resonance phenomenon. The actual turn voltage was evaluated by the voltage measurements and the circuit simulation. The actual voltage of the large power supply, JT-60U power supply, was measured. The results indicated that the actual maximum voltage increase the 20% from rated voltage of the power supply. The margin of the insulation voltage is generally much larger than 20%. This amplitude of the voltage fluctuations is permissible for the design of the turn insulation. The resonance characteristic test was performed using the copper dummy pancake. The test results suggested that the resonance frequency of the EF4 pancake was about 56 kHz. The resonance characteristics of the real coil was also evaluated by the numerical method based on the lumped parameter circuit model. The resonance frequencies of the EF4 coil were calculated as 180 kHz, 300 kHz and the several higher frequencies. The main frequency component of the power supply is less than 5 kHz. The influence of the resonance phenomenon on the maximum turn voltage is negligible small.

Effects of Tensile and Compressive Strain on Critical Currents of

Effects of tensile and compressive strain on critical currents (I c) of Nb3Al strand and cable-in-conduit (CIC) conductor were investigated using a newly developed apparatus. The Nb 3Al strand manufactured by jelly roll process was 0.74 mm in diameter with a copper/noncopper ratio of 4.05. The CIC conductor sample which consists of two Nb3Al strands and one copper wire inserted into a stainless steel conduit was prepared. The effective strain and magnetic field ranges for the CIC conductor sample were from -0.91% to +0.26% and from 6 T to 11 T, respectively. It is clearly shown that the Ic decrease of the CIC conductor sample by longitudinal strain is relieved considerably compared with the strand sample, indicating the relaxation of intrinsic strain in Nb3Al filaments in CIC conductor due to the cabling effect. The relaxation effect is about +0.1% for -0.8% in effective strain of CIC conductor