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JT-60SA is a superconducting tokamak in the framework of the broader approach agreement between Europe and Japan. For this international project among its various procurements, the Italian National Agency for new technology energy and sustainable economic development (ENEA) is providing four ac/dc converters for the central solenoid (CS) superconducting magnets, two ac/dc converters for the equilibrium field (EF) superconducting magnets, and two ac/dc converters for the fast plasma position control coils (FPPCC). Furthermore, ENEA is also providing six converter transformers of which four are of the dry type for FPPCC power supplies (PSs) and two of oil type for CS PSs. CS/EF PSs feed poloidal superconducting magnets of JT-60SA, whose purpose is the generation of strong magnetic fields able to induce and confine the plasma current in a prefixed geometrical configuration. The control system of CS/EF PSs has to be able to reproduce in real-time a reference current or voltage profile, known as “scenario” and these are particularly critical in these systems for the high currents required and for the high magnetic energy stored in the superconducting magnets. All performed type tests, carried out in accordance to the IEC60146 Standards, pointed toward a good dynamic behavior of the control system both for forward and for reverse rectifier bridges, assuring the smooth transitions between different operating modes: single mode, circulating current mode and dual mode. This is the focus of this paper. [ABSTRACT FROM AUTHOR]

The superconducting toroidal field (TF) magnets of JT-60SA are designed to conduct a 25.7 kA current to generate a steady magnetic field for plasma confinement. In case of loss of their superconductive status, a fast discharge of the current is operated and the TF magnets are subjected to electromagnetic transient phenomena in a range of frequencies up to several tens of kHz. The TF coils show an involved frequency-dependent behavior as a consequence of eddy currents induced in the casing, stray capacitances, as well as electrical skin-effect in the jacket enclosing the strands; therefore, electrical models derived considering the coil characteristics at a single frequency may not be suitable to perform reliable analyses of the fast discharge event. A broadband electromagnetic model was generated and validated against experimental data. Such a model can reproduce with sufficient accuracy the behavior of the TF magnets in the frequency range of interest. This model was employed to simulate the transient of the voltage distribution inside the coils during a fast discharge and to evaluate the maximum voltage difference between adjacent turns. The model was also employed to make predictions about the detectability of internal short-circuit in a coil by means of impedance spectrum measurements. It was possible to conclude that the highest sensitivity to short circuits can be achieved looking for impedance variations in proximity of the antiresonant frequency. [ABSTRACT FROM PUBLISHER]

This paper describes the detailed design of the quench protection circuits (QPCs) for the superconducting toroidal field (TF) and poloidal field (PF) magnets of the satellite tokamak JT-60SA, which will be installed in Naka, Japan. 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 the 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. [ABSTRACT FROM PUBLISHER]

This paper presents the development and testing of the prototype of a hybrid mechanical–static dc circuit breaker (CB) for quench protection of superconducting magnets based on a mechanical switch paralleled to a static CB. In normal operation, the mechanical switch is closed and it handles the continuous flow of dc current, minimizing the on-state losses. In case of intervention, the mechanical switch opens, commutating the current into the static CB that quickly interrupts and transfers the current into a discharge resistor, withstanding the reapplied voltage. This paper was carried out to face and solve some issues of this design solution being not available in the literature examples of similar applications rated for significant power levels. The operation of the hybrid CB has been successfully tested up to a current of 10 kA with a recovery voltage of about 1 kV. Special tests have been dedicated to characterize the reliability and the repeatability of the current commutation from the mechanical switch to the static CB and to characterize the arc voltage under different conditions. [ABSTRACT FROM AUTHOR]

This paper describes the feasibility studies of a hybrid dc circuit breaker (CB) for quench protection of superconducting magnets rated for current up to some tens of kiloamperes (kA) and voltage of several kilovolts (kV). The proposed scheme is based on a mechanical switch paralleled to a fully controlled static CB; integrated gate commutated thyristors (IGCTs) were selected. In normal operation, the mechanical switch handles the continuous flow of dc current minimizing the on-state losses. When protection against quench is requested, the current is diverted into the static breaker that quickly interrupts the current, withstanding the reapplied voltage of several kV. The reliable turn-ON of several IGCTs in parallel with low voltage applied between anode and cathode is an issue to be assessed to confirm the feasibility of this design solution. The paper highlights that the components commonly used as snubber or protection circuit can guarantee the turn-ON of all the devices in parallel and that the IGCTs can be turned-ON with few volts applied; this latter aspect was proved by means of specific tests. Finally, a comparison between a static only and a hybrid CB is performed, highlighting the benefits introduced by the hybrid solution. [ABSTRACT FROM AUTHOR]

Abstract: In JT-60SA, the new satellite tokamak to be built in Naka, Japan, the magnetic field for plasma generation and confinement is produced by a number of superconducting coils whose rated current ranges from 20kA to 25.7kA, with a nominal voltage up to 5kV. This paper deals with the conceptual design of the Quench Protection Circuits (QPC) of JT-60SA, which must provide a fast removal of the energy stored in the superconducting coils in case of quench. The core of the QPC units is based on a dc current breaker, which diverts the coil current into a discharge resistor. Different concepts of dc circuit breaker for QPC were considered, including vacuum and static circuit breakers; then, a hybrid solution, composed of a mechanical bypass switch paralleled to a static switch based on Integrated Gate Commutated Thyristor (IGCT) technology, has been worked out. A pyrobreaker is added in series to the hybrid switch as a backup protection. The resulting design allows benefiting from the fast breaking and very low maintenance requirement of the static switches, besides maintaining the advantage of the much lower power losses of the mechanical bypass in normal operation. After a brief presentation of the preliminary studies, the selected design solution for JT-60SA is presented in detail. [Copyright &y& Elsevier]

Supervisory control system for superconducting magnet power supplies in JT-60SA is designed and developed. This system is in charge of many kinds of control; discharge sequence control, real-time magnet current control, data acquisition, machine protection and human safety. In the design, communication interface has to be optimized according to all requirements. The most important feature is real-time control with 4 kHz which is required for equilibrium control of plasma. In order to satisfy this requirement, reflective memory (RFM) network is applied for high speed data sharing with multiple controllers. This paper describes the architecture of the control system and the operation of the power supply system under the supervision. [ABSTRACT FROM AUTHOR]