Speaker
Description
One of the main objectives of JT-60SA, the satellite tokamak under construction in Naka (Japan), is to confine steady-state high-beta plasmas. To reach the desired plasma performance, the control of the instabilities called Resistive Wall Modes (RWMs) is crucial. At this purpose, besides the combination of an in-vessel passive structure (stabilizing plate) and plasma rotation, a dedicated active control system based on 18 in-vessel sector coils has been devised. These coils are placed just behind the tiles of the first wall and around the ports; therefore, due to the plasma proximity and the low shielding effect of the surrounding passive structures, they can efficiently generate fast magnetic fields if properly driven. The strategy is to succeed in controlling RWMs (which grow exponentially) when their amplitude is still low, so that low magnetic field components and relevant current to produce them are sufficient. As a consequence, the power rating of the power supply system is not so demanding, but at the price of high dynamic performance. To achieve an effective control, each coil will be independently fed by a dedicated inverter, rated for 300 A, 240 V, with a closed-loop bandwidth of 3 kHz and a latency between reference and output voltage lower than 50 µs. These performances exceed those of standard industrial products and call for the adoption of innovative solutions. To exploit the simple and compact H-bridge topology, a switching frequency higher than that sustainable by standard silicon IGBTs is necessary; the adoption of new power semiconductors based on Silicon-Carbide (SiC), capable to switch at 30 kHz (60 kHz equivalent at the output), and a very fast control system have been the key design choices to satisfy the requirements. To prove the feasibility at reasonable cost of this design, an inverter prototype has been developed in 2014, whose performance was very satisfactory. Therefore, the same inverter design has been confirmed also for the final power supply system (called RWM-PS), which is now under manufacturing. This will be composed of an ac disconnector, a step-down transformer, two ac/dc rectifiers, a distributed dc-link capacitor bank, 18 water-cooled fast inverters and a Local Control Cubicle. Both differential-mode and common-mode filters are foreseen at the inverter outputs; their main aims are to reduce the current ripple and minimize the electro-magnetic interferences due to the very long coaxial cables connecting the fast inverters to the coils. The control system supervises all the plant. Each inverter can be individually controlled in current control loop or in open-loop, the reference being arbitrary and generated by the JT-60SA MHD Controller.
The RWM-PS will be the first power supply system for fast control of plasma instabilities in fusion experiments adopting SiC semiconductors. The paper will give an overview of the final design of the RWM-PS, with particular emphasis on its special features and the solutions adopted to satisfy the critical requirements. The interface issues for an effective integration with the JT-60SA power and control systems will be also treated.
| Eligible for student paper award? | No |
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