THE POWER-SUPPLY SYSTEM OF RFX

The RFX power supply system is a combination of electrical devices, such as a.c.-d.c. converters, direct current interrupters, large capacitor banks and fast closing switches, whose coordinated operation allows the investigation of plasma in the reverse field pinch (RFP) configuration. The large electrical power, required for the pulses, is derived partly from the 400 kV high voltage Italian grid and partly by the transfer to the plasma of energy stored before the pulse in the capacitor banks or in the machine magnets. The circuit operation phases to obtain RFP configurations are initially described: after premagnetizing the machine and charging the capacitor banks, the plasma current start-up follows (between 5 and 50 ms long) during which the energy stored is transferred to the plasma. Then, by means of the a.c.-d.c. converters, the steady state of the plasma configuration is sustained for a maximum duration of 0.25 s. In the design of the power supply units a few aspects have been taken into account which have a great impact on the technical solutions: the possible fault conditions, which can in a few cases heavily damage the devices; the presence of high electromagnetic interference (EMI) which can induce noises in signals and errors in the circuit operations; the request for easy reconfigurations of the circuits, to cope with different types of plasma investigations. The fault protection led in RFX to the development of an active logic able to command the required protective actions, which have to operate within 1 ms of the fault. The EMI reduction was obtained with a careful analysis of the EMI source, related to the magnetic stray field caused by the machine magnetizing winding and by the operation of converters, capacitor banks, interrupters and ignitrons; screening of the area with control equipment, the signal insulation and a particular solution for the ground system was adopted. Finally, a high level of plant flexibility was reached by adopting a modular solution in the design of the a.c.-d.c. converter system and of the toroidal capacitor bank. In the paper the design criteria of the power supply system units are presented, together with a discussion concerning the main choices; the structures of the power supply units (the a.c. voltage network, the a.c.-d.c. converters, the capacitor bank, the energy transfer and the switching units and the pulse discharge cleaning system) are described in detail. Finally, the results of the commissioning and first operation of the whole power supply system with dummy loads are also reported.