Upgrade of the RFX energy transfer system for a reliable 35 kV, 50 kA DC-current interruption

In the reversed field pinch experiment (RFX), the energy transfer system (ETS) is operated so as to divert the magnetising current into the transfer resistors, in order to ionise the gas and start the plasma current. In the next experimental campaigns, the current in the magnetising circuit will be increased to the design target of 50 kA; this will lead to unbearable conditions for the vacuum interrupters, in terms of specific energy (I2t) that can be dissipated in the closed contacts, before opening; an upgrade of the ETS is therefore necessary. The paper, after giving a survey of possible solutions, discusses the choice made for RFX, which consisted in connecting new vacuum breakers in parallel to the existing ones, so as to increase the interruption capabilities; then, the implementation of the new components is described, in particular with reference to the control and protection upgrade. Finally, the tests carried out on one of the four units are described. The positive results of the tests allowed programming the extension of the modifications to the other units during the experiment shut-down taking place in late summer 2008.