Enhancement of the power supply systems in RFX-mod towards 2 MA plasma current

The recent start-up improvements brought to RFX-mod have allowed increasing the plasma current in the range 1.7-1.9 MA, but still they proved to be not sufficient to routinely and reliably reach higher values of plasma current that would be very useful for the present studies on enhanced confinement. The need for higher poloidal flux variation together with the overabundant available toroidal flux has led to the study of a rearrangement of the power supply system with the aim of increasing the first at the expenses of the latter. Thanks to the flexibility of the power supply system, composed of modular thyristor converters, the rearrangement has been easily obtained and the resulting enhanced poloidal flux capability scenario described in the paper allows a two step plasma current ramp-up. In order to increase as much as possible the plasma current up to the nominal value of 2 MA it is necessary to fully exploit the thyristor converter power capability, in particular in the first ramp-up phase. In the paper a detailed verification of their maximum performance is presented. The first results obtained after power supply reconfiguration are very good, with plasma current kept constant at about 2 MA for 100 ms. The enabled operation at very high plasma current requires also an optimization of the magneto hydro dynamics (MHD) mode coil power supplies that are required to provide higher current values. Therefore a dedicated study of their control system has been worked out, which allowed understanding how to increase as much as possible the output current without losing the dynamic performance, so keeping the efficiency in the control of dominant and secondary modes, which is essential to obtain good and reproducible discharges.