The role of power electronics in the advances on plasma performance and control in the RFX-mod experiment

This paper highlights the role of power electronics and digital control in the performance improvements of RFX (Reverse Field eXperiment), a toroidal device for plasma physics studies related to nuclear fusion research. Designed and built in the `80s, it started operation in early `90s, with the mission to explore the physics of Reverse Field Pinch (RFP) configuration at high plasma current, up to 2 MA. Significant enhancements both to the machine assembly and power supply systems were introduced at the beginning of 2000s, when it was clear that active feedback control of plasma MagnetoHydroDynamics (MHD) was crucial for the improvement of plasma performance. The new design was the result of a synergic process of understanding the relevant physics and of a skilful exploitation of the new enhanced performance and flexibility given by a strong use of state of art power electronics and digital controls. Aim of the paper is also to underline the peculiarity of the power electronics applications in fusion experiments, based on an advanced use of industrial components, improved and upgraded to satisfy the unconventional requirements coming from the physics experiments. For this purpose, the paper describes the specific features of the RFX new circuit topologies and the peculiar requirements related to the integrated operation which characterizes them with respect to more conventional applications of similar power. Finally, we emphasize the role of the higher flexibility in controlling the coil currents on the progress of RFX performance in terms of MHD instability control and exploration of new regimes, which strongly improved plasma confinement in the last years.