Adaptive plasma current control in RFX-mod

Plasma current control was experimented in the RFX machine in early nineties with unsatisfactory results due to the frequent onset of unstable regimes leading to increasing voltage demand and saturation of the flat-top power supply. More recently, taking advantage of the new operating conditions disclosed by the active control system of MHD modes and the flexibility of digital systems, the plasma current control has appeared feasible again. A design of the controller according to adaptive control techniques is presented along with the definition of transition rules through the different phases of the pulse. A zero-dimensional model has been used for the design of a regulator for flat-top operation. The digital implementation has allowed a flexible control strategy depending on the actual values of selected plasma parameters. In particular, a real-time estimation of the plasma resistance has been used to drive the control system through different regulators in a stable transfer mode. The paper presents, in addition, the results of experimental campaigns which have been carried out using the new plasma current controller.