Overview of the RFX-mod Results

With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and promising perspectives for the Reversed Field Pinch (RFP). A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. We present results, which are relevant for the understanding of the RFP physics and confinement performance, and contribute to the solution of issues for the optimization of ITER construction and operation. RFX-mod is a large ohmically heated RFP (R = 2 m, a = 0.457 m, volume 10 m3), equipped with an advanced system for feedback control of MHD instabilities, based on 192 independently driven coils covering the whole plasma boundary. An axisymmetric magnetic boundary is produced, with differential rotation of tearing modes and full stabilization of multiple RWM. The operation above 1 MA (up to 1.6 MA reported at this conference) has revealed a new self-organised helical equilibrium (the Single Helical Axis - SHAx - state), which is the preferred state at high current. This regime is characterized by strong core electron transport barriers, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor four improvement in confinement time with respect to the standard RFP. The electron temperature shows peak values in the 1 keV range, linearly increasing with plasma current with no degradation of beta. The RFP dominated by magnetic chaos belongs to the past, and the new findings open an exciting and clear path, which deserved to be explored. The achievements in the field of MHD stability real-time control have been of high quality. Progresses in control of tearing modes and RWM have been reported. Besides providing an improved magnetic boundary for the high current operation, the feedback experiments address general issues like multi-mode control, tools for mode unlocking and for forcing mode rotation, implementation of advanced control theory, development of a plant model including features of the plasma, wall, sensors, actuators, field error environment, noise environment and controller dynamics, incorporation of 3-d effects due to a non-axisymmetric wall, benchmarking of numerical stability codes also used for ITER. All these results give strong confidence in the RFX-mod future programme.