Overview of RFX-mod results with active MHD control

Plasma experiments resumed in December 2004 on RFX-mod. The machine now has a thin (3mm) Cu shell with one overlapped poloidal gap and one toroidal gap. Shell penetration time for Bv has been lowered from 450 to 50 ms and shell/plasma proximity from b/a=1.24 to 1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide a better control of the toroidal field. Newly designed graphite tiles protect the vessel from highly localized power deposition. The MHD Control System, MHD-CS, a set of 192 external saddle coils controlled by a digital feedback system, is used to control radial fields due to field errors, MHD modes and Resistive Wall Modes (RWMs). A dramatic improvement of plasma performance was obtained by using the MHD-CS to cancel all of the radial field components, an operational mode dubbed Virtual Shell (VS). The toroidal loop voltage was lowered by more than 40% and the plasma pulse duration tripled. In practice, steady state RFP pulses are now limited only by the applied volt-seconds. Hence RFX-mod initial operation demonstrated the possibility to operate a large RFP without a thick conducting shell, and opened enhanced RFP scenarios. Indeed the improved magnetic boundary in VS mode, which mimics an ideal closely fitting shell, has an effect on the tearing modes underlying the sustainment of the RFP configuration, the so-called dynamo modes, which are also responsible for field line stochastization in the plasma core and confinement limitation. With the VS the amplitude of such modes in the plasma centre was nearly halved. As expected, this led to improved particle and energy confinement. For instance, peak electron temperature in reference pulses at 600 kA was increased from 200 to 300 eV with more peaked profiles, which corresponds to a reduction of the thermal conductivity by a factor 2 in the region r/a < 0.9. The MHD-CS is extremely flexible and can be used for a variety of mode control experiments. The most important result already obtained was the demonstration of the active control of RWMs. We found that full VS control completely inhibits the growth of RWMs, whereas such modes are indeed seen to grow in agreement with the theoretical prediction if the MHD-CS operated in Selective VS mode, i.e. leaving one or more mode helicity uncontrolled.