Overview of RFX-mod results with active MHD control

The paper presents an overview of the main results from the modified RFX since its restart in December 2004. The new machine has a thin Cu shell with vertical field penetration time τS, lowered from 450 to 50 ms and shell/plasma proximity from b/a = 1.24 to 1.1. Toroidal equilibrium is feedback-controlle d and new power supplies provide better B φ control. Newly designed graphite tiles protect the vessel from highly localized power deposition. A mesh of 192 external saddle coils independently controlled by a digital feedback system, controls radial fields due to field errors and MHD modes. A dramatic improvement of plasma performance was obt ained by using the saddle coils to cancel all the r adial field components, an operation mode dubbed Virtual S he l. The toroidal voltage was lowered by more than 10 V and the pulse length was tripled, i.e. up to 7 time s the τS. Steady-state RFP pulses are now limited only by the applied volt-seconds. The improved magnetic boundar y h s also an effect on the tearing modes underlyin g the sustainment of the RFP, the dynamo modes, whose core amplitude is more than halved. This results in a 1 00% increase of the particle and energy confinement tim e relative to the previous experiment. RFX-mod is now a very flexible machine used for a variety of mode control experiments, such as Oscillating Poloidal Current d rive and Quasi Single Helicity studies. One of the most impo rtant results obtained is the feedback stabilizatio n f Resistive Wall Modes. Hence RFX-mod initial operation demonstrated the possibility to operate a large RFP without a thick conducting shell, actually greatly improving its performance in terms of plasma parame ters and confinement and, moreover, opened the possibility t o further develop advanced RFP scenarios.