Transport and Confinement Studies in RFX-mod Reversed-Field Pinch Experiment

In reversed-field pinch (RFP) devices the magnetic configuration is characterized by a toroidal magnetic field whose direction in the plasma edge is opposite that in the core. With an edge safety factor qa? - 0.02 ( q = aBt/RBp), the toroidal magnetic field in the RFP is much smaller than in the Tokamak and stability is ensured by the large magnetic shear (r/q dq/dr). Since in the RFP the safety factor is everywhere less than one, a large number of MHD modes are resonant and their amplitude largely affect the confinement. In RFX, as usual in RFP devices, MHD modes amplitude had been controlled by the use of a passive thick conductive shell that vanish the radial magnetic field at the surface. In RFX-mod a different approach has been followed: radial magnetic field control is obtained by external magnetic field coils and a close fitting thin conductive shell. In the so-called Virtual Shell operation (VS), radial field zeroing at the thin shell radius is stationary provided by the externally controlled coils. First experiments of RFX-mod proved the capability of the active scheme to control the radial magnetic field at any time. Furthermore it has been found that such edge magnetic field control extends its beneficial effects to the whole plasma, leading to a stationary 2 to 3-fold reduction of the core Br field amplitude. The reduction of field fluctuations positively reflects on confinement. In fact, a strong reduction of the loop voltage is observed and correspondingly a 3-fold increase in pulse length is achieved by using the same poloidal flux swing. Temperature and particle measurements confirm the improved confinement properties of the virtual shell operation. With a lower ohmic input power, higher electron temperature in the plasma core and a steeper profile is measured. Particle and heat transport have been studied by means of a 1-d code. Local power balance was used to compute the heat conductivity profile: for the VS discharges a lower conductivity over a significant region of the plasma is found. The improved properties of RFX-mod VS operation provide a better confinement scaling both in terms of plasma current and density. The results show that compared to the thick shell solution, a significant confinement improvement can be obtained under stationary conditions by actively controlling the plasma magnetic boundary.