Tokamak experiments in RFX-mod with polarized insertable electrode

The RFX-mod device (R=2m, a=0.459m) can be operated as an ohmic tokamak with a toroidal field up to 0.55T for 1 sec. Thanks to its MHD control system and to the close fitting vacuum vessel, it can systematically operate circular cross section discharges up to very low q, q(a)<2, regimes [1]. Moreover, taking advantage of its flexible system of power supplies and field shaping windings, discharges with non-circular cross section have been realized [2, 3]. Recently, an insertable polarized graphite electrode made has been implemented in order to act on the edge electric field and investigate transitions to enhanced confinement regimes comparing different scenarios achievable in RFX-mod, namely Single Null q95 ?3, circular discharges with q(a) ?3 and q(a)<2. The required current to the electrode was provided by one of the RFX-mod power supply unit. The electrode graphite head has been monitored by a fast thermal camera and a safety algorithm has been integrated into the real-time RFX-mod control system. Furthermore, a density control feedback loop, based on real-time measurements of diamagnetic flux and double filter SXR electron temperature, has been implemented. No transitions to high confinement have been observed so far when the electrode is positively polarized with respect to the first wall, while several signatures of H-mode occur with negative polarization: sudden reduction of D? signals; sudden increase of electrode voltage Vel and decrease of electrode current Iel; increase of plasma density even without puffing; increase of stored energy; steepening of edge density (as measured with insertable probes and microwave reflectometer) and increase of the edge radial electric field; reduction of magnetic and electrostatic fluctuation level. LH transitions occur if electron density and polarization voltage exceed threshold values. Negative polarization corresponds to an increase of the natural floating potential. Ion toroidal flow, measured by passive spectroscopy, is affected by the electrode polarization consistently with the ExB sign. Transition phases can be characterized by a dithering behavior whose duration is related to the rate of change of the voltage waveform. During the dithering phases, D?, Vel and Iel signals oscillate between the low and high confinement values consistently with fast sequences of L-H transitions and back-transitions. These strong fluctuations appear associated to the onset of MHD activity, whose behavior as well as the filamentary structures in the fast turbulence electromagnetic fluctuations will be analyzed and discussed in the paper.