Search

The occurrence of improved confinement regime, H - mode, in magnetic plasma confinement devices is characterized by the creation of an edge transport barrier and a reduction in the turbulence level s. Such reduction is usually associated with a massive shear in plasma flow at the edge, impacting on dynamic and features of blobs or filament. These structures emerging from turbulence background in the SOL are responsible for a huge loss of particles an d energy . Edge Localized Modes (ELMs) large filaments often appear at the plasma edge during the H - mode and are related to a quasi - periodic relaxation of the transport barrier. Although much information about ELMs has been gathered in the last few years, e nabling even the successful mitigation of them, their underlying physical characteristics ( i.g. their electromagnetic behaviour in different scales) are, in a certain sense, still obscure. Filament behaviour has been reported in the ELMs, bringing an inter esting parallel with the turbulent structures in the L - mode . In this contribution w e have been studying transition s to H - mode induced by electrode biasing at the edge of the RFX - mod experiment operating in tokamak configuration. The electromagnetic fluctua tion s and the equilibrium of the plasma parameters at edge were measured by insertable probe head, U - probe, before and during the biasing phase. They have been addressed to determine in which conditions the H mode is reached in the RFX (i.g. if there are i ntermediate phases ) . The different st ructures occurring during the H - Mode phase have been studied, including ELM and the inter ELM phase . Advanced statistical methods have been applied to simultaneous analysis of different plasma parameters such as the electron temperature , the electron density and the vorticity, as well as magnetic signals measured by a set of pick - up coils installed in the probe head . A detailed comparison of the electromagnetic features of the filaments occurre nce before and during the H - mode has been done, with a special attention to the electromagnetic ELM fine structure , through the analysis in different scales.

In a Reversed Field Pinch discharge (RFP), magnetic reconnection events are related to selforganized processes[1]. Along with the action of the continuous dynamo process, discrete quasicyclic relaxation events, leading to rearrangement of magnetic field topology through magnetic reconnection phenomena, have been seen in RFP plasmas, with a dynamics resembling that of sawtooth in tokamak plasmas. In RFX-mod (R = 2m,a = 0.459m), it was shown that a relaxation event is associated with the rapid formation of a localized magnetic perturbation characterized by a main m=0 periodicity [2], resonant on the reversal surface where q = 0. This perturbation corresponds to the formation of a poloidal current sheet. The m=0 activity starts in a localized region around a given toroidal position and then moves with the plasma in a direction opposite to the toroidal current. In this contribution, a characterization of the m=0 perturbations, through the use of a statistical method for the detection of intermittent structures, is presented. Data analysis here reported refers to the toroidal magnetic field perturbations collected by ISIS (Integrated System of Internal Sensors [3]), whose magnetic sensors are placed just below the graphite first-wall (r = 1.03a). A statistical analysis on the magnetic field intermittent structures suggests that current sheets, associated with magnetic reconnection phenomena, are subject to a fragmentation process. This result is in agreement with what predicted by simulations presented in the literature [4], which highlight two important aspects: a current sheet could be naturally subject to fragmentation and a fragmented current sheet represents a more efficient particle accelerator with respect to a monolithic one, because it offers multiple particle acceleration sites [5]. Indeed, we found a relationship between relaxation events in RFP discharges and the neutron fluxes due to DD fusion reactions, suggesting the coupling among relaxation events, particle acceleration and magnetic reconnection phenomena.