Current sheet fragmentation following magnetic reconnection in RFP plasmas

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.