Characterization of Particle Dynamics and Magnetic Reconnection in the RFX-mod Plasmas

An experimental analysis of particle dynamics is presented in both tokamak and reversed-field pinch (RFP) Deuterium plasmas produced in the RFX-mod device. Particular attention is dedicated to effect of magnetic reconnection processes occurring spontaneously in the plasma. The diagnostics is made of a Neutral Particle Analyzer (NPA) and of a couple of neutron and gamma-ray detectors. The NPA can resolve the energy distribution of the neutral atoms produced by charge-exchange processes and exiting the plasma on the low field side on equatorial plane of the machine on 11 energy channels, being the energy and mass dispersion produced by a E//B combination. Neutron and gamma-ray fluxes are measured by means of 2 scintillators (EJ-301 liquid and NaI(Tl)) coupled to flat-panel photomultipliers, suitable for operation under magnetic fields. The production of neutrons and gamma rays in RFP plasmas is found to be strongly influenced by the Ohmic input power, with a threshold value of about 1.2MA in terms of plasma current level. Below such current value a low rate of gamma rays and almost no neutrons are detected. Neutral Deuterium atoms fluxes, neutron and gamma productions are strongly determined by the MHD activity measured in the plasma. In particular, a bursty behavior of neutron yields, correlated to the spontaneous magnetic reconnection events, occurring almost cyclically in the RFP plasmas are observed. Magnetic reconnection also modifies the energy distribution of neutral atoms having impact on both the Maxwellian component and on the observed higher energy tail. This is true also in tokamak plasmas during the sawtoothing activity mostly seen in low q(a) condition. The role of particle acceleration and ion heating during such events is discussed.