The phenomenology of fast reconnection events in the reversed field pinch

Reconnection is a basic phenomenon in magnetized plasmas by which the plasma relaxes to a state oflower magnetic energy. It plays a fundamental role in a variety of instabilities common to most fusiondevices, as sawtooth or tearing modes. Often it occurs in impulsive events (crash phase), following alonger period in which the magnetic fields evolve slowly. Energy conversion and topological change are two highly linked aspects of reconnection.Hot fusion plasmas may contain multiple reconnection sites and this is particularly true for the Reversed Field Pinch (RFP), due to the peculiar current density distribution, associated to a variety ofresonant tearing modes. In addition, large RFPs involve large plasma currents and therefore a largeamount of available magnetic energy. This makes the RFP a good device to observe and analyze reconnection events.In this contribution the phenomenology of the impulsive and periodic reconnections in the RFX-mod device is analyzed in detail, through an ensemble average of the data from many reconnection eventsin high plasma current discharges (1.5 MA). Spatial and temporal scales are invoked as fundamental observables. Roughly speaking, an almost stationary helical state is considered typical of phases before the crash. The crash destroys this symmetry, and the plasma topology undergoes a transition to a more chaotic state with broad MHD spectra. The crash evolution is described as a sequence of events. A trigger of the crash has been associated for the first time to a slight change in the helical equilibrium, which pushes closer the inner most important resonant flux surfaces in the core. Reconnection starts when the resonant surfaces – and therefore the intense related current sheets- are close enough and a precursor of the crash is found in the locking in phase of these resonant modes. After these slowly evolving phases, fast reconnection takes place, which lasts about 1 ms.Plasma parameters and magnetic measurements suggest that crashes are related with the interaction between tearing modes at different radii, and the reconnection process evolves according to the safety factor profile.