Neutron Production and Fast Particle Dynamics in Reversed-Field Pinch Plasmas for FFH

An experimental study of neutron production and particle dynamics in Deuterium plasmas is presented in the RFX-mod device operated in reversed-field pinch (RFP) configuration. In the RFP, the toroidal magnetic field is one order of magnitude smaller than in a tokamak, and is mainly generated by currents flowing in the plasma itself, thus reducing the need for superconducting coils. This feature underlies the main potential advantage as a reactor concept of the RFP, namely the capability of achieving fusion conditions with purely ohmic heating in a much simpler and compact device. The present analysis is mainly based on data collected by means of a couple of neutron and gammaray detectors and of a neutral particle analyzer (NPA). The NPA resolves the energy distribution of the neutral atoms produced by charge-exchange processes, exiting the plasma on the low field side and on the equatorial plane of the machine. Energy and mass dispersion are produced by a combination of electrostatic and magnetic fields. 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 in a noisy magnetic environment. The production of neutrons from D-D fusion reactions and gamma rays in RFP plasmas is found to be determined by the ohmic input power, with a threshold value of about 1.2MA in terms of plasma current. Above this threshold, neutron and gamma fluxes become strong function of the plasma current. Neutron and gamma production dynamics is largely influenced by the MHD activity of the RFP plasmas. In particular, a bursty enhancement of neutron yields, correlated with the spontaneous magnetic reconnection processes, occurs almost cyclically in the RFP plasmas. Magnetic reconnection, which is known to be associated to heating and particle acceleration, indeed is also found to modify the energy distribution of the collected neutral atoms having impact on both the Maxwellian component and on the observed higher energy tail. The spontaneous generation of fast particles is of particular interest in RFP plasmas, as it has been proven by means of external beams that high energy ions exhibit good confinement properties, with a classical behavior and no enhanced radial transport. The possible role of such particles in the destabilization of the observed Alfvén eigenmodes is also discussed. Data from RFP experiments are compared to those obtained in low-current tokamak campaigns also performed in RFX-mod.