Experimental particle transport studies by pellet injection in helical equilibria

Abstract

Helical equilibria have been experimentally found in the RFX-mod reversed field pinch both as spontaneous and induced states. These states are associated with a dominant helicity in the spectrum of magnetic field fluctuations producing a helical deformation of magnetic surfaces in a large fraction of the plasma core. In particular, they are also characterized by an improvement in global plasma performances in terms of electron temperature and energy confinement time with respect to the standard axisymmetric configuration with multiple helicities in the magnetic spectrum. These observations suggest that one should also expect an improvement in terms of particle confinement although this has never been experimentally observed due to the lack of a particle source in the core of RFX-mod plasmas. To address this point perturbative experiments were done in RFX-mod by injecting pellets (a known particle source) both inside and outside the helical magnetic structure present during such states. These experiments show that plasma density asymmetries and variations in the ablation rate of pellets are correlated with the internal magnetic field structure obtained by means of a line tracing code based on an equilibrium reconstructed in toroidal geometry using external measurements. In particular, they prove that particle transport is significantly reduced in the helical states with respect to axisymmetric configurations. An increase by a factor 2–3 was determined for the global electron particle confinement time calculated with a zero-dimensional model taking into account the helical geometry.