Inward turbulent particle transport in a helical plasma heated by neutral beam injection

In neutral-beam-injection (NBI) heated plasmas of the Compact Helical System (CHS) heliotron/torsatron [K. Matsuoka et al., Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441], inward turbulent particle flux is observed at a particular radial location near the plasma edge. The observed reversal of the particle flux is connected to the change in the cross-phase between electron density and poloidal electric field fluctuations. This phenomenon is observed in the parameter regime of low collisionality at the electron temperature Te>15 eV, electron density ne<3×1018 m−3, which approximately corresponds to the effective collision frequency normalized by the transit frequency of a circulating particle ν*≲1. The large inward particle flux (≳1×1020 m−2 s−1) correlates with the positive shear of the radial electric field Er′ (≳1×106 V m−2), but does not have clear correlation with the radial electric field Er and the curvature Er″.