Role of turbulent transport in plasma confinement in the H-1 heliac

Experimental studies in turbulence and fluctuation-driven transport in magnetically confined plasmas are regaining their central positions in the plasma confinement physics. This is primarily due to the role the turbulent transport seems to play during formation of the edge and internal transport barriers. The formation of the transport barriers, followed by transitions to improved plasma confinement regimes, nearly always correlates with the turbulence modifications. A model of the turbulence suppression by sheared ExB flows remains one of the most likely candidates to explain the turbulence reduction at the plasma edge during low-to-high (L-H) confinement transitions. After the discovery of the improved confinement in the discharges with negative magnetic shear, characterised by the formation of the internal (r/a < 0.5) transport barriers, numerous attempts have been made to explain this effect using a slightly modified ExB shear suppression model. However, due to apparent difficulties in the experimental characterisation of the fluctuation-driven transport at the internal transport barriers, the results are somewhat contradictory. In some experiments, the turbulence level seems to reduce across the transition, while some observations show no significant reduction in the fluctuation amplitude. Considering that the fluctuation-produced flux is proportional not only to the density fluctuation level (the quantity most often used to experimentally characterise the turbulence), the conclusion about the fluctuation suppression by ExB sheared flows seems to be a bit premature and requires further experimental tests. In this paper we address several questions with regard to modifications in the fluctuation-driven particle flux across the confining magnetic field in the H-1 heliac. It has been shown, that confinement improvement across the L-H transitions in H-1 is correlated with the suppression of the fluctuations and associated with them radial outward-directed particle flux. However, it has been discovered later, that it is not only the level of the turbulent particle flux, but also its direction (from outward to inward) that may change across transitions to improved confinement