Density profile and particle transport of NBI-heated plasmas in the low-aspect-ratio heliotron/torsatron CHS

Transport phenomenon is the critical issue in a low-aspect-ratio helical device like CHS against its favorable MHD characteristics. Much effort has been made to evaluate local Transport coefficients experimentally in CHS for neutral beam heated plasmas. The electron thermal diffusivity was found to be higher than the neoclassical prediction in the plateau regime by a factor of 20 to 50. The ion thermal diffusivity had less anomaly but had the same order of magnitude (χe ∼ χi ∼ 5-10 m2/s at 1 Tesla). The parallel viscosity was studied experimentally by observing toroidal rotation profile, in which necessity of introducing the anomalous perpendicular viscosity (momentum diffusivity) of the order of the thermal diffusivity was pointed out. The particle diffusivity so far has not been evaluated mainly due to a difficulty in the determination of local particle sources. Especially in the magnetic configuration with an inward-shifted magnetic axis, where global confinement time was optimum in CHS, the plasma was wall-limited and strong recycling occurred on the inboard wall. On the other hand, a peaked density profile was observed in the inward-shifted magnetic axis, while the density profile flattened in outward-shifted one. Two explanations for the density peaking are possible: The particle diffusivity is reduced with an inward-shifted magnetic axis. The inward flow velocity is increased with an inward-shifted magnetic axis. In order to clarify this point, it is necessary to study particle transport by determining the particle diffusivity and the convective velocity separately. The first effort along the line is described in this paper, where the relationship between the density profiles and the local particle transport is examined for the steady state neutral beam heated plasma in a plateau regime based on new experimental observation of neutral hydrogen density. (author) 5 refs., 2 figs