Influence of magnetic configuration on edge turbulence and transport in the H-1 Heliac

Abstract

The role of the rotational transform (ι) profile on fluctuations and transport is investigated in the H-1 Heliac by means of dynamic (i.e. changing during a shot) and static (fixed during a shot) scans of rotational transform through a range of values where the electron density drops markedly and which correspond to having the point of ℓ located near r/a = 0.75 in a region of magnetic well (such that the surface averaged magnetic field strength increases with radius). The gap is near the ℓ = 4/3 resonance, but as the resonance is not in the plasma for more than half the gap it is not clear that this is relevant. Although this drop is clearly driven by the variation of helical current, under particular circumstances, similar density changes occur spontaneously. Plasma currents are measured throughout the scan and are found to slightly affect the rotational transform profile, and reverse about the configuration of minimum confinement, while induced currents through a toroidal loop voltage in the dynamical scans are not found to be significant. The confinement and fluctuation properties are studied by means of 2D movable Langmuir probes. Large near edge-localised dithering quasi-coherent fluctuations at ∼ 6 kHz develop in a strong density gradient region with low magnetic shear as ι is scanned up to a point where the density collapses in the outer region. This dithering corresponds to an m = 3 mode comprising of standing and propagating components. The net and fluctuation-induced transport components are measured near the plasma edge in a similar discharge, and it is found that fluctuation-induced transport driven by these low frequency coherent modes dominates the particle balance during the low density phase but is only a small component of the net flux when the density is higher. Show more