The electron internal transport barrier (ITB) is formed by an edge perturbation induced by a tracer encapsulated solid pellet injection in a net-current free LHD plasma. The ITB trigger by an edge perturbation takes place in the low density regime and the ITB is not accompanied by a significant change in the density. At the onset of the core temperature rise, there is a strong correlation between the heat flux in the central region and the temperature gradient at a distance half of the plasma radius away. A radial temperature profile with a concave curvature abruptly appears in the growing phase of temperature gradient. Turbulence suppression mechanisms via flow and magnetic shears are not sufficient to explain these features.
Abrupt reduction of core electron heat transport in response to edge cooling on the Large Helical Device