Fusion Science and Technology / Volume 58 / Number 1 / July 2010 / Pages 150-159
Chapter 3. Confinement and Transport / Special Issue on Large Helical Device (LHD)
The interlinkage of particle, momentum, and heat transport in plasmas appears as a nondiffusive term of each transport equation. The physical mechanisms determining the diffusive and nondiffusive terms of particle, momentum, and heat transports are described. The nondiffusive term in the particle transport and impurity transport, which causes an inward pinch or outward flux, is driven by the temperature gradient and the magnetic field curvature. One significant piece of evidence of the nondiffusive term of particle transport is observed in the impurity transport as an impurity hole, where the impurity profiles become extremely hollow and the inward flow due to the density gradient is balanced with the outward flow driven by the ion temperature gradients. The outward convection of impurity observed contradicts the neoclassical prediction but is expected to contribute to the purity of plasma in the ion root even if the radial electric field is negative. The nondiffusive term in the momentum transport, which drives spontaneous toroidal rotation, is also observed in the plasmas in the Large Helical Device (LHD). The spontaneous rotations are driven by the electric field near the plasma edge and the ion temperature gradient at the midradius in the plasma. In the heat transport, no clear nondiffusive term is observed, and it is considered to be diffusive. The temperature and temperature gradient dependences of the diffusive terms are studied with the perturbation transport study and the slow transition between two transport branches that have a weak and a strong temperature dependence of thermal diffusivity.