Effect of Rotational Transform on Thermal Transport in Stellarator–Heliotron Plasmas on LHD

Experimental evidence that indicates a positive effect of rotational transform on thermal transport has been shown for electron cyclotron heated plasmas on large helical device (LHD). Although this positive dependence was suggested by earlier scaling studies on energy confinement time, there was a concern that rotational transform is strongly correlated with another major non-dimensional parameter, that is, aspect ratio, in stellarator–heliotron systems. A careful experiment to exclude correlation between these two non-dimensional parameters was carried out on LHD by means of combining helical coil pitch control and limiter insertion. Plasmas with similar aspect ratio but different rotational transform have been compared in terms of global energy confinement time and local heat diffusivity. Energy confinement time increases with the rotational transform. Also the comparison of plasmas dimensionally similar in terms of normalized gyro-radius, collisionality, normalized pressure and aspect ratio has indicated that thermal transport improves with rotational transform. Since the plasmas studied here are dominated by turbulent transport rather than neoclassical transport, the identified feature, common to toroidal plasmas with tokamak, will stimulate the challenge to resolve the origin of the favorable effect of poloidal field and the compatibility with drift turbulence theory.