Effect of radial electric field and bulk plasma velocity shear on ion thermal transport in Heliotron-E

Abstract

The effect of radial electric field and velocity shear on thermal transport is studied in Heliotron-E. When neutral beams are injected into the target plasma produced by electron cyclotron heating (high mode), or an ice pellet is injected into an NBI plasma (pellet injection mode), the central ion temperature increases in time up to 0.7 - 0.8 keV. These high plasmas are characterized by a peaked ion temperature profile and are associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling, or by pellet fuelling. The global energy confinement is improved compared with L-mode plasmas by a factor of 1.4 for the same line-averaged electron density. The observed improvement in ion heat transport is related to the radial electric field shear rather than to the rotation velocity shear in the bulk plasma.