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

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 T-i mode), or an ice pellet is injected into an NBI plasma (pellet injection mode), the central ion temperature T-i(0) increases in time up to 0.7-0.8 keV. These high T-i 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.