Energy Transport in Tokamak Plasmas with Central Current Density Control Using Fast Waves

Fast wave current drive has been used to substantially modify the central current density profile in a tokamak plasma. Counter-fast wave current drive (FWCD) applied to discharges with negative central magnetic shear enhances the shear reversal and leads to a distinct transition to a mode of improved core confinement. In this state, the electron thermal diffusivity decreases by $(50\pm 20)\%$ and the ion diffusivity by $(80\pm 20)\%$, compared to just before the transition. The FWCD and electron heating elucidates the role of the current profile on confinement and stability.