STATIONARY MAGNETIC SHEAR REVERSAL EXPERIMENTS IN TORE-SUPRA

Stable and stationary states with hollow current density profiles have been achieved in Tore Supra with lower hybrid current drive (LHCD) du ring reduced toroidal magnetic field operation (B-t less than or equal to 2 T) and in weak LH absorption regimes. For these plasma condition s, off-axis LH power deposition profiles are obtained in a reproducibl e manner when the internal LH caustics prevent central absorption of t he waves. In the multipass LH wave propagation regime, the validity of the statistical treatment of stochastic wave diffusion is shown both theoretically and experimentally. When a large fraction of the plasma current (above 50%) is non-inductively sustained by the LH waves, the magnetic shear is reversed in the plasma core, i.e. inside a normalize d plasma radius of the order of 0.4. The resulting hollow current dens ity profiles have led to an enhancement of the total electron thermal energy content, up to a factor of 1.6 compared with L-mode discharges. The confinement improvement is attributed to a strong reduction of th e electron thermal diffusivity in the central reversed shear region, n early down to its neoclassical level.