Hybrid advanced scenarios: perspectives for ITER and new experiments with dominant RF heating
A 'hybrid' scenario for ITER is defined through its objectives: a large fusion yield for a long time duration. In many tokamaks, discharges characterized by a stationary current density profile, enclosing a large volume of low magnetic shear with q(0) near 1, have achieved improved confinement and higher beta limits. Their extrapolation to ITER from existing data corresponds to the ITER hybrid scenario. These discharges are characterized by soft MHD events. Physics issues relevant to the existence and extrapolation of this scenario will be addressed. New JET experiments with a large component of RF heating have answered some of these issues: injected momentum is not essential, hybrid scenarios are achievable at low rho(*), hybrid regimes have been achieved with ITER-relevant T-e/T-i and they are compatible with a very low edge activity/low pressure pedestal. Data from pure RF discharges in other tokamaks (FTU, TS, TCV) seem to confirm that a large volume of low magnetic shear with q(0) close to 1 is the key to achieving hybrid scenarios. Issues needing resolution in the extrapolation to ITER are discussed. The present understanding provides encouraging prospects for the use of this scenario in ITER.