Fusion alpha performance in advanced scenario plasmas based on reversed central magnetic shear

In this paper we provide evidence that, for fusion energy production, advanced scenarios based on the presence of an internal transport barrier (ITB) have some advantages compared with the standard H-mode operation. This is due to their magnetic properties that affect alpha particle heating and helium exhaust. With identical density and temperature profiles, a qs-profile with reversed central shear, typical of an ITB plasma, is found to provide the same amount of total heating as a monotonous qs-profile, but with a broader heating profile. The overall confinement of alphas is almost the same for both profiles, but the neoclassical, energy-dependent alpha transport barrier due to stagnation orbits is closer to the magnetic axis for the monotonous profile. Therefore, the reversed profile helps to confine 3.5 MeV alphas born over a wider region while letting through the cooled-down He-ash.