Consistency of proposed burning plasma scenarios with alpha-particle transport induced by Alfvenic instabilities

The stability of proposed burning-plasma scenarios (for ITER-FEAT, IGNITOR and FIRE) with respect to shear-Alfven modes possibly driven by fusion-produced alpha particles is investigated along with the effects of such modes on the transport and confinement properties of the alpha particles themselves. The results of numerical particle-in-cell model simulations show that one of the proposed scenarios (namely, the ITER-FEAT reversed-shear case) could be characterized by fast-growing energetic-particle modes, saturating via a convective displacement of the energetic ions. The consequent broadening pressure profile of the alpha particles and their additional diffusion due to scattering by the saturated electromagnetic fields show that the envisaged scenario cannot be considered consistent with the constraints imposed by nonlinear energetic-particle interactions with the Alfven modes. The validity of the above results is discussed with reference to the approximations used in the model.