Abstract: Two aspects that are peculiar to burning plasmas and require a conceptual step in the analysis of magnetically confined plasmas, are investigated in this work. The first one is related to the fact that, in order to achieve reactor relevant conditions, it is necessary that fast ions (MeV energies) and charged fusion products (hereafter generically referred to as energetic ions) are sufficiently well confined that they transfer their energy and/or momentum to the thermal plasma without appreciable degradation due to collective modes. The identification of burning plasma stability boundaries with respect to energetic ion collective mode excitations and their nonlinear dynamic behaviors above the stability thresholds obviously impact the operation-space boundaries, since energy and momentum fluxes due to collective losses may lead to significant wall loading and damaging of plasma facing materials. Such analyses can be performed, at least in part, in present day experiments and provide nice examples of mutual positive feedbacks between theory, simulation and experiment. In a burning plasma, however, energetic ion power density profiles and characteristic wavelengths of the collective modes will be unique and not reproducible in existing experiments: the important implications of the predictive capabilities based on numerical simulations and modeling will be emphasized here along with the necessity of using existing and future experimental evidences for modeling verification and validation.

The second aspect is related with plasma turbulence and turbulent transport in a burning plasma. The presence of MeV ion energy tails does not only introduce a dominant electron heating contribution on the local power balance and a different weighting of the electron driven micro-turbulence with respect to present experiments. It also generates long time-scale nonlinear behaviors typical of self-organized complex systems, which are due to mutual interactions between collective modes and energetic ion dynamics on the one side and drift wave turbulence and turbulent transport on the other side. It is important that these interactions do not deteriorate the thermonuclear efficiency of the considered system on long time scales. These issues will be analyzed starting from their first principle theoretical grounds and introducing the possibility of investigating them via low-dimensional nonlinear dynamic models that can be formulated via formal theoretical-analytical approaches.