DTT NBI fast particle modelling with Monte Carlo ASCOT code

Abstract

The present thesis deals with the analysis and modelling of the behavior of Energetic Particles (EPs) injected by a neutral beam in a tokamak plasma. By Neutral Beam Injection (NBI), it is possible to achieve the high temperatures needed for fusion reactions in plasmas, but also to drive current and provide torque. In order to study EPs, the orbit-following ASCOT Monte Carlo code is used. A good confinement of EPs is essential both for plasma performances and to avoid potentially harmful EP losses from confined plasma to the machine first wall. For this reason, EP modelling is used to predict their interaction with the plasma and to eventually set limitations for NBI use depending on plasma parameters. In particular, fast ion losses can be caused by particle orbits that cross the plasma boundary (orbit losses) or from injected neutral particles not ionized in the plasma (shine-through losses). After a brief introduction presenting the foreseen advantages of employing fusion energy and the relevant concepts of plasma physics for this thesis, the theory of fast ion confinement and orbits is reviewed. The case of the Divertor Tokamak Test, an experimental device in construction in Frascati (IT), is then analyzed, with a classification of possible EP orbits through a topological map in the phase space defined by EP constants of motion and adiabatic invariant of the system. EP orbit topologies are shown for different DTT plasmas and for different EP injection energies, giving already a grasp of expected EPs confinement and losses in the approximation of collisionless orbits. ASCOT modelling is then used to verify the different situations analyzed and to understand the role of EP collisions. ASCOT numerical results give also estimations of EP loss channels different from orbit losses, as for instance shine-through losses, showing their dependence on the plasma density as foreseen by theory. The theoretical study of EP orbits and numerical modelling of NBI-plasma interaction contribute to the understanding of predicted EP confinement and losses for the forthcoming DTT device, in order to allow for the most effective application of NBI in future DTT operations