Resumen de 3D hybrid kinetic-MHD modelling of the interaction between Edge Localised Modes and Energetic Particles in the ASDEX Upgrade tokamak
Jesús José Domínguez-Palacios Durán
Nuclear fusion is a clean and virtually unlimited energy source that might meet the large energy demands in the near future. For the successful realization of a future fusion reactor, Edge Localized Modes [ELMs, periodic magnetohydrodynamic (MHD) instabilities that expel particles and energy from the plasma in a similar way to solar flares from the edge of the Sun] must be kept under control to avoid the large heat fluxes onto the plasma facing components, which will reduce the lifetime of the reactor. Although the ELM nature is well understood, its behavior and consequences in a burning plasma with a significant fraction of energetic (supra-thermal) ions is still missing. Energetic ions, which are produced by auxiliary heating systems or by the fusion reaction, are an essential source of momentum and energy that must be kept well confined until they slow down to the plasma bulk through Coulomb collisions. However, energetic ions are prone to a rich variety of wave-particle interactions due to their large velocities and long mean free paths, that can lead to an efficient exchange of energy and momentum with a broad spectrum of MHD fluctuations.
This thesis shows the first nonlinear hybrid kinetic-MHD simulations of ELMs, aimed to study the self-consistent interaction between ELMs and fast-ions, applying the nonlinear hybrid kinetic-MHD code MEGA to an ASDEX Upgrade plasma. During this thesis, the numerical set up of MEGA code has been modified to study both the thermal plasma and fast-ion dynamics at the plasma edge. First, simulations without fast-ions are performed to simulate the basic physics of an ELM crash. In the linear phase, it is found that high-n ballooning modes are more unstable. The nonlinear coupling between the modes is observed in the early non-linear phase. The ELM crash is finally simulated, observing the plasma filaments that are ejected from the plasma region, with the consequence of the flattening of the driving sources.
Fast-ions are then included in the model and different parameters of the fast-ion distribution have been scanned to understand how fast-ions and ELMs interact with each other. First, single-n simulations, which includes n = 0, 10 modes, are performed. When the fast-ion distribution peaks at the plasma core, both the ELM at the plasma edge and an Energetic Particle Mode (EPM) at the plasma core are simulated. Only when the fast-ion distribution is closer to the plasma edge, a strong interaction between ELMs and fast-ions is observed. In such a case, the simulations indicate that fast-ion kinetic effects have a strong impact on the spatio-temporal structure of the ELM. The interaction mechanism between the ELM and fast-ions has been analyzed studying the nature of the power exchange between ELMs and fast-ions in the phase-space of the energetic particles. Although the ELM is driven by the thermal plasma pressure gradient, a resonant interaction between the drift orbits of the edge fast-ion population and the ELM electromagnetic perturbation leads to a net wave-particle energy and momentum exchange that determines the resulting ELM spatio-temporal structure. An Energetic particle driven Geodesic Acoustic Mode (EGAM) appears after the ELM in the hybrid kinetic-MHD simulations, whose mode structure is strongly impacted by the ELM and that might help to understand the frequency pattern of the n = 0 mode observed in NBI heated plasmas.
A hybrid kinetic MHD multi-n simulation of ELM, which includes n = 0, …, 10 modes, has also been performed to account for the wave-wave coupling in the presence of fast-ions. Without fast-ions, n = 9, 10 modes are the most unstable modes. In the presence of fast-ions, the most unstable mode number is n = 8 with an energy almost 5 times larger than the mode energies obtained in the MHD multi-n simulation without fast-ions. The shifting of the most unstable mode is due to a large energy exchange between n = 8 mode and energetic ions. The impact of the fast-ion kinetic effects on the spatio-temporal structure of high-n modes is qualitatively the same in both the multi-n and single-n hybrid kinetic-MHD simulations. A strong power exchange between the ELM and fast-ions is also found in the multi-n simulations. In this simulation, a resonance overlap between the different toroidal modes is probably taking place, given the closeness of the resonances associated to the different modes. Additionally, in the multi-n simulations, a strong impact on the spatio-temporal structure of low-n harmonics has been observed. The simulations presented in this manuscript reproduce some outstanding ELM observations in low collisionality plasmas with large fast-ion contents that feature abrupt and large ELM crashes.
