Abstract
Economical viability of future fusion reactor will require an efficient plasma heating provided by different mechanisms, including the ion cyclotron resonance heating (ICRH). Fish-bones are destabilized in JET Deuterium-Tritium (DT) by the minority population of energetic trapped Hydrogen generated by the ICRH. Fish-bones recursive destabilization causes significant alpha particle losses and a partial de-confinement of the thermal plasma, leading to a decrease of JET DT discharge performance. The aim of the present study is to analyze the destabilization of Fish-bones in the soft (local softening of profiles gradient) and hard (global burst-like relaxation) MHD limits, that is to say, analyzing configurations showing a benign or abrupt linear and saturation mode phases using the gyro-fluid code FAR3d. The transition from the soft to the hard MHD limit is reproduced in the simulations by increasing the EP drive (larger EP β by up scaling the EP density profile). The analysis indicates the Fish-bone destabilized in the soft MHD limit shows a linear evolution of the mode amplitude and EP perturbation energy in time. The mode causes negligible EP losses despite an outwards redistribution of the energetic Hydrogen. The EP flux towards the middle plasma induces a radial electric field with a magnitude around 20 kV/m that generates shear flows located in the inner plasma. Thermal plasma pressure and q profiles are marginally affected, indicating a weak perturbation of the flux and magnetic surfaces. On the other hand, fish-bones triggered in the hard MHD limit lead to an exponential evolution of the mode amplitude and perturbation energy in time. On top of that, EP losses and radial redistribution are stronger as well as the deformation of the flux and magnetic surfaces compared to the soft MHD limit, leading to a flattening of the thermal plasma pressure in the inner plasma, a significant excursion of the q profile induced by zonal currents as well as the generation of more intense shear flow by a radial electric field with a magnitude close to 80 kV/m. Consequently, operation scenarios may avoid the destabilization of Fish-bones in the hard MHD limit that can cause a significant decrease of the reactor performance. In contrast, Fish-bones in the soft MHD limit may improve the thermal plasma confinement linked to the generation of shear flows in the inner plasma (potentially reducing the plasma turbulence) while avoiding large EP losses and a large perturbation of the thermal plasma.
