Effect of multiple energetic particle populations in burning plasma: consequences on JET D-T campaign and ITER*

Future nuclear fusion reactors will be heated by neutral beam injectors (NBI) and high frequency electromagnetic waves as well as fusion born alpha particles. Consequently, a reactor relevant plasma hosts multiple energetic particle (EP) populations that will interact non-linearly between them and the thermal plasma, affecting the system stability and leading to potential harmful effects on the device performance. The present study is dedicated to the analysis of the effect of multiple EP species on JET D-T plasma on Tokamaks, as well as the consequences on the alpha particle confinement. Linear and nonlinear simulations are performed with the gyro-fluid FAR3d code based on the JET D-T discharge #99896. The analysis indicates fish-bones can induce important alpha particle losses although the losses caused by Toroidal Alfven Eigenmodes (TAEs) are negligible, consistent with the experimental observations. EPs have a stabilizing effect on tearing mode instabilities, both in the linear and saturation phases. This is explained by the mode poloidal rotation induced by the EPs, narrowing its radial width, as well as the extraction of the perturbation energy in the EPs resonance with the magnetic field lines. Optimized configurations with reduced tearing mode amplitude and EP losses are identified with respect to the EP density below the EP β threshold required to trigger AE. For reactor relevant plasmas including alpha particle and NBI EP populations, the simulations show important changes on the tearing modes and AE stability compared to single EP species, indicating the important role of the nonlinear interaction between different EP populations. Particularly, multiple EP species show significant alpha particle losses in configurations with improved tearing mode stability following single EP species. This result may indicate present day experiments with negligible alpha particle populations cannot fully explore the stability of reactor relevant plasma. Another consequence applies to the alpha power build-up in burning plasma, notably during the ramp-up phase. The interaction between a rather low alpha particle population with current driven modes near the q=1 surface and in the presence of other EP population could lead to enhanced losses and lower heating power of the alpha particles, reducing the device performance in the flat-top phase.