Mitigation of the toroidal Alfvén eigenmodes in negative triangularity plasmas in the TCV tokamak*

Fast ion confinement, and particularly alpha particles, is key for the development of a successful fusion power plant (FPP), as they should be the main heating and current drive mechanism. Instabilities in plasmas, like Alfvén eigenmodes, can be excited by large fast ion populations and can result in an untamable loss of power, as well as damage to the reactor vessel [1]. 3D Non-linear hybrid kinetic simulations performed with the MEGA [2] code have revealed a mitigation of the Toroidicity-induced Alfvén Eigenmodes (TAEs) in TCV plasma when operating in negative triangularity (NT) plasmas [3], compared to positive triangularity (PT). The full kinetic treatment of the fast ions is key in reproducing the underlying wave-particle resonance structures in the phase space. About 30% less energy is found to be exchanged in the NT case. The change in particle orbit topology due to the triangularity significantly changes the energy exchange between the modes and the particles, damping it in the NT case, highlighting the need for kinetic treatment. A 3-fold reduction of the AE-induced fast ion losses has been observed in the self-consistent MEGA simulation in the NT case, in comparison to the PT plasma. This trend is further confirmed with purely kinetic simulations using the ASCOT5 code [4], which shows that for even more marked negative triangularities, fast ion losses reduce even further. 

[1] ITER Physics Expert Group on Energetic Particles Heating and Drive and ITER Physics Basis Editors, Nucl Fus, 39 (1999)

[2] Y. Todo et al, PoP, 5 (1998)

[3] C. Paz-Soldan et al., PPCF, 63 (2021)

[4] E. Hirvijoki et al., Comp Phys Comm, 185 (2014)