Review: Review of advances in the understanding of energetic ions and impact on plasma confinement in DT burning plasmas

Self-plasma heating by MeV fusion-born alpha particles from Deuterium-Tritium (DT) reactions is one of the essential pillars of magnetically confined plasmas as a realistic energy source. However, energetic particles have some effects that are not optimum from the point of view of fusion energy generation. For example, they transfer their energy mostly to electrons rather than thermal ions.  In addition, they can resonate with Alfven waves, leading to turbulent transport that can degrade fusion power output.  In this talk, we review theoretical and experimental progress that has contributed to a significant improvement in understanding the impact of energetic ions on the confinement of thermal plasmas in fusion devices. Dedicated studies performed in a broad variety of plasma operating conditions with Neutral Beam Injection or Ion Cyclotron Resonant Heating in several international tokamak devices (e.g. JET, ASDEX-U and DIII-D) have shown that ion thermal energy confinement can actually improve in the presence of energetic ions. Especially relevant for burning plasmas, in the recent JET DTE2 campaign, improved confinement was found in DT plasmas in the presence of MeV ions, high electron heating, low rotation, and destabilized Fishbone and Toroidal Alfvén Eigenmodes (TAEs).  Several mechanisms play a role in the reduction of turbulent transport in the presence of energetic ions, and state-of-the-art gyrofluid and gyrokinetic turbulence simulations have revealed the physics behind the unexpected impact of MeV ions on plasma confinement. Plasmas with MeV ions are fundamentally a complex nonlinear multiscale system, spanning small electron to large Alfven scales, in which thermal ion turbulent transport can be reduced due to enhancement of stabilizing zonal flows in the presence of Fishbone and TAEs. The presence of T in D-T plasmas further enhances this mechanism. Such numerical results have been corroborated by reflectometer measurements in JET, showing nonlinear coupling between TAEs and zonal flows. Nevertheless, strong perturbations associated with MeV ions must also be avoided as they can lead to large alpha particle losses, as deduced from experimentally validated simulations. This talk reviews the previous findings, as well as recent JET DTE2 insights, in the context of future D-T in fusion reactors.