Alfven spectrum and eigenmodes in the advanced steady-state-tokamak scenario with negative triangularity

In the earlier studies [Fundamental Plasma Phys. special issue: Plasma Science Crossroads 2023, vol.10, 100051 (2024)], the negative triangularity tokamak has revealed outstanding features for plasma confinement. In addition to alleviating concerns about divertor heat load, it can have a higher beta limit in the nearly steady-state regime, about twice the Troyon limit compared to the positive triangularity case.

In this work, we extend the negative triangularity investigation to the Alfven spectrum and toroidal Alfven eigenmodes (TAEs, submitted to Phys. Plasmas) as a step to assessing energetic particle confinement in this configuration under an advanced scenario. We show that, because of the high safety factor and reversed magnetic shear in the core plasma region, the TAE gap tends to be closed or have only short radial extension between the continua in the core region. The gap is open primarily in the off-axis region, where TAEs reside. That suggests that energetic particle confinement in the core, where fusion reactions prevail, should not be seriously affected by the TAEs. The configuration with off-axis TAEs alleviates the confinement of energetic particles compared to the case of near-axis TAEs. We thus expect that the advanced steady-state tokamak scenario with negative triangularity can be favorable for energetic particle confinement.