Alfvén mode spectrum in the advanced steady-state tokamak scenario with negative triangularity

In the earlier studies [Zheng et al. Fundam. Plasma Phys. 10, 100051 (2024)], the negative triangularity tokamak was shown to have a unique and outstanding feature for the steady-state confinement. In addition to relieving the divertor heat load concern, it can reach a much higher beta limit in the nearly steady-state confinement, about twice the Troyon limit in the positive triangularity case. In this work, we extend the investigation to the Alfvén spectrum and toroidal Alfvén eigenmodes (TAEs). This helps to evaluate the property of energetic particle confinement in this advanced scenario of a negative triangularity tokamak. We found that, because of the large safety factor and reversed magnetic shear in the core plasma region near the magnetic axis, the TAE gap tends to be closed or have only a short radial extension between the continua. in the on-axis core region and open primarily in the off-axis region, where TAEs are shown to be present. This indicates that energetic particle confinement in the core region near the magnetic axis, where nuclear fusion reactions prevail, is not seriously affected by the excitation of TAEs in this scenario. The configuration with off-axis TAEs helps the confinement of energetic particles as compared to the configuration with near-axis TAEs. The results show that the advanced steady-state tokamak scenario with negative triangularity, because of the rapid variation of the safety factor, also has a favorable feature for energetic particle confinement.. Authors: Zheng, Linjin [The University of Texas at Austin, TX (United States)] (ORCID:0000000151774602); Kotschenreuther, M. T. [The University of Texas at Austin, TX (United States)] (ORCID:000000016327877X); Waelbroeck, F. L. [The University of Texas at Austin, TX (United States)] (ORCID:0000000193243690); Breizman, B. [The University of Texas at Austin, TX (United States)] (ORCID:0000000279086497). DOE Contract: FG02-04ER54742. Subjects: Fourier analysis; Heat transfer; Magnetic fields; Magnetohydrodynamics; Nuclear fusion; Plasma confinement; Plasma properties and parameters; Plasma waves; Tokamaks