Speaker
Description
Good plasma confinement is crucial to harness fusion energy. Experiments on the TCV [1] and DIII-D [2] tokamaks have shown that negative triangularity (NT) reduces the turbulent transport, hence improving confinement. Trapped Electron Modes (TEM) are thought to play an important role in this process. A full understanding of the underlying physics is necessary for assuring NT plasmas to be reactor relevant, but is still missing. To identify the key physical mechanisms driving the improvement of confinement in TEMs dominated NT plasmas, a reduced analytical model has been recently derived, focusing on TEMs linear stability [3]. In this contribution, the model of [3] is extended by including a simplified model for ions without accounting for their resonant response - thus excluding ITGs. This extension enables a qualitative agreement of the model with gyrokinetic (GK) simulations performed with GYSELA [4] and GENE [5] in the linear regime, and an evaluation of the impact of Finite Larmor Radius effects on stability.
The model highlights the key role of "Finite Mode Width" (FMW) effects, which is the poloidal localization of the linear modes – ballooning character. A key quantity for understanding the importance of FMW effects is the precession frequency, the growth rate in the fluid limit scaling like its square root. The ballooning of the instability gives more weight to deeply trapped electrons, whose precession frequency is lower for NT than for positive triangularity (PT). On the other hand, the benchmark with GENE highlights the key role played by the trapped electrons parallel dynamic (related to their bouncing frequency) and by passing electrons in the shear dependence of the different linear stability of TEMs in negative and positive triangularity.
Although it provides precious insights into the underlying physics, a linear analysis is not sufficient to explain the experimental results [1][2]. Therefore, a nonlinear analysis has been carried out using the gyrokinetic code GYSELA. In particular, the electric and diamagnetic components of the Reynolds stress, which drive the zonal flows, will be shown to exhibit significant differences in NT and PT configurations.
References
[1] Y. Camenen et al., Nucl. Fusion 47 (2007)
[2] M. Austin et al., Phys. Rev. Lett. 122 (2019)
[3] X. Garbet et al., Nucl. Fusion 64 (2024) [4] V. Grandgirard et al., Comput. Phys. Comm. 207 (2016)
[5] G. Merlo et F. Jenko, Journ. Plasma Phys. 89 (2023)
