On the interplay between negative triangularity and micro-tearing mode turbulence

The conceptualization of a Negative Triangularity (NT) Fusion Power Plant is still at an early stage and important questions remain unanswered. Most notably, will the confinement benefits observed in existing NT experiments persist in higher performance devices? Reactor-relevant plasmas are expected to achieve larger values of β, which can destabilize electromagnetic instabilities, such as Micro-Tearing Modes (MTMs).

We present a thorough study of the impact of plasma triangularity on MTM turbulence. We consider several different NT and Positive Triangularity (PT) scenarios taken from existing tokamaks (TCV, DIII-D, MAST-U and SMART) and EU-DEMO. With linear and nonlinear local gradient-driven gyrokinetic GENE simulations we scan relevant parameters for MTMs. Regardless of aspect ratio, MTMs are much stronger in NT than in PT. At sufficiently large β, large magnetic shear, a relatively flat density gradient and large electron temperature gradient, MTMs dominate turbulent transport in NT, leading to larger growth rates and heat fluxes than in PT. For these same parameters, PT scenarios are dominated by electrostatic turbulence. We observe that the values of β needed to destabilize MTMs in NT are only achieved in spherical tokamaks and not in conventional tokamaks. In addition, we demonstrate that stronger MTM-dominated transport in NT is entirely due to faster magnetic drift velocities.

Lastly, we use the increased strength of MTMs in NT to explain why NT plasmas do not transition to H-mode.