Regimes of weak ITG/TEM modes to enable strong transport barriers for fusion

Recent gyrokinetic calculations find that low velocity shear in burning tokamak H-modes can enable the reemergence of electrostatic modes in the pedestal, that are suppressed in present tokamaks, resulting in excessive transport^1,2. Here we explicate regimes that hugely weaken these modes to enable the low transport needed for burning plasmas. We employ GENE and analytic theories. This result is unexpected, given the diversity of destabilization mechanisms, and the expectation of strong driving from steep gradients. Very different properties are found for Ion Temperature Gradient Modes coupled to Trapped Electron Modes (ITG/TEM), compared to the core.  Simulations are deciphered using a Simplified KInetic Model (SKIM), which includes the panoply of destabilizing effects: passing resonances for ions and electrons, trapped electron effects, and curvature effects. SKIM includes critical physical effects with exceptional transparency, agrees well with GENE, and substantiates the robustness of the regimes.

¹ M Kotschenreuther, D R Hatch, S Mahajan, et. al. , Nucl. Fusion 57 (2017) 064001

² D R Hatch , R D Hazeltine, M Kotschenreuther, S Mahajan PPCF 60 (2018) 084003