Gyrokinetic studies of Kinetic Ballooning Modes (KBMs) in quasi-axisymmetric stellarator equilibria at finite beta

Under reactor-relevant, high-beta conditions, a steep plasma pressure gradient can excite Kinetic Ballooning Modes (KBMs) while suppressing other microinstabilities such as the Ion Temperature Gradient (ITG) and Trapped Electron Modes (TEM). In this research, KBMs are investigated in quasi-axisymmetric (QA) stellarator equilibria using the GENE code across various parameter scans. Through linear gyrokinetic simulations, KBMs were identified over a range of ky values. The critical beta threshold for KBMs was determined by scanning a series of equilibria with different pressure profiles and is compared with the threshold for Ideal Ballooning Modes calculated with the DESC code, providing a new perspective on turbulence optimization in the high-beta regime. Furthermore, the dependence of the beta limit on the radial location of the flux tube has been investigated. Finally, nonlinear gyrokinetic simulations are used to calculate heat and particle fluxes for a deeper analysis of the underlying nonlinear KBMs, including comparisons of the nonlinear saturation mechanisms and resulting transport levels.