Significance of Microtearing Turbulence in Turbulence-Reduced High-Density-Gradient Plasmas in Wendelstein 7-X

Gyrokinetic simulations of a turbulence-reduced Wendelstein 7-X discharge-characterized by a steep density gradient, moderate temperature gradients, and low plasma beta-show that microtearing mode (MTM) turbulence dominates transport. The simulated heat and particle fluxes agree with experimental measurements, leaving MTMs as the only mode consistent with the data. These conditions, the quasi-isodynamic and nearly max-J magnetic configuration of the Wendelstein 7-X stabilize ion temperature gradient modes and density-gradient-driven trapped-electron modes, while moderate collisionality and low magnetic shear further enable MTM growth. Further nonlinear scans of the density gradient reveal a significant reduction in turbulent transport at the experimentally observed threshold, which we identify as an ion temperature gradient to MTM-dominated turbulence transition. These findings provide a robust explanation for the turbulence suppression and deepen our understanding of low turbulent transport regimes in optimized stellarators.