  In the present Letter, first-of-its-kind computer simulations predicting
plasma profiles for modern optimized stellarators -- while self-consistently
retaining neoclassical transport, turbulent transport with 3D effects, and
external physical sources -- are presented. These simulations exploit a newly
developed coupling framework involving the global gyrokinetic turbulence code
GENE-3D, the neoclassical transport code KNOSOS, and the 1D transport solver
TANGO. This framework is used to analyze the recently observed degradation of
energy confinement in electron-heated plasmas in the Wendelstein 7-X
stellarator, where the central ion temperature was "clamped" to $T_i \approx
1.5$ keV regardless of the external heating power. By performing
first-principles based simulations, we provide key evidence to understand this
effect, namely the inefficient thermal coupling between electrons and ions in a
turbulence-dominated regime, which is exacerbated by the large $T_e/T_i$
ratios, and show that a more efficient ion heat source, such as direct ion
heating, will increase the on-axis ion temperature. This work paves the way
towards the use of high-fidelity models for the development of the next
generation of stellarators, in which neoclassical and turbulent transport are
optimized simultaneously.
