  High-performance fusion plasmas, requiring high pressure $\beta$, are not
well-understood in stellarator-type experiments. Here, the effect of $\beta$ on
ion-temperature-gradient-driven (ITG) turbulence is studied in Wendelstein 7-X
(W7-X), showing that subdominant kinetic ballooning modes (KBMs) are unstable
well below the ideal MHD threshold and get strongly excited in the turbulence.
By zonal-flow erosion, these sub-threshold KBMs (stKBMs) affect ITG saturation
and enable higher heat fluxes. Controlling stKBMs will be essential to allow
W7-X and future stellarators to achieve maximum performance.
