  In the complex 3D magnetic fields of stellarators, ion-temperature-gradient
turbulence is shown to have two distinct saturation regimes, as revealed by
petascale numerical simulations, and explained by a simple turbulence theory.
The first regime is marked by strong zonal flows, and matches previous
observations in tokamaks. The newly observed second regime, in contrast,
exhibits small- scale quasi-two-dimensional turbulence, negligible zonal flows,
and, surprisingly, a weaker heat flux scaling. Our findings suggest that key
details of the magnetic geometry control turbulence in stellarators.
