  In fusion devices, the geometry of the confining magnetic field has a
significant impact on the instabilities that drive turbulent heat loss. This is
especially true of stellarators, where the "trapped electron mode" (TEM) is
stabilised if specific optimisation criteria are satisfied, as in the
Wendelstein 7-X experiment (W7-X). Here we find, by numerical simulation, that
W7-X indeed has low TEM-driven transport, and also benefits from stabilisation
of the ion-temperature-gradient mode, giving theoretical support for the
existence of enhanced confinement regimes at finite density gradients.
