Turbulence mitigation in maximum-J stellarators with electron-density gradient

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 density-gradient-driven branch of the 'trapped electron mode' (TEM) is predicted to be linearly stable if the magnetic field has the maximum-J property, as is very approximately the case in certain magnetic configurations of the Wendelstein 7-X experiment (W7-X). Here we show, using both analytical theory and simulations, that the benefits of the optimisation of W7-X also serve to mitigate ion-temperature-gradient (ITG) modes as long as an electron density gradient is present. We find that the effect indeed carries over to nonlinear numerical simulations, where W7-X has low TEM-driven transport, and reduced ITG turbulence in the presence of a density gradient, giving theoretical support for the existence of enhanced confinement regimes, in the presence of strong density gradients (e.g. hydrogen pellet or neutral beam injection).

Automated summary

In fusion devices, the geometry of the confining magnetic field plays a crucial role in the instabilities that cause turbulent heat loss. A study on the Wendelstein 7-X experiment shows that optimizing its magnetic field configuration can not only reduce the transport driven by trapped electron modes, but also mitigate the ion-temperature-gradient modes as long as an electron density gradient exists. This finding provides theoretical support for the existence of enhanced confinement regimes in the presence of strong density gradients.