Abstract
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).
Highlights
In magnetic confinement fusion devices, there are usually three processes limiting the energy confinement: radiation losses, neoclassical transport, which encompasses collisional diffusion – including the effect of particle drifts that arise due to gradients and curvature of the confining magnetic field – and turbulence
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)
We find that the effect 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
Summary
In magnetic confinement fusion devices, there are usually three processes limiting the energy confinement: radiation losses, neoclassical transport, which encompasses collisional diffusion – including the effect of particle drifts that arise due to gradients and curvature of the confining magnetic field – and turbulence. While the theory states that this resilience should hold for perfectly quasi-isodynamic stellarators with the maximum-J property, linear simulations (Proll, Xanthopoulos & Helander 2013; Alcusón et al 2020) showed that W7-X, which is only approximately quasi-isodynamic, benefits from reduced TEM growth rates, too. These results are only linear and raise the question whether the enhanced stability results in less turbulent transport. We provide analytical arguments for why, in quasi-isodynamic configurations, trapped electrons have a stabilising property for ITG modes
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