Abstract
We demonstrate favorable stability properties of maximum-J stellarators, exemplified by the Wendelstein 7-X (W7-X) device, in scenarios with low plasma beta. A large number of electrostatic linear gyrokinetic simulations are conducted to scan the relevant parameter space for different configurations, resulting in stability maps that account for the key micro-instabilities thought to drive turbulent transport. These maps exhibit a ‘stability valley’ in the region where the normalized ion temperature gradient is roughly equal to the normalized density gradient. In this valley, the electrostatic instabilities are partly suppressed thanks to the maximum-J property of the W7-X field. This property varies across different W7-X configurations, and this measurable difference is demonstrated to affect the size of the stability valley. Finally, the impact of the isotope effect and collisions on the valley is examined.
Highlights
Energy and particle losses in magnetically confined fusion reactors are attributed to turbulent transport, driven by microinstabilities, and neoclassical transport (NC), due to collisions
Regions of trapped electrons and regions of bad curvature are not coincident, which theoretically should lead to resilience against density-gradient-driven trapped electron modes (TEMs) [4, 5], which are replaced by relatively weak ion-driven TEMs [6, 7]
In this study we have investigated a region of the electrostatic instability space in maximum-J stellarators which we term ‘stability valley’, with suppressed micro-instabilities on scales comparable to, or smaller than the ion gyroradius but smaller than the electron gyroradius
Summary
Energy and particle losses in magnetically confined fusion reactors are attributed to turbulent transport, driven by microinstabilities, and neoclassical transport (NC), due to collisions. It is expected that the turbulent transport is the main cause of radial losses in discharges where the NC transport is not sufficient to explain the experimental results [10,11,12], as in the core of tokamak configurations, where the reduced NC contribution leads to a transport dominated by turbulence. The turbulence is generated by micro-instabilities which are driven by gradients of the electron and ion temperatures, and the plasma density [13, 14]. The central result of this work is that, contrary to tokamaks and non-maximum-J stellarators, W7-X has a wide region in the space of possible profiles—where the transition between the ion temperature gradient (ITG) instability and the trapped electron mode (TEM) occurs—with distinctly reduced linear growth rates, forming a ‘stability valley’.
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