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Effect of the ambipolar potential on stellarator confinement

The confinement properties of reactor-scale stellarators in the presence of a self-consistent ambipolar potential Φ are examined in the light of the current theoretical expectations for the dependence of the transport coefficients on collisionality and radial electric field. It is found that stellarators have sufficiently good confinement to be viable as reactors. In addition, it is found that multiple roots of the ambipolarity constraint can exist in stellarators (as in other devices) and that, for appropriate parameters, confinement can be improved by operating at a root different from the one usually considered.