Simulation of the poloidal rotation shear layer for stellarators

In the neoclassical theory based on the first order expansion of the distribution function, the radial electric field, Er, is calculated by the roots of the ambipolarity condition of the local particle fluxes: Γe = ZiΓi with Γα = -nα · [D11α (nα,/nα - qαEr/Tα) + D12αTα,/Tα] with α = e, i (impurity ion fluxes are disregarded), qα being the particle charge. In the particle flux densities, Γα, the Ware pinch term is omitted. In this context, additional 'anomalous' contributions are assumed to be intrinsically ambipolar. For given density and temperature profiles, Er is estimated separately for each flux surface. As the neoclassical particle transport coefficients depend on Er (and quite differently for ion and electrons in the different regimes of collisionality), multiple roots of the ambipolarity condition can exist. Especially when both the electrons and the ions are in the LMFP regime three roots can appear: the 'ion root', Eri, at small Er values, and the strongly positive 'electron root', Ere. An unstable root of the ambipolarity condition exists in between, while both the 'ion' and the 'electron root' are stable. At outer radii with higher collisionality, typically only the 'ion root' with typically Eri<0 can exist. (author) 9 refs., 4 figs