Speaker
Dr
Felix Warmer
(Max Planck Institute for Plasma Physics)
Description
With the recent start of the optimized modern stellarator Wendelstein 7-X, the upgrade of the Large Helical Device and the advent of gyrokinetic simulations in stellarator geometry, for the first time it has now become possible to study and disentangle neoclassical and turbulent transport effects in stellarator geometry.
ECRH experiments conducted in the latest campaigns of W7-X and LHD give first indications towards a better understanding of the interplay of neoclassical and turbulent transport in stellarators. The strong ECRH heating gives rise to electron-root conditions in both devices with strongly peaked electron temperatures and flat ion temperatures as well as a strong positive radial electric field in the region where Te is peaked.
In both devices it has been observed that the measured radial electric field is in excellent agreement with neoclassical theory despite the fact that the neoclassical energy and particle fluxes are insufficient to explain the experimental fluxes over a wide range of parameters. Thus, it seems that the radial electric field in stellarators is well described by enforcing the ambipolarity constraint on the neoclassical particle fluxes as provided by standard neoclassical theory. This means that the remaining turbulent transport must be intrinsically ambipolar which is consistent with gyrokinetic theory for electromagnetic micro-turbulence. The fraction of turbulent energy flux, however, seems to vary with plasma parameters. In standard performance scenarios, the neoclassical energy flux can explain up to half the energy flux and becomes more dominant in high-performance, high-temperature scenarios due its strong temperature dependence.
First gyrokinetic simulations indicate that W7-X electron-root plasmas are dominated in the plasma centre by ETG while in similar LHD plasma scenarios, TEM seems to be the main drive in the plasma centre but not at the plasma edge. A general statement about the micro-turbulence characteristics cannot be easily given due the strong dependence on the magnetic geometry.
While the scales of neoclassical and turbulent transport are well separated, they still condition each other due to their interdependency resulting from plasma profile and gradient modifications. A specific influence has the radial electric field and its shear, which is determined by the ambipolarity condition (but is connected to the plasma profiles via the neoclassical particle fluxes) and directly acts as an external constraint on micro-turbulence. But also the neoclassical transport coefficients themselves are strongly affected by the radial electric field.
An often neglected effect is the total particle transport. The combination of the neoclassical and turbulent particle fluxes as well as particle sources determine the density profile and its gradient, which has a strong impact on e.g. stabilization of ITG or the drive of TEM. Consequently, a first-principle understanding will always result in a connection of particle and energy transport.
Primary author
Dr
Felix Warmer
(Max Planck Institute for Plasma Physics)
