Optimum-confinement in the wendelstein 7-AS stellarator

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Abstract
Optimum confinement is realized in Wendelstein 7-AS by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. The effective heating of net current free plasmas by ECRF and neutral beam injection (NBI) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios. An extended parameter range with electron temperatures of 200 eVei 19 e20 m-3 was accessible. The characteristics of the energy confinement and the particle and impurity transport are described and related to the specific heat and particle sources. The investigations comprise the analysis of electron and ion heat conductivity, particle transport modelling and impurity transport studies by laser blow-off-experiments. The influence of the ambipolar electric field is discussed.