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Optimum-confinement in the wendelstein 7-AS stellarator

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 eV<or=Te<or=3 keV, ion temperatures of 100 eV<or=Ti <or= 0.7 keV and electron densities of 1019 <or= ne<or=3.1020 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.