Electric Field and Transport in W7-AS

At W7-AS, confinement properties are analysed and compared mainly with neoclassical predictions for quite different conditions. Low-density electron cyclotron resonance heating (ECRH) discharges allow access to the very-long-mean-free-path regime for electrons (T e up to 6 keV) whereas pure neutral beam injections (NBI) and combined NBI/ECRH discharges at high density (T i T e ≥ I keV at n e 10 20 m -3 ) lead to high performance (τ E up to 50 ms). Depending on the achieved temperatures, the experimental transport analysis in the plasma core is consistent with the neoclassical predictions. The experimentally observed 'electron root' feature with strong E r > 0 is driven by the convective flux of ripple-trapped suprathermal electrons generated by the ECRH absorption. 'Optimum' confinement is obtained in discharges with narrow density, but broad temperature profiles with steep gradients in the region of low densities and strong E r < 0 close to the plasma edge. The large radial electric fields, both positive and negative, strongly reduce neoclassical transport. The achieved temperatures, however, are limited by the strong temperature dependence of the neoclassical transport.