Electric field and transport in W7-AS

At W7-AS, confinement properties are;analysed and compared mainly with neoc lassical predictions for quite different conditions. Low-density electron c yclotron 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 approximate to T-c greater than or equal to 1 keV at n(c) approximate to 10 (20) m(-3)) lead to high performance (tau(E) up to 50 ms). Depending on the achieved temperatures, the experimental transport analysis in the plasma c ore is consistent with the neoclassical predictions. The experimentally obs erved 'electron root' feature with strong E-r > 0 is driven by the convecti ve flux of ripple-trapped suprathermal electrons generated by the ECRH abso rption. '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 ele ctric fields, both positive and negative, strongly reduce neoclassical tran sport. The achieved temperatures, however, are limited by the strong temper ature dependence of the neoclassical transport.