CONFINEMENT IN W7-AS AND THE ROLE OF RADIAL ELECTRIC-FIELD AND MAGNETIC SHEAR

Improved neoclassical electron confinement in the centre of low-densit y ECRH plasmas has been observed in the presence of a strong positive radial electric field, which resembles the electron root solution of t he neoclassical ambipolarity condition but is obviously driven by the loss of ECRH-generated suprathermal electrons. At higher densities and with NBI heating, a high confinement regime substantially above the I SS95-scaling and different from the H-mode is established with a stron gly sheared negative radial electric field at the boundary. The applic ation of plasma-current induced magnetic shear reveals that confinemen t in W7-AS is essentially determined by perturbations at high-order ra tional surfaces. For optimum confinement, these resonances have either to be avoided in the boundary region or magnetic shear must be suffic iently large. Independent of its sign, magnetic shear can reduce elect ron energy transport which is enhanced in the presence of such resonan ces to the neoclassical level.