OHMIC H-MODE AND CONFINEMENT IN TCV

The unique flexibility of TCV for the creation of a wide variety of pl asma shapes has been exploited to address some aspects of tokamak phys ics for which the shape may play an important role. The electron energ y confinement time in limited ohmic L-mode plasmas whose elongation an d triangularity have been varied (kappa = 1.3 - 1.9, delta = 0.1 - 0.7 ) has been observed to improve with elongation as kappa(0.5) but to de grade with triangularity as (1 - 0.8 delta), for fixed safety factor. Ohmic II-modes have,been obtained in several diverted and limited conf igurations, with some of the diverted discharges featuring large ELMs whose effects on the global confinement have been quantified. These ef fects depend on the configuration: in double null (DN) equilibria, a s ingle ELM expels on average 2%, 6% and 2.5% of the particle, impurity and thermal energy content respectively, whilst in single null (SN) co nfigurations, the corresponding numbers are 3.5%, 7% and 9%, indicativ e of larger ELM effects. The presence or absence of large ELMs in DN d ischarges has been actively controlled in a single discharge by altern ately forcing one or other of the two X-points to lie on the separatri x, permitting stationary density and impurity content (Z(eff) approxim ate to 1.6) in long II-modes (1.5 s).