Effect of plasma shape on confinement and MHD behaviour in the TCV tokamak
The energy confinement time of TCV ohmic L mode discharges depends strongly on plasma shape. For fixed average current and electron densities, confinement times increase with plasma elongation and decrease with (positive) plasma triangularity. This dependence can be explained by the geometrical effects of flux surface expansion and compression on the temperature gradients together with the effect of power degradation, without the need to invoke a shape dependence of the transport coefficients. A global factor of merit, the shape enhancement factor H-s, is introduced to quantify this geometrical effect. The shape enhancement factor also has the potential to improve the description of the shape dependence in existing interdevice scaling laws. Modified versions of Neo-Alcator scaling and of Rebut-Lallia-Watkins scaling provide successful descriptions of ohmic L mode confinement for a large variety of plasma shapes in TCV by making use of H-s. Magnetohydrodynamic activity is also strongly dependent on plasma shape. Sawtooth amplitudes are largest at positive triangularity and sometimes vanish at negative triangularity where the amplitude of MHD modes is highest. It is shown that the changes in MHD behaviour are to a large extent consequences of the confinement changes produced in these shaping experiments.