Effect of plasma shape on confinement and MHD behaviour in TCV

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Abstract
The TCV tokamak (, a < 0.25 m) has produced a wide variety of plasma configurations, both diverted and limited, with elongations ranging from 0.9 to 2.58, triangularities from -0.7 to 1 as well as discharges with nearly rectangular cross sections. Plasma currents of 1 MA have been obtained in elongated discharges . Ohmic discharges with have smaller sawteeth and higher levels of MHD mode activity than plasmas with . The main change in MHD behaviour when elongation is increased beyond two is an increase in the relative importance of modes with m,n < 1 and a reduction of sawtooth amplitudes. Confinement is strongly dependent on plasma shape. In ohmic limiter L-modes energy confinement times improve typically by a factor of two as the plasma triangularity is reduced from 0.5 to 0 at constant . There is also an improvement of confinement as the elongation is increased. In most discharges the changes in confinement are explained by a combination of geometrical effects and power degradation. A global factor of merit (shape enhancement factor) has been introduced to quantify the effect of flux surface geometry. The introduction of into well known confinement scaling expressions such as Neo-Alcator and Rebut - Lallia - Watkins scaling leads to improved descriptions of the effect of shape for a given confinement mode. In some cases with limited ohmic L-modes undergo a slow transition to a confinement regime with an energy confinement improved by a factor of up to 1.5 and higher particle confinement. First experiments to study the effect of shape in ECRH at a frequency of 83 GHz (second harmonic) have been undertaken with 500 kW of additional power.

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