EFFECT OF PLASMA SHAPE ON CONFINEMENT AND MHD BEHAVIOR IN TCV

The TCV tokamak (B-T < 1.5 T, R approximate to 0.88 m, a < 0.25 m) has produced a wide variety of plasma configurations, both diverted and l imited, with elongations kappa(a), ranging from 0.9 to 2.58, triangula rities delta(a) from -0.7 to 1 as well as discharges with nearly recta ngular cross sections. Plasma currents of 1 MA have been obtained in e longated discharges (kappa(a) approximate to 2.3). Ohmic discharges wi th delta(a) < 0 have smaller sawteeth and higher levels of MHD mode ac tivity than plasmas with delta(a) > 0. The main change in MHD behaviou r when elongation is increased beyond two is an increase in the relati ve importance of modes with m, n < 1 and a reduction of sawtooth ampli tudes. Confinement is strongly dependent on plasma shape. In ohmic lim iter 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 q(a). There is also an improvement of confinement as the elongation is increased. In most discharges the changes in confinement are explaine d by a combination of geometrical effects and power degradation. A glo bal factor of merit H-s (shape enhancement factor) has been introduced to quantify the effect of Bur surface geometry. The introduction of H -s into well known confinement scaling expressions such as Neo-Alcator and Rebut-Lallia-Watkins scaling leads to improved descriptions of th e effect of shape for a given confinement mode. In some cases with kap pa(a) greater than or equal to 1.7 limited ohmic L-modes undergo a slo w transition to a confinement regime with an energy confinement improv ed by a factor of up to 1.5 and higher particle confinement. First exp eriments to study the effect of shape in ECRH at a frequency of 83 GHz (second harmonic) have been undertaken with 500 kW of additional powe r.