Effect of Plasma Shape on Confinement and MHD Behaviour in TCV
The TCV tokamak (B(T) < 1.5 T, R ÿ 0.88 m, a < 0.25 m) has produced a wide variety of plasma configurations, both diverted and limited, with elongations ka ranging from 0.9 to 2.58, triangularities da from -0.7 to 1 as well as discharges with nearly rectangular cross sections. Plasma currents of I MA have been obtained in elongated discharges (Ka ÿ 2.3). Ohmic discharges with da < 0 have smaller sawteeth and higher levels of MHD mode activity than plasmas with da > 0. The main change in MHD behaviour when elongation is increased beyond two is an increase in the relative importance of modes with m, n < I 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 q(a) .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 Hs (shape enhancement factor) has been introduced to quantify the effect of flux surface geometry. The introduction of Hs 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 ?(a) = 1.7 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.
Bibliographic Reference: An article published in: Plasma Physics Controlled Fusion, 39 (1997) B135-B144
Record Number: 200013657 / Last updated on: 2001-09-18
Original language: en
Available languages: en