M. Anton

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 Hs, 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 Hs. 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

Vertical instability in TCV: Comparison of experimental and theoretical growth rates

Note: 38th Annual Meeting, APS Division of Plasma Physics, Denver, CO, USA, November 1996 Reference CRPP-CONF-1996-056 Record created on 2008-05-13, modified on 2017-05-12

Ohmic H-mode and confinement in TCV

The unique flexibility of TCV for the creation of a wide variety of plasma shapes has been exploited to address some aspects of tokamak physics for which the shape may play an important role. The electron energy confinement time in limited ohmic L-mode plasmas whose elongation and 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 degrade with triangularity as (1-0.8 delta ), for fixed safety factor. Ohmic H-modes have been obtained in several diverted and limited configurations, with some of the diverted discharges featuring large ELMs whose effects on the global confinement have been quantified. These effects depend on the configuration: in double null (DN) equilibria, a single ELM expels on average 2%, 6% and 2.5% of the particle, impurity and thermal energy content respectively, whilst in single null (SN) configurations, the corresponding numbers are 3.5%, 7% and 9%, indicative of larger ELM effects. The presence or absence of large ELMs in DN discharges has been actively controlled in a single discharge by alternately forcing one or other of the two X-points to lie on the separatrix, permitting stationary density and impurity content (Zeff=1.6) in long H-modes (1.5 s).