During the first year of operation, the TCV tokamak has produced a large variety of plasma shapes and magnetic configurations, with 1 . O B J1.46T, I <800kA, ~S2.05, -0.7G%0.7. A new shape control algorithm, Eased on a finite element reconstruction of the plasma current in real time, has been implemented. Vertical growth rates of 800 sec-', corresponding to a stability margin f=l.IS, have been stabilized. Ohmic H-modes, with energy confinement times reaching 8 h s , normalized beta (p ,aB/I> of 1.9 and z P R 8 9 -P of 2.4 have been obtained in singlenuB X-point deuterium discharges with the ion grad B drift towards the X-point. Limiter H-modes with maximum line averaged electron densities of 1 . 7~1 0~~m -~ have been observed in D-shaped plasmas with 360kASIp&00kA.

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ABSTRACT. 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,, 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 Nec-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,. 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. EXPERIMENTAL CONDITIONSThe distinctive feature of TCV is a vacuum vessel with a height to width ratio of 3 surrounded by 16 poloidal field shaping coils (Fig. 1). This construction permits the creation of highly elongated and strongly shaped plasmas [l, 21. The machine therefore offers a unique capability to extend the world confinement database and to improve our understanding of transport in shaped plasmas. The basic parameters in the experiments reported are Ro = 0.88 m, a = 0.25 m for the major and minor radius, respectively, and BT = 1.43 T for the toroidal magnetic field.In the work presented the influence of plasma shape on the energy confinement and MHD behaviour has been investigated by means of a large variation of the plasma shape, from circular to highly elongated equilibria and from strongly D shaped (positive triangularity) to inverse D shaped equilibria (negative triangularity). The discharges investigated consist of 232 limited ohmic L mode plasmas in which elongation triangularity 6, and edge safety factor * Present ufiliution: Max-Planck-Institut fur Plasmaphysik, Garching, Germany. qa at the last closed flux surface and line averaged density ii, were systematically scanned in the range and 2.85 x lo1' m-3 < ii, < 8.5 x m-3. Plasmas were created with all possible combinations from four values of each of these parameters, with the exception of some 20 combinations that were inaccessible. All data shown and discussed in this article are from this dataset of 232 discharge conditions. Unless otherwise indicated, the figures show all available data for which the relevant quantities could be obtained.The TCV shape control system was programmed to produce equilibria such that the shape of the l...

Confinement in TCV electron cyclotron heated discharges was studied as a function of plasma shape, i.e. as a function of elongation, 1.1 < κ < 2.15, and triangularity, −0.65 ≤ δ ≤ 0.55. The electron energy confinement time was found to increase with elongation, owing in part to the increase of plasma current with elongation. The beneficial effect of negative triangularities was most effective at low power and tended to decrease at the higher powers used. The large variety of sawtooth types observed in TCV for different power deposition locations, from on-axis to the q = 1 region, was simulated with a model that included local power deposition, a growing m/n = 1 island (convection and reconnection), plasma rotation and finite heat diffusivity across flux surfaces. Furthermore, a model with local magnetic shear reproduced the experimental observation that the sawtooth period is at a maximum when the heating is close to the q = 1 surface.

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion ∇B drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors (β tor aB/I p ) up to 2 and line average densities up to 2.2×10 20 m −3 , corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary Hmodes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the 'active' X-point in unbalanced double-null discharges and the plasma-wall separation in single-null discharges. These dependencies have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.