The TCV tokamak offers outstanding variability of the plasma shape. Using x-ray tomography, the shape of the inner flux surfaces of a poloidal cross section of the plasma can be reconstructed, including fast variations due to MHD activity. Both the hardware and the software of the 200 channel system developed for TCV are described.A new 'dynamical' calibration takes actual plasma parameters into account to determine the spectrum-dependent detector efficiency, resulting in an enhanced quality of reconstructions.Tomographic inversions are obtained using a variety of methods such as maximum entropy, linear regularization and a newly developed method based on the Fisher information. The merits of the different algorithms, which have been implemented as MATLAB functions, are compared.Inversion results are analysed with the help of singular-value decomposition, allowing, for example, identification of MHD modes without using any a priori information on the poloidal mode structure.Recent results on the dependence of sawtooth activity on the plasma triangularity are presented to demonstrate the performance of the soft x-ray tomography system.
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.
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...
A new vertical position control system, including an internal active coil, has become operational on TCV. The new system has made it possible to stabilize plasmas with open loop growth rates up to 4400 s −1 , currents up to 1.0 MA and elongations up to 2.58. The closed loop stability of the new system has been analysed with a numerical model in which the plasma is assumed to be undeformable, and the power supply outputs are delayed with respect to their inputs. Model predictions agree with the main experimental results.
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