[en] We analyse the effect of the separatrix shape on stochasticity onset and on diffusion in the two waves system, and in particular the role of the separatrix angle at the X-point. Introducing anharmonicity in the expression of the perturbation, we can adjust this angle, and eventually have it go to zero. We show that, in this latter case, stochasticity appears much sooner and that the diffusion coefficient is strongly enhanced. Applications to specific physical problems are discussed

[en] Recent operation of JET with centrally strongly reversed magnetic shear, produced with the help of lower hybrid current drive, has extended the domain in which Internal Transport Barriers (ITBs) can be formed in JET. Performance is frequently limited by MagnetoHydroDynamic (MHD) instabilities in these reversed shear regimes. The most severe limit is a pressure driven kink mode which leads to a disruption. This disruptive limit is essentially the same in ITB plasmas with low or strongly reversed shear. Unique to the reversed shear regime is a dominantly n=1 mode, which has multiple harmonics. This mode is a common limit to performance, in the highest performance plasmas. Also unique to the reversed shear regime are q>1 sawteeth events, which can in turn trigger n=1 post-cursor oscillations. In general these post-cursor oscillations are benign but do provide valuable information on the q-profile. Other instabilities, including 'snakes' at the outer q=3 surface, are also observed to limit the performance of reversed magnetic shear ITB regimes. (author)

[en] This paper presents the first observation of geodesic acoustic modes (GAMs) on Tore Supra plasmas. Using the Doppler backscattering system, the oscillations of the plasma flow velocity, localized between r/a = 0.85 and r/a = 0.95, and with a frequency, typically around 10 kHz, have been observed at the plasma edge in numerous discharges. When the additional heating power is varied, the frequency is found to scale with C_s/R. The MUltiple SIgnal Classification (MUSIC) algorithm is employed to access the temporal evolution of the perpendicular velocity of density fluctuations. The method is presented in some detail, and is validated and compared against standard methods, such as the conventional fast Fourier transform method, using a synthetic signal. It stands out as a powerful data analysis method to follow the Doppler frequency with a high temporal resolution, which is important in order to extract the dynamics of GAMs. (paper)

[en] Fluctuation measurements provide key elements for turbulent transport analysis. In scaling experiments, they bring complementary local information for validating confinement dependence on non-dimensional parameters. The scaling of spatial and temporal characteristics of the fluctuations allows a more detailed comparison with transport models. This article will review the fluctuation observations of the scaling experiments in different tokamaks. They concern mainly scaling laws with the normalised gyroradius ρ^*. Most observations are favorable and corroborate the predictions from simulations. This article also presents another aspect of fluctuations dependence, the scaling with the wavenumber, which provides information on the nonlinear turbulence mechanisms. Fluctuations at high wavenumber k show a departure from classical k-spectrum power law, with a typical spectral index larger than usual observations and predictions, which is still unresolved. The resulting very low level of fluctuations in this range does not indicate a strong effect of electron temperature gradient modes

[en] Fusion plasma confinement is limited by radial energy losses across the magnetic field. In the frame of weakly out-of-equilibrium thermodynamics, cross B energy flux is proportional to the temperature gradient, times a constant coefficient which depends only on the equilibrium properties. For turbulent fluids, the 'turbulent' transport is also assumed to be proportional to the temperature gradient, although the transport coefficient is much larger than the equilibrium one, and cannot be a-priori predicted. An appropriate control of a fusion plasma requires an access to this coefficient. Until now, two types of procedures, active and passive, have been applied. The active method uses a strong energy deposition perturbation (additional heating modulation, sawtooth disruption) and observes the heat pulse cross-B propagation. The passive method requires a fully diagnosed plasma, where the temperature as well as a detailed power density deposition profiles are simultaneously available. An energy balance numerical model then provides the heat transport coefficient profile. These two methods however are either strongly perturbative, or indirect, and require expensive power losses or extensive diagnostics. The collective scattering of electromagnetic waves can directly provide a localized, instant measurement of a plasma turbulent transport coefficient. Plasma non-uniformities provoke the scattering of a monochromatic light beam. Since in addition the scattered light frequency is shifted from the incident light frequency by plasma motion, a detailed analysis of the scattered light frequency spectrum provides direct informations on this plasma motion. (author) 5 refs., 4 figs

[en] The received date was inadvertently omitted. It should have read 'Received 23 February 1999, in final form 24 May 1999'. (author)

[en] Measurement of turbulence properties provides key insight to understand anomalous transport in magnetic fusion devices. On Tore Supra, scattering diagnostics and reflectometers have been used to measure density fluctuations in the plasma core. A cross-polarization scattering diagnostic was also the first diagnostic to measure the turbulence magnetic fluctuations in a fusion plasma core. This paper presents the principle and the experimental setup of these diagnostics, with chosen results illustrating their capabilities to determine the spatial structure of the turbulence and to assess the link between energy transport and fluctuations. These flexible and complementary, measurements made it possible to analyze the confinement and fluctuation scaling laws with non-dimensional parameters. which requires a wide variety of plasma conditions

[en] Recent operation of JET with centrally strongly reversed magnetic shear, produced with the help of lower hybrid current drive, has extended the domain in which internal transport barriers (ITBs) can be formed in JET. Performance is frequently limited by magnetohydrodynamic (MHD) instabilities in these reversed shear regimes. The most severe limit is a pressure driven kink mode which leads to a disruption. This disruptive limit is essentially the same in ITB plasmas with low or strongly reversed shear. Unique to the reversed shear regime is a dominantly n=1 mode, which has multiple harmonics. This mode is a seemingly common limit to performance, in the highest performance plasmas. Also unique to the reversed shear regime are q>1 sawteeth events, which can in turn trigger n=1 post-cursor oscillations. In general, these post-cursor oscillations are benign but do provide valuable information on the q-profile. Other instabilities, including 'snakes' at the outer q=3 surface, are also observed to limit the performance of reversed magnetic shear ITB regimes. (author)

[en] Microwave beam backscattering near the cut-off layer appears to be the most interesting diagnostic to observe density fluctuation time evolution for a given localization in the plasma and at a defined wave vector. It also provides perpendicular plasma velocity. Scattering only occurs when the Bragg selection rule is fulfilled, i.e. when the scattering wave vector is almost perpendicular to the magnetic field. In order to evaluate these scattering conditions, ray tracing is required. 3D geometry is necessary to evaluate the angle between the magnetic field and the wave vector at the reflection. The ripple effect on the iso-index layer curve cannot be neglected. Scattering localization and wave vector resolution can be approached if single ray tracing is replaced with quasi-optical beam tracing. Optical propagation is still considered in the WKB approximation but the beam is described as multiple connected rays. The beam radial expansion due to diffraction is well described. This approach allows one to compute beam parameters for all data acquisitions (50 triggers per shot) and all shots (40 shots per day) during the following night on a recent personal computer with MatLab (copyright)

[en] The effect of the ergodic divertor on the plasma edge in Tore Supra is to enhance the perpendicular transport through ergodization of the magnetic field lines [Ph. Ghendrih et al., Contrib. Plasma Phys. 32 (3 and 4) (1992) 179]. Nevertheless, the hot spots observed on the divertor plates during ergodic divertor operation indicate that the cross-field transport driven by the fluctuations is still playing an important role, although measurements by CO₂ laser scattering and reflectometry show a decrease of the turbulence level [J. Payan, X. Garbet, J.H. Chatenet et al., Nucl. Fusion 35 (1995) 1357; P. Beyer, X. Garbet, P. Ghendrih, Phys. Plasmas 5 (12) (1998) 4271]. In order to gain more understanding, fluctuation level and poloidal velocity have been measured with a reciprocating Langmuir probe biased to collect the ion saturation current (j_sat) and with a CO₂ laser scattering diagnostic. Though the relative fluctuation level behaves as previously observed at low density, a new interesting result is that this picture is gradually modified when the density is increased. Both diagnostics observe an increase of δn/n with density in the ergodic region, which is not the usual behavior observed in limiter configuration. This increase is detected on both sides of the E_r inversion radius and is therefore also affecting the plasma bulk. Finally, the confinement time is found to follow an L-mode law at all densities indicating that the ergodic divertor does not change the global confinement properties of the plasma