T. Aniel

Radial electric field measurement in a tokamak with magnetic field ripple

In the regions of the Tore Supra tokamak with significant ripple it is expected that a radial electric field (Er) ensures the ambipolarity of fluxes of thermal particles trapped in ripple wells. A neoclassical calculation (Connor and Hastie 1973 Nucl. Fusion 13 221, Stringer 1972 Nucl. Fusion 12 689) shows that Er is related to ion temperature and density gradients. The validity of this relation is investigated in a series of Tore Supra L-mode discharges without external momentum input. Doppler reflectometry measurements of fluctuations perpendicular velocity, which is dominated by the Er × B drift, are found to be in good agreement with the predicted neoclassical Er.

On plasma rotation with toroidal magnetic field ripple and no external momentum input

Ripple-induced thermal loss effect on plasma rotation is investigated in a set of Ohmic L-mode plasmas performed in Tore Supra, and comparisons with neoclassical predictions including ripple are performed. Adjusting the size of the plasma, the ripple amplitude has been varied from 0.5% to 5.5% at the plasma boundary, keeping the edge safety factor constant. The toroidal flow dynamics is understood as being likely dominated by turbulence transport driven processes at low ripple amplitude, while the ripple-induced toroidal friction becomes dominant at high ripple. In the latter case, the velocity tends remarkably towards the neoclassical prediction (counter-current rotation). The radial electric field is not affected by the ripple variation and remains well described by its neoclassical prediction. Finally, the poloidal velocity is of the order of the neoclassical prediction at high ripple amplitude, but significantly departs from it at low ripple.

Validation of a new mixed Bohm/gyro-Bohm model for electron and ion heat transport against the ITER, Tore Supra and START database discharges

A new model based on a combination of a Bohm-like term plus a gyro-Bohm-like term is proposed for the electron and ion heat diffusivity in the L mode regime, which is the commonest regime of operation of tokamaks. This model is derived using the dimensionless analysis technique taking into account the indications of scaling laws for the global confinement time and other experimental constraints on the diffusivity. The model has been successfully tested against data from several different experiments from the ITER database and the local Tore Supra database. Statistical analysis has shown it to perform better than purely Bohm or gyro-Bohm models and global scaling laws in the chosen dataset.

Comparative modelling of lower hybrid current drive with two launcher designs in the Tore Supra tokamak

Fully non-inductive operation with lower hybrid current drive (LHCD) in the Tore Supra tokamak is achieved using either a fully active multijunction (FAM) launcher or a more recent ITER-relevant passive active multijunction (PAM) launcher, or both launchers simultaneously. While both antennas show comparable experimental efficiencies, the analysis of stability properties in long discharges suggest different current profiles. We present comparative modelling of LHCD with the two different launchers to characterize the effect of the respective antenna spectra on the driven current profile. The interpretative modelling of LHCD is carried out using a chain of codes calculating, respectively, the global discharge evolution (tokamak simulator METIS), the spectrum at the antenna mouth (LH coupling code ALOHA), the LH wave propagation (ray-tracing code C3PO), and the distribution function (3D Fokker-Planck code LUKE). Essential aspects of the fast electron dynamics in time, space and energy are obtained from hard x-ray measurements of fast electron bremsstrahlung emission using a dedicated tomographic system. LHCD simulations are validated by systematic comparisons between these experimental measurements and the reconstructed signal calculated by the code R5X2 from the LUKE electron distribution. An excellent agreement is obtained in the presence of strong Landau damping (found under low density and high-power conditions in Tore Supra) for which the ray-tracing model is valid for modelling the LH wave propagation. Two aspects of the antenna spectra are found to have a significant effect on LHCD. First, the driven current is found to be proportional to the directivity, which depends upon the respective weight of the main positive and main negative lobes and is particularly sensitive to the density in front of the antenna. Second, the position of the main negative lobe in the spectrum is different for the two launchers. As this lobe drives a counter-current, the resulting driven current profile is also different for the FAM and PAM launchers.

q-profile evolution and improved core electron confinement in the full current drive operation on Tore Supra

The formation of a core region with improved electron confinement is reported in the recent full current drive operation of Tore Supra, where the plasma current is sustained with the lower hybrid (LH) wave. Current profile evolution and thermal electron transport coefficients are assessed directly by using the data of the new fast electron bremsstrahlung tomography that provides the most accurate determination of the LH current and power deposition profiles. The spontaneous rise of the core electron temperature observed a few seconds after the application of the LH power is ascribed to a bifurcation towards a state of reduced electron transport. The role of the magnetic shear is invoked to partly stabilize the anomalous electron turbulence. The electron temperature transition occurs when the q-profile evolves towards a non-inductive state with a non-monotonic shape, i.e. when the magnetic shear in the plasma core is reduced to close to zero. The improved core confinement phase is often terminated by a sudden MHD activity when the minimum q approaches 2.

Experimental observation of m/n = 1/1 mode behaviour during sawtooth activity and its manifestations in tokamak plasmas*

To shed some light on the development of the fast m/n = 1/1 precursor to the sawtooth crash and its influence on plasma transport properties in the vicinity of the q = 1 surface, series of dedicated experiments have been conducted on the Tore Supra and TEXTOR tokamaks. It has been concluded that, before a crash, the hot core gets displaced with respect to the magnetic axis, drifts outwards by as much as 8–10 cm and may change its shape. Observation of the magnetic reconnection process has been made by means of electron cyclotron emission diagnostics. The heat pulse is seen far outside the inversion radius. The colder plasma develops a magnetic island on the former magnetic axis, after the hot core expulsion. Different kinds of behaviour of the m = 1 precursor before the crash, with respect to the displacement of the hot core and the duration of the oscillating phase, have been observed. An ideal kink model alone cannot be used for explanation; therefore, resistive effects play an important role in the mode development. Possible mechanisms that lead an m = 1 mode to such behaviour, and their links to the change in the central q-profile, are discussed. Results have been discussed in the light of various theoretical models of the sawtooth.

Lower hybrid current drive experiments on Tore Supra in the ergodic divertor configuration

Lower hybrid current drive experiments in the ergodic divertor (ED) configuration have been carried out on Tore Supra for 9 years (1991-1999). This paper gives an overview of the results regarding both the current drive and the divertor efficiencies: waves coupling, current drive efficiency and fast particle confinement, energy confinement, thermal load on the ED plasma facing components, impurity screening. It is shown that the coupling remains good enough for high power transmission up to 26 MW m -2 . The current drive efficiency, measured on not fully non-inductive discharges, is only marginally affected by the loss of fast electrons due to the perturbed magnetic structure which is documented from hard x-ray measurements. Electron confinement follows, as in the limiter case, the Rebut-Lallia-Watkins scaling without any losses related to the width of the ergodic layer extending to 15% of the minor radius. The heat flux on the divertor neutralizers from both thermal particles and fast electrons accelerated near the antennae is detailed. Plasma contamination due to carbon and oxygen is analysed from a large database: it is shown that, for a large enough gap between the ED modules and the antennae, the contamination by these species is reduced by a factor of ∼3 with respect to the limiter case.

Increased understanding of the dynamics and transport in ITB plasmas from multi-machine comparisons

OAK A271 INCREASED UNDERSTANDING OF THE DYNAMICS AND TRANSPORT IN ITB PLASMAS FROM MULTI-MACHINE COMPARISONS.

Thermal electron transport in regimes with low and negative magnetic shear in Tore Supra

The magnetic shear effect on thermal electron transport is studied in a large variety of non-inductive plasmas in Tore Supra. An improved confinement in the region of low and negative shear was observed and quantified with an exponential dependence on the magnetic shear (Litaudon, et al., Fusion Energy 1996 (Proc. 16th Int; Conf. Montreal, 1996), vol. 1, IAEA, Vienna (1997) 669). This is interpreted as a consequence of a decoupling of the global modes (Romanelli and Zonca, Phys. Fluids B 5 (1993) 4081) that are thought to be responsible for anomalous transport. This dependence is proposed in order to complete the Bohm-like L mode local electron thermal diffusivity so as to describe the transition from Bohm-like to gyroBohm transport in the plasma core. The good agreement between the predictive simulations of the different Tore Supra regimes (hot core lower hybrid enhanced performance, reversed shear plasmas and combined lower hybrid current drive and fast wave electron heating) and experimental data provides a basis for extrapolation of this magnetic shear dependence in the local transport coefficients to future machines. As an example, a scenario for non-inductive current profile optimization and control in ITER is presented

Electron and ion internal transport barriers in Tore Supra and JET

Formation of core regions in Tore Supra and JET tokamaks with reduced transport coefficients is reported. Characteristics of the enhanced confinement regions and the physics process involved in their formation and maintenance should be considered separately when the electron or ion components are predominantly heated. In Tore Supra and JET, central electron temperature transitions are observed by injecting lower hybrid waves at modest power levels during the current ramp-up phase of the discharges. Transport analyses stress the importance of the low magnetic shear in the core to explain the anomalous electron transport reduction. With high-power dominant ion heating schemes in JET (neutral beam injection and ion cyclotron resonance heating), internal transport barriers have been obtained in plasmas fuelled with a mixture of deuterium-tritium (D-T) ions leading to a successful production of fusion power (8.2 MW) in this regime. Similar additional power levels to those applied in pure deuterium (D-D) plasmas are required to establish internal transport barriers in D-T plasmas. In D-D and D-T plasmas, ion thermal diffusivities are reduced close to their neoclassical levels in the plasma core and electron thermal diffusivities decrease by one order of magnitude at midplasma radius. The combined role of magnetic shear and velocity shear can explain the formation and evolution of plasma core regions with low energy transport coefficients.