14 research outputs found
First Measurements of Electron Temperature Fluctuations by Correlation ECE on Tore Supra
Electron temperature fluctuation studies can help to understand the nature of
the turbulent transport in to-kamak plasmas. At Tore Supra, a 32-channel
heterodyne ECE radiometer has been upgraded with two chan-nels of 100 MHz
bandwidth and tunable central frequencies allowing the shift of the plasma
sample volume in the radial direction. With the sufficiently large video
bandwidth and the long sampling time, it is possible to reduce significantly
the thermal noise and to identify "true" high frequency components up to 200
kHz from the cross-correlation between these channels. First results of
temperature fluctuation measurements on Tore Supra are reported in this paper.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Non-linear temperature oscillations in the plasma centre on Tore Supra and their interplay with MHD
Regular oscillations of the central electron temperature have been observed
by means of ECE and SXR diagnostics during non-inductively driven discharges on
Tore Supra. These oscillations are sustained by LHCD, do not have a helical
structure and, therefore, cannot be ascribed as MHD phenomena. The most
probable explanation of this oscillating regime (O-regime) is the assumption
that the plasma current density (and, thus, the q-profile) and the electron
temperature evolve as a non-linearly coupled predator-pray system. The
integrated modelling code CRONOS has been used to demonstrate that the coupled
heat transport and resistive diffusion equations admit solutions for the
electron temperature and the current density which have a cyclic behaviour.
Recent experimental results in which the O-regime co-exists with MHD modes will
be presented. Because both phenomena are linked to details of the q-profile,
some interplay between MHD and oscillations may occur. The localisation of
magnetic islands allows to obtain an accurate picture of the q-profile in the
plasma core. In some case, MHD-driven reconnection helps in maintaining a
weakly inverted q-profile that is found to be, in the CRONOS simulations, a
necessary condition to trigger the oscillations.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Colloquy
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Current ramps in tokamaks: from present experiments to ITER scenarios
In order to prepare adequate current ramp-up and ramp-down scenarios for ITER, present experiments from various tokamaks have been analysed by means of integrated modelling in view of determining relevant heat transport models for these operation phases. A set of empirical heat transport models for L-mode (namely, the Bohm-gyroBohm model and scaling based models with a specific fixed radial shape and energy confinement time factors of H(96-L) = 0.6 or H(IPB98) = 0.4) has been validated on a multi-machine experimental dataset for predicting the l(i) dynamics within +/- 0.15 accuracy during current ramp-up and ramp-down phases. Simulations using the Coppi-Tang or GLF23 models (applied up to the LCFS) overestimate or underestimate the internal inductance beyond this accuracy (more than +/- 0.2 discrepancy in some cases). The most accurate heat transport models are then applied to projections to ITER current ramp-up, focusing on the baseline inductive scenario (main heating plateau current of I(p) = 15 MA). These projections include a sensitivity study to various assumptions of the simulation. While the heat transport model is at the heart of such simulations (because of the intrinsic dependence of the plasma resistivity on electron temperature, among other parameters), more comprehensive simulations are required to test all operational aspects of the current ramp-up and ramp-down phases of ITER scenarios. Recent examples of such simulations, involving coupled core transport codes, free-boundary equilibrium solvers and a poloidal field (PF) systems controller are also described, focusing on ITER current ramp-down.</p
Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra
International audienc
On Plasma Rotation with Toroidal Magnetic Field Ripple and no External Momentum Input
International audienc
On Plasma Rotation with Toroidal Magnetic Field Ripple and no External Momentum Input
International audienc
Localized reversal of the perpendicular velocity in Tore Supra ohmic, L-mode, limited plasmas
International audienceIn Tore Supra plasmas, the perpendicular velocity measured by Doppler reflectometry was observed to reverse in a localized zone close to a normalized radius???0.5?0.6, changing from a negative value (corresponding to a negative radial electric field E r ) to a positive value ( ##IMG## [http://ej.iop.org/images/0029-5515/57/4/046021/nfaa59bbieqn001.gif] E_\textr>0 ). This occurs in L-mode, ohmic plasmas with a negligible external momentum input, a non-circular limited cross-section, and an edge safety factor close to 3. This reversal is favoured by a decrease in the magnetic field, or an increase in density. It is accompanied by a characteristic behaviour of the MHD activity signal, whose amplitude decrease during a ramp-down of the edge safety factor as it approaches ##IMG## [http://ej.iop.org/images/0029-5515/57/4/046021/nfaa59bbieqn002.gif] q_a∼ 3.1 ?3.2. A m / n ??=??2/1 mode is involved in the mechanism causing these observations