A Truc

ALTAIR: An infrared laser scattering diagnostic on the TORE SUPRA tokamak

A collective laser light scattering diagnostic ALTAIR (a french acronym for local analysis of anomalous transport using infrared light), using a CO2 laser beam (λ=10.6 μm) has been realized to measure plasma density fluctuations in the TORE SUPRA tokamak. This article describes in detail the optical setup, the signal processing, acquisition, and control systems required for this experiment. As the density fluctuations propagating in tokamaks have small wave numbers and require small scattering angles, such scattering experiments are considered as having no resolution along the beam. However, taking advantage of the pitch angle variation of the magnetic field lines around the magnetic axis along a vertical chord, it has been possible to obtain partial spatial localization of the scattering volume by rotating the direction of the analyzed wave vector in a horizontal plane. Heterodyne detection is used to determine the fluctuations propagation direction. The experiment has been tested on acoustic waves and t...

Density Fluctuation Measurement by Far Infrared Light Scattering

Density fluctuation observation can be obtained by scattering of electromagnetic radiation. New developments and applications are presented. By heterodyne detection the scattered signal is shown to be the time variation of a space Fourier transform of the density. The antenna beam, that especially defines the sensitivity, is experimentally investigated. The time variation is used in two occurrences, to check quantum efficiency uniformity on detectors area, and in an air turbulence where the interspectrum is obtained between two scattering devices looking at different regions. Calibration formulas are given for quantitative measurement of density fluctuation spectrum. Observations are also reported on a tokamak plasma, on the drift wave spectrum, and on a new second order group of oscillations that were observed during ion cyclotron radio-frequency heating. The analysis shows this new group is a forced oscillation by beating of the drift waves with the high-frequency heating wave, and its amplitude provides a measurement of the HF wave electric field.

Statistical study of density fluctuations in the Tore Supra tokamak

It is believed that radial anomalous transport in tokamaks is caused by plasma turbulence. Using infra-red laser scattering techniques on the Tore Supra tokamak, statistical properties of the density fluctuations are studied as a function of the scales in ohmic as well as additional heating regimes using the lower hybrid or the ion cyclotron frequencies. The probability distributions are compared to a Gaussian in order to estimate the role of intermittency which is found to be negligible. The temporal behaviour of the three-dimensional spectrum is thoroughly discussed; its multifractal character is reflected in the singularity spectrum. The auto-correlation coefficient is shown to reflect a short-time coherence of the dissipative structures as well as their long-time incoherence and statistical independence. We also put forward the existence of fluctuations transfer between two distinct but close wavenumbers. A rather clearer image is thus obtained about the way energy is transferred through the turbulent scales.

Temporal separation of the density fluctuation signal measured by light scattering

On Tore Supra, the frequency spectra of the turbulent fluctuations often have two peaks centred on some positive and negative values. These two peaks correspond to a poloidal motion of the electrons in opposite directions. In this paper, a criterion is elaborated which allows us to distinguish, in time, the density fluctuations convected in the parallel direction from those in the anti-parallel direction with respect to the analysing wavevector. Two signals are thus extracted out of one. The validity of our model is experimentally checked by comparing the auto-correlation coefficients and the frequency spectra computed for the whole and the separated signals. Consequently, the frequency spectrum is studied in detail as a function of the analysing wavenumber leading to an accurate determination of some plasma properties.

Fluctuations and associated transport in the L mode in Tore Supra

A parametric experimental study of fluctuations has been performed in L-mode plasmas during steady-state phases in Tore Supra. Simultaneous measurements of density and magnetic fluctuations in the gradient region of the discharge show that both components are strong functions of the local electron temperature decreasing lengths. The wavenumber spectrum of density fluctuations is obtained with the scattering diagnostic both in the bulk and at the edge of the discharge. In the bulk, in the L-mode compared to ohmic plasmas, the k spectrum increases below and is globally shifted towards low k values. At the edge (r/a > 0.9) an increase on all k values is observed. At r/a = 0.5, the level of both types of fluctuations is found to be linearly correlated with the local heat diffusivity . The heat flux ratio advected by electrostatic eddies in the bulk of the discharge is estimated by using fluctuation measurements and generic predictions for the parametric dependency of the thermal flux. The calculated level of density fluctuation necessary to account for the heat transport at midradius is found to be compatible with values measured in other tokamaks. It is also found that the heat flux ratio behaviour conducted by electrostatic fluctuations is compatible with most of the experimental heat flux ratio behaviour in the plasma core (0.4 < r/a < 0.6). This property does not hold at the edge of the plasma (r/a = 0.7). The similar parametric dependency of magnetic and density fluctuation level does not allow one to make a clear statement about their relative importance in the transport process and suggests that they could be coupled.

The ion feature in a laboratory plasma: Theory and experiment using CO2 laser light scattering

The electron density fluctuation spectrum in the range of frequencies below the ion plasma frequency (the ‘‘ion feature’’) is investigated in plasmas where the ion acoustic waves are weakly damped. The situation typical in laboratory plasmas where the electron temperature is larger than the ion temperature is considered in detail. Analytical expressions are given in the limit of weak damping and the variations of the spectrum with the relevant parameters are readily obtained, including the dependence on frequency, wavenumber, temperature ratio, drift velocity, and the concentrations of light impurity ions. Velocity distributions with non‐Maxwellian tails are also considered. An experiment is described in which the scattering of CO2 laser radiation from a discharge plasma is studied. The observed fluctuation spectrum corresponds to the ion feature, in that the dependence of the observed frequencies on wavenumber corresponds to the dispersion relation of ion acoustic waves. They are only observed, however, in the edge plasma where drifting ions enhance the fluctuation level, and not in the plasma center, where light impurities in the argon plasma damp out the ion resonance.

Estimate of the correlation dimension for Tokamak plasma turbulence

The coherent CO2 laser scattering signals from TFR Tokamak plasmas have been analysed using a variant of the algorithm proposed by Grassberger and Procaccia (1983) to determine the correlation exponent of these measured plasma turbulence signals. By comparing the results obtained with a randomly generated test signal, the authors see that the plasma turbulence may be considered a dynamical system which possesses a low correlation exponent (the value of which depends on wavenumber of the measured turbulence), thus indicating the possible existence of a low dimensional attractor. They also show by using different signals acquired at different sampling rates that this low dimensional structure is only observed when the acquisition rate of the turbulence signal is sufficiently rapid.

Density fluctuations during the sawtooth crash in the Tore Supra tokamak

Using a laser scattering experiment, transient density fluctuations related to the sawtooth crash were measured, for about 100 sawteeth, during similar and stationary ohmic discharges of the Tore Supra tokamak. Simultaneously, the evolutions of electron temperature, density and soft x-ray emission were recorded to analyse the plasma behaviour during the crash. In particular, tomographic reconstructions of soft x-ray signals were used to draw emissivity maps and to investigate in detail the sawtooth collapse. Two bursts of density fluctuations, spatially localized within the q = 1 surface, have been identified. A first burst appears together with the fast displacement of the hot core at the beginning of the crash (phase I), and a second stronger burst develops when the central temperature collapses (phase II). The levels and propagation directions of these fluctuations have been systematically compared with the direction of the initial hot core motion. From this analysis, it is deduced that fluctuations propagate from the hot core compression zone along the q = 1 separatrix with a velocity that is about five to 10 times larger than the hot core motion. Levels are strongly correlated to the hot core motion, indicating spatially localized fluctuations. Using these results and a numerical model, it is shown that fluctuations detected during phase I are principally located in the vicinity of the hot core compression zone while those correlated to phase II are in two more central regions, where the temperature collapses.

Transport and turbulence in Tore Supra

Several discharges have been analysed in Tore Supra in order to improve the understanding of anomalous transport. In ohmic discharges, it has been shown that both particle and heat diffusion coefficients improve with increasing density, then saturate. The level of edge density fluctuations behave in the same way. In the presence of additional heating, the bulk and the edge turbulences behave differently. The level of edge density fluctuations remains constant in most discharges with additional heating (up to 8 MW). In practice, it only depends on edge conditions. On the contrary, the level of core magnetic fluctuations increases systematically with any additional power. Finally, a lowering of the level of the edge density fluctuations has been observed when using the Tore Supra ergodic divertor. This improvement is accompanied by the onset of a temperature pedestal between the plasma bulk and the ergodic layer. This explains why the global confinement remains unchanged in presence of the ergodic divertor in spite of the reduced radius of the last closed magnetic surface.

Density fluctuations at high density in the ergodic divertor configuration of Tore Supra

Abstract 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&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 CO 2 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 CO 2 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.