Statistical study of density fluctuations in the Tore Supra tokamak

Abstract

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 autocorrelation 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.

Study of edge turbulence in dimensionally similar laboratory plasmas

“…Additionally, the three-dimensional geometry should further contribute to deviations from pure 2D fluid turbulence. Spectral indices of −3 and −4 have been found for the density fluctuation power spectra in TEXTOR [30] and Tore Supra [31]. Other theoretical treatments of fusion plasma turbulence predict a value of −4 [32,33].…”

Nonlocal k-space interaction and the rare existence of coherent structures in the tokamak edge plasma

“…Accordingly, it is not surprising that when using a much smaller number of points, reported in an earlier paper, 9 we were unable to unveil clearly the presence of coherent structures. However, no significant time delay is recorded, suggesting that the mechanism behind this cross correlation is coherent structures coupling rather than an energy transfer in the k space.…”

“…6 the cross-correlation coefficients for ⌬kϭϮ3 and Ϯ4 cm Ϫ1 . 9 and replotted for the same data sets, hence using the same time resolution, in Fig. 5.…”

“…A first attempt to characterize the correlation between two scales in Fourier space in tokamaks was performed by Antar et al 9 We concluded that the interaction is local in the sense that as the difference between the two analyzing wave numbers increases, the cross correlation decreased. However, these measurements suffered from a lack of time resolution and a limited number of points (3.2ϫ10 4 ).…”

Kolmogorov-Kraichnan Scaling in the Inverse Energy Cascade of Two-Dimensional Plasma Turbulence

“…Other investigations on the Tore Supra tokamak showed that the autocorrelation time of the three-dimensional energy, which could be linked to the eddy turnover time, decreases with decreasing scale. Because in the enstrophy subrange the eddy turnover time does not depend on the scales, it appears that the studied fluctuations, between 0.4 and 2 cm, lie in the energy cascade subrange [25].…”