Localized measurements of turbulence in the TORE SUPRA tokamak

Abstract

A collective infraGred laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations. For the range of wavenumbers explored (3-15cm-'), the scattering angles are very weak (-1 mrad). Consequently, the scattering signals are averaged along the whole observation chord, resulting in poor longitudinal spatial localization. However, by virtue of the pitch angle variation of the magnetic field lines in the tokamak, and of the perpendicularity of the turbulence wavevector to these field lines, it has been possible to obtain partial spatial resolution along the direction of the beam. Good agreement between the experimental and theoretical angular resolution of the diagnostic as well as the results of cross-correlation performed on the signals obtained by two simultaneous probing beams also justiiy this novel concept.From the variation of the fluctuation power with the orientation angle of the observed wavevector, it has been possible to deduce the radial fluctuation profile by a deconvolution procedure, showing that the fluctuations increase sharply near the edge.The k-spectrum was also measured and shows a k-3 dependence for k>6cm-'. Experimental evidences are put foward to show that the k-spectrum is neither purely poloidal nor purely radial in the (k,k,) plane.

Electron transport and the critical temperature gradient

“…In previous electron heating experiments, the existence of such a threshold is suggested by the electron transport analysis [13][14][15][16][17] and by the measurements of both density and magnetic fluctuations [21][22][23][24]. Examples of rising levels of magnetic fluctuations [22], with increasing midradius electron temperature gradients, are shown in Fig [25] has reported that for short…”

Localized Measurement of Turbulent Fluctuations in Tokamaks with Coherent Scattering of Electromagnetic Waves

“…11-15) the length of the scattering region along the direction of the probing beam is similar to that of previous fluctuation measurements in small tokamaks. [3][4][5][6][7] Consequently, since for a given field of turbulent fluctuations and probing beam the scattered power is mainly limited by this geometrical parameter, we conclude that the proposed scheme is capable of detecting at least the same level of fluctuations as in previous measurements -but with a much improved radial localization.…”

“…For example, wave scattering measurements, that were so prominent in early fluctuation studies, [3][4][5][6][7] have a poor spatial resolution -very often larger than the plasma minor radius. The method of Beam Emission Spectroscopy 8 requires a perturbing neutral beam, has serious difficulties in detecting plasma fluctuations in the central region of large tokamaks, and is sensitive only to relatively large-scale fluctuations.…”

Recent Advancements in Microwave Imaging Plasma Diagnostics

“…This was discussed in Ref. [60], where it was shown that the radial resolution is given by an instrumental selectivity function with the same value of k ⊥ , i.e., the radial resolution is the product of this ratio with the radial separation of the points of origin along the trajectory of the probing beam (=3.25 cm).…”