Long-range correlation properties of quasi-coherent modes at TEXTOR

Abstract

The paper reports on the first measurements of long-range correlations along field lines in the center of ohmic plasmas at TEXTOR. The measurements are performed with correlation reflectometry. For the first time the poloidal, toroidal and radial extension of quasi-coherent modes are simultaneously investigated using correlation reflectometry at two poloidally and toroidally separated positions. The experiments also allow the local determination of the toroidal velocity from a combination of delay time estimation and safety factor measurements from the same instrument. The paper investigates whether turbulence and which kind of turbulence, with respect to the frequency range, is aligned along field lines and embedded on iso-density surfaces in ohmic plasmas. It especially addresses the question of the properties of coherent structures in the vicinity of the q = 1 surface by analyzing propagation times and coherence levels from short-and long-range correlations. The results are discussed within the context of quasi-coherent modes as observed at T-10, TEXTOR, Tore Supra and recently JET and which are linked to trapped electron modes. Furthermore, the experiment tries to investigate whether a treatment of turbulence in two dimensions is sufficient or if 3D geometry has to be used for the description of turbulence stimulated transport. This is of importance for the development of turbulence codes and simulations.

Magnetic field pitch angle and perpendicular velocity measurements from multi-point time-delay estimation of poloidal correlation reflectometry

“…Only a few publications report on measurements of parallel correlation length [14,15,16,17] with values between 4 and 40 m reported. Recent parallel correlation length measurements from the TEXTOR tokamak give values between 11 and 22 m [20] indicating that the effect of structure declination is small. Conversely, if the real magnetic field pitch angle can be determined independently (i.e.…”

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…This has recently allowed to evidence their burst like character and their 3-dimensional nature [8].…”

Parametrization of reflectometry fluctuation frequency spectra for systematic study of fusion plasma turbulence

“…In previous works 11,12,14,15,20 several components were distinguished in fluctuation spectra associated to specific physical phenomena: the direct current (DC) component 15 , lowfrequency (LF) fluctuations 14 , broadband (BB) fluctuations and in some cases quasi-coherent (QC) oscillations 20 . The BB fluctuations, which cover the whole frequency range, have a short correlation length 14,15 and have been attributed to turbulence, to be called BB The QC oscillations can be observed in the LFS and are linked to drift wave instabilities. 13,14 In addition, the noise (N) level should be considered as another component for completeness.…”

“…The parametrization method relies on a decomposition of the turbulence spectrum. This decomposition method is based on the pioneering works of Vershkov et al 14 and Krämer-Flecken et al 15 Through radial, poloidal and long-range correlation, they have investigated different components of the turbulence spectrum and their properties. Our work is based on such an approach, using an extensive database of Tore Supra plasmas and a parametrization of frequency spectra obtained from reflectometry measurements.…”