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