Spatial structure of drift-wave turbulence and transport in a stellarator

Abstract

Abstract. The three-dimensional structure of drift-wave turbulence and turbulent transport is investigated in plasmas of the stellarator experiment TJ-K. By means of two poloidal Langmuir probe arrays placed at different toroidal positions, density and potential fluctuations are recorded simultaneously at 128 positions on a single flux surface. From this data, the spatial drift-wave turbulence pattern including perpendicular and parallel structure sizes are obtained using a cross-correlation technique. A comparison with the magnetic field structure indicates an initially perfect alignment of turbulent structures with magnetic field lines. Passing over regions with different field line pitches according to the local variation of the rotational transform, however, results in a measured displacement of turbulent structures with respect to the field lines during their radial propagation. A reduction of the perpendicular correlation lengths in regions of high absolute values of local magnetic shear is found. Prominent and poloidally narrow turbulent transport maxima are measured at different toroidal positions. They are connected by the magnetic field lines and located in regions of negative normal curvature. The poloidal propagation pattern of turbulent structures and the exact position of the transport maximum depend on the magnetic field direction.

Spatio-temporal structure of turbulent Reynolds stress zonal flow drive in 3D magnetic configuration

“…In this region the normal curvature is still negative whereas the geodesic curvature is positive. A similar behaviour has been found for the radial cross-field transport G = á ñ   v n r [25,34]. These investigations have shown that the turbulent transport is peaked in regions with the above mentioned combination of curvature parameters (k < 0 n , k > 0 g ).…”

“…Although the plasma parameters are comparatively low, it has been shown that normalised quantities are similar to those in fusion edge plasmas [20,21]. Furthermore, many studies demonstrated the drift-wave nature of the plasma turbulence in the TJ-Kdevice with a density-potential cross phase close to zero and finite parallel wavelength [22][23][24][25]. Especially for small ion masses the r s scaling was found to be close to predictions for drift-wave turbulence [26].…”

“…The average poloidal probe spacing is D = x 1.4, 1.5, 1.6, and 1.7 cm on the four different flux surfaces at relative radius -= R R 9.5 0 , 10.0, 10.5, and 11.0 cm. Also, with a spatial uncertainty of 2 mm, the distances are still below the typical structure size of 3-5 cm [25,28,29]. The array makes it possible to measure the flux surface averaged Reynolds stress (indicated by á ñ .…”

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

“…The decrease of the difference in α towards the core could be due to an increase in parallel correlation length according to equation 16. A parallel correlation length of 2 m would be rather small compared to result from previous studies [14,15,16,17](but not impossible).…”

Spectral properties of electrostatic drift wave turbulence in the laboratory and the ionosphere

“…Investigations of turbulent drift waves have been stimulated by their importance for anomalous transport in magnetized plasmas and fusion plasma experiments in particular (Liewer, 1985;Wootton et al, 1990;Misguich et al, 1987;Horton, 1990Horton, , 1999. The present summary implicitly assumes locally homogeneous magnetic fields, but drift wave turbulence has been observed in more complex magnetic field configurations such as stellarators (Bol, 1964;Birkenmeier et al, 2013).…”