Turbulence investigations during ergodic divertor operation with induced 2/1 tearing mode

Abstract

Within the last year of TEXTOR operation a major part of the experiments were dedicated to the interaction of the ergodized plasma edge, induced by the dynamic ergodic divertor (DED), with the core plasma. Systematic experiments are performed in the 3/1 configuration of the DED. Depending on β, the DED generates a locked 2/1 mode in the plasma at a critical current in the DED coils. Different diagnostics were used to measure the plasma response in the core and edge. The investigations in this paper are focused on the measurement of density fluctuation and the analysis of the coherent-mode structure outside the q = 2 surface using O-mode poloidal correlation reflectometry. A reduction of the quasi-coherent mode amplitude and frequency is observed, caused by the 2/1 mode together with a decrease of the density scale length. Moreover, the deduced poloidal rotation velocity at the q = 3 surface changes from the electron to the ion diamagnetic drift direction with the onset of the 2/1 island. Also a reduction of the density fluctuations is observed as well as a decrease in the poloidal correlation length. From Ar-injection a delayed inward propagation of the emission maximum is observed for different Ar ionization stages when the 2/1 mode is generated.Similar experiments with a slowly rotating DED show that the steepened density gradients are spatially localized and confirm the hypothesis that the 2/1 mode is responsible for the observations. PACS : 52.25. Gj, 52.30.Cv, 52.35.Ra, 52.55.Fa, 52.55.Tn

Effect of the dynamic ergodic divertor in the TEXTOR tokamak on MHD stability, plasma rotation and transport

“…Further increasing the DED current, an m/n = 3/1 mode forms, and the plasma even starts to rotate in the opposite poloidal direction (now the corresponding E × B is outward directed). (c) Deducing the radial electric field from a poloidal correlation reflectometer, a similar picture arises [20]. While the initial increase of the DED current hardly causes a change, the onset of the m/n = 2/1 tearing mode results in a change of the sign of E r between the normalized radii of 0.8 and 0.9 (from −4 to +4 kV m −1 ).…”

“…In conjunction with the formation of the m/n = 2/1 tearing mode there are more direct indications for a change of the radial electric field, given by three independent measurements of the edge electric field or components thereof. Details of these experiments are discussed elsewhere [11,20]. Here, only the main results are summarized: (a) the floating potential measured by a rake probe and covering about 10% of the minor radius in the plasma edge clearly shows a change of the radial electric field with increasing DED current and thus ergodization [11].…”

Transport of argon and iron during a resonant magnetic perturbation at TEXTOR-DED

“…The analysis of the discharge with an m/n = 3/1 TM at r/a ≈ 0.8 requires the discussion of the influence of an island on the impurity transport. In plasmas similar to the reference discharge a radial electric field of ≈-50 V cm −1 appears at the q = 3 magnetic flux surface which generates an electron flow towards the wall which disappears after the onset of an m/n = 2/1 TM [20]. One possible explanation is that the m/n = 2/1 TM causes a plasma ion flow towards the wall due to an increase of the particle transport.…”