Influence of the Static Dynamic Ergodic Divertor on Edge Turbulence Properties in TEXTOR

Abstract

Systematic measurements on the edge turbulence and turbulent transport have been made by Langmuir probe arrays on TEXTOR under various static Dynamic Ergodic Divertor (DED) configurations. Common features are observed. With the DED, in the ergodic zone the local turbulent flux reverses sign from radially outwards to inwards. The turbulence properties are profoundly modified by energy redistribution in frequency spectra and suppression of large scale eddies. The fluctuation poloidal phase velocity changes direction from electron to ion diamagnetic drift, consistent with the observed reversal of the E r B flow. In the laminar region, the turbulence is found to react to an observed reduced flow shear. [4] have demonstrated that an ergodized magnetic boundary can be effective to optimize the plasma-wall interaction. Meanwhile, the local effects of the magnetic ergodization on edge turbulence and turbulent cross-field transport have also been studied both experimentally [1,5,6] and theoretically [7,8]. It has been observed on TEXT [1] and Tore Supra [2,5,6] that in the ergodic divertor (ED) configuration the edge density fluctuations are decreased whereas the turbulent cross-field diffusivity is less affected. However, a systematic investigation of the turbulence properties, such as frequency and wave-number spectra and the fluctuation propagations, has not been made or was done for a reduced set of wavenumber values [5]. A distinct description in the ergodic and laminar zones was also not given.Recently, on the tokamak TEXTOR the Dynamic Ergodic Divertor (DED) [4] has been installed at the high-field side of the torus (R=a 1:75=0:47 m), contrary to other machines where the ED coils were mounted at the low-field side [1][2][3]. The DED consists of 16 perturbation coils oriented parallel to the field lines on the magnetic flux surface with a safety factor q 3. With different current distributions in the coils, the base poloidal/toroidal modes, m=n, can be adjusted as 12=4, 6=2, and 3=1. The penetration depth into the plasma depends on m: In 12=4 the influence is restricted to the plasma boundary, while in 3=1 it can reach much deeper. In the outer plasma layer, DED induces stochastization of the magnetic field lines, including an ergodic zone with long and a laminar zone with short connection lengths to the wall [9]. In this Letter, we present the first systematic measurements by Langmuir probe arrays on the edge turbulence properties and fluctuation-driven transport in the presence of various static DED configurations (dc current on the coils).To get effective impacts of the DED at the plasma boundary, the discharge conditions have been optimized as follows: For m=n 12=4, I p 250 kA, B T 1:4 T, R=a 1:73=0:46 m, dc DED current I DED 12 kA; for 6=2, I p 270 kA, B T 1:9 T, R=a 1:73=0:46 m, I DED 6 kA; and for 3=1, I p 250 kA, B T 1:9 T, R=a 1:75=0:48 m, I DED 1 kA. The I DED is applied during the stationary phase of the Ohmic discharge. In all cases, no external tearing modes are excited, the lineaveraged plasma densit...

Helical modulation of the electrostatic plasma potential due to edge magnetic islands induced by resonant magnetic perturbation fields at TEXTOR

“…In particular, a region of large positive E r , along the LCFS, can be noticed. This is a confirmation of the well-known presence of a positive E r in the stochastic edge of a tokamak [22,23,38,63,64,65,66,67,68]. But, if we focus on this region, we can note also a modulation in the poloidal angle, strictly linked to the magnetic topology, too: E r has a minimum in between the XP and the OP, and an absolute maximum in correspondence of the XP.…”

Changes in particle transport as a result of resonant magnetic perturbations in DIII-D

“…19 particle flux as a result of RMPs was performed in low confinement L-mode. 20,21 The only work on fluctuation changes as a result of RMPs in H-mode was performed on DIII-D. 19,22,23 Moyer et al 22 showed at high collisionality that although magnetic fluctuations increase, there is no change to the radial electric field. However for low collisionality, the radial electric field in the pedestal area becomes more positive and at the same time the far infrared (FIR) coherent scattering (a non-localized measurement) shows that there is an increase in turbulence in the plasma edge.…”

Impact of large island perturbations on turbulent blob transport in tokamaks

“…density, can be applied to a possible suppression of instabilities due to impeding ١ ʈ n = 0. Finally it should be mentioned that in experiments 4 the turbulent E ϫ B-flux is not only reduced, it actually changes sign at least at certain locations.…”

“…The interest in three-dimensional turbulence simulations including complex magnetic topologies like open field lines, islands, and ergodic structures have been stimulated further by recent observations at DIII-D, 1,2 JET, 3 and at TEXTOR-DED. 4,5 There it has been proved that resonant magnetic perturbation fields have a beneficial effect for the control of bursty particle and heat transport. Typically the radial transport of particles and heat in the tokamak scrapeoff layer ͑SOL͒ is not smooth and steady but rather exhibits an intermittent, bursty behavior which occurs due to the presence of the last closed flux surface ͑LCFS͒, i.e., a boundary where an abrupt change from closed to open magnetic field lines occurs.…”