Grulke, O.

Klinger, T.

Windisch, T.

English

MPG.PuRe

Intermittent radial transport in the linear VINETA deviceT. Windisch, O. Grulke, T. KlingerMax-Planck Institute for Plasma Physics, EURATOM Association,D-17491 Greifswald, GermanyMotivated by the observation of particle and energy losses perpendicular to the magneticfield in fusion devices, the fluctuation induced cross-field plasma flow into the far scrape-offlayer (SOL) has been subject of intense investigation in thelast two decades [1, 2]. As a re-sult of this transport, the formation of a pronounced shoulder in the radial density profile andenhanced recycling of plasma particles at the first wall is observed [3], which has a strong im-pact on the divertor efficiency. A characteristic feature ofthis convective particle flux in theSOL are intermittent density bursts, characterized by a non-Gaussian amplitude probabilitydistribution function (PDF), which are ascribed to large-amplitude turbulent structures, calledblobs, propagating radially outwards with a velocity of typically v ∼ 10 % of the ion soundspeed [2]. It was shown that blobs contribute considerably to the radial transport in the SOL[4]. Common models addressing the radial blob propagation inthe SOL rely mainly on cur-vature of the magnetic field, which causes either polarization of the blobs [5] or gives rise toelectrostatic interchange dynamics [6]. However, intermittent density fluctuations in the plasmaedge and the formation and radial propagation of spatiotemporal structures have recently alsobeen observed in linear devices with homogeneous magnetic fild geometry [7]. In this pa-per we characterize the formation and propagation of turbulent structures in drift wave turbu-lence in the linearly magnetized VINETA device [8]. As a general feature of helicon sources arather dense plasma (npeak≤ 2·1019m−3) at low electron temperatures (Te ∼ 3eV) is produced.0 50 10000.51r  /mmdensity  /1018m-3101520plasma potential  / Vdensityplasma potentialA B CFigure 1: Time averaged plasma density andplasma potential profiles.The results presented in the present paperwere obtained in an Argon helicon plasmawith a neutral gas pressure ofp = 0.1Pa, amagnetic field ofB = 60mT induction, anda RF−power ofPr f = 1.8kW. Typical valuesfor the drift scaleρs and for the peak plasma-βpeakareρs = 18mm andβpeak= 1%, respec-tively. Time averaged radial profiles of theplasma density density and the plasma poten-tial as measured with rf-compensated Lang-muir probes are shown in Fig. 1. Both, the33rd EPS Conference on Plasma Phys. Rome, 19 - 23 June 2006 ECA Vol.30I, P-2.121  (2006)-202-40480 5 10 15 20-40480101-5 0 501-500-500100101102-500time /ms f /kHzPDFS /dBn /s~PDFPDFS /dBS /dBn /s~n /s~n /s~a1b1c1 c2a3b2a2b3c3Figure 2: Plasma density fluctuations normalized to standard eviation, respective probabilitydistribution function (PDF) and frequency power spectrum atradial positions A (a.1-a.3), B(b.1-b.3) and C (c.1-c.3) (cf. Fig. 1).plasma density and the plasma potential profile are characterized by an exponential shape withdifferent 1/e–folding lengths (46 mm for plasma density and and 169 mm for plasma potentialprofile). Plasma density fluctuations are measured with uncompensated Langmuir probes. Theprobes are operated in the ion saturation regime at a probe bias ofU = −100 V with respect tothe plasma vessel and fluctuations of the ion saturation current are taken as directly proportionalto plasma density fluctuations (electron temperature fluctuations are assumed to be negligible).To characterize the evolution of plasma density fluctuations across the radial plasma densityprofile single-point measurements at three different radialpositions were done. The positionsare indicated in Fig. 1 and in the following are referred to aspo itions A, B and C. Position A isat the position of the maximum radial density gradient, positi n B is in the plasma edge, wherethe density decreases to 7% of the peak density, and in the farplasma edge at position C, thedensity decreased to 4% of the peak plasma density. In Fig. 2 measurements of time series ofdensity fluctuations normalized to standard deviationσ , the respective probability distributionfunction (PDF), and the frequency power spectrum at those three positions A, B, and C areshown. In the radial density gradient region (pos. A) coherent d sity fluctuations of am = 1drift wave mode with relatively small maximum amplitudes (< 2σ) dominate the time series(Fig. 2.a1). This is supported by the peak in the frequency power spectrum (Fig. 2.a3) at a fre-quency of 4.1kHz. For higher frequencies above 8kHz a power law decay is observed in thespectrum. The PDF is symmetric and double-humped. In the plasma edge at (pos. B) the den-sity fluctuations display positive density bursts with maximum amplitudes of 4σ (Fig. 2.b1),33rd EPS 2006; T.Windisch et al. : Intermittent radial transport in the linear VINETA device 2 of 4x  /mmy  /mm-150 -100 -50 0-50050x  /mm-150 -100 -50 0x  /mm-150 -100 -50 0x  /mm-150 -100 -50 0-202t=-80 µs t=-40 µs t=0 µs t=40 µsamplitudeFigure 3: Conditional average (CA) of density fluctuations forfour different time lagsτ in theazimuthal plane. The amplitude condition applied to the refrence probe, which is located in theplasma edge (r = −80mm) was chosen to p=3σ . The CA amplitude is normalized to standarddeviation. The black dashed line indicates the azimuthal direction, the black solid line indicatesthe position of the maximum radial density gradient.which result in a skewed PDF having a pronounced positive tail. The occurrence of positivedensity bursts is not completely random in time but has a characte istic frequency, which leadsto a broadened peak in the frequency power spectrum at a frequency of∼ 1.5kHz (Fig. 2.b3).The intermittent character of density fluctuations is even more pronounced the far plasma edge(pos. C, Fig. 2.c), where the normalized maximum amplitude ofthe positive density bursts is7σ . The corresponding PDF is strongly skewed and peaked with values of the higher order mo-ments skewnessS and kurtosisK of S = 2.9 andK = 11. The amplitude of the positive densitybursts is comparable to the density fluctuations in the maximum radial gradient region, whichindicates that the the large fluctuations in the far plasma edge originate in the density gradientregion.To gain insight into the spatiotemporal dynamics in the azimuthal plane we arranged 16 sin-gle probes as a vertical array with a spacing ofd = ρs/2= 9mm. The array scans in consecutivemeasurements the azimuthal cross sections. The spatiotempral evolution of the large-amplitudeplasma density fluctuations in the azimuthal plane is reconstructed by use of the conditional av-erage (CA) technique [9]. The reference probe for this technique s located in the plasma edgeat position B and also measures density fluctuations. The conditi for the CA analysis is anamplitude threshold condition ofp = 3σ ±5% at falling slope to extract the coherent part ofthe positive large-amplitude fluctuations. The results of the CA procedure is shown in Fig. 3for four different time lagsτ . A coherent turbulent structure can be observed, which propa-gates mainly azimuthally in the direction of the backgroundE×B-drift (counter-clockwise inthis representation). The turbulent structure is slightlydistorted with an azimuthal correlationlength oflcorrθ ≈ 70mm = 3.8ρs and a radial correlation length oflcorrr ≈ 50mm = 3.2ρs. How-33rd EPS 2006; T.Windisch et al. : Intermittent radial transport in the linear VINETA device 3 of 4ever, the propagation of the coherent structure is not purely in azimuthal direction (indicated bythe dashed lines in Fig. 3) but has also a radial component. Toquantify the structure’s propaga-tion behavior the trajectory of the center-of-mass of the coherent structure was tracked in timefrom τ = −40µs to τ = 40µs, which yields a constant azimuthal velocity ofvθ = 760ms−1.This velocity results in an azimuthal turn-over time of the struc ure, which corresponds to afrequency of 1.3kHz and agrees with the burst frequency of the single-pointmeasurement (cf.Fig. 2.c1). The obtained radial velocity isvr = 230ms−1 ≈ 10%cs, which is much smaller thanthe azimuthal velocity.In conclusion the radial propagation of turbulent structures is not intrinsically tight to a cur-vature of the magnetic field but is also observed in linear magnetic field geometry. In azimuthaldirection the propagation speed of turbulent structures isdetermined by the backgroundE ×Bdrift. In radial direction structures propagate self-consistently with velocities, which are muchsmaller than in azimuthal direction. Thus, the picture emerges that turbulent structures spiral ra-dially outwards, thereby considerably contributing to radial fluctuation induced transport. Thedetails of the underlying mechanisms leading to radial propagation is subject of further inves-tigations. As was suggested by Krasheninnikov and co-workers [10] vertical polarization ofdensity fluctuation structures in linear magnetic geometrycan be caused by a gradient in thevelocity of the neutrals, the so-called neutral wind. For the present plasma conditions a radialvelocity caused by neutral wind ofvnwr = 270ms−1 is obtained, which is comparable with ourexperimental findings.References[1] P. W. Terry, Rev. Mod. Phys.,72, 109 (2000)[2] S. J. Zweben, et al., Nucl. Fusion,44, 134 (2004)[3] B. LaBombard, et al., Nucl. Fusion,40, 2041 (2000)[4] J. A. Boedo, et al., Phys. Plasmas,8, 4826 (2001)[5] S. I. Krasheninnikov, Physics Letters A,283, 368 (2001)[6] O. E. Garcia, V. Naulin, et al., Phys. Rev. Lett.,92, 165003 (2004)[7] T. A. Carter, Phys. Plasmas,13, 010701 (2006)[8] C. M. Franck, O. Grulke, T. Klinger Phys. Plasmas,10 323 (2003)[9] R. J. Adrian, Phys. Fluids,22, 2065 (1979)[10] S. I. Krasheninnikov, A. I. Smolyakov, Phys. Plasmas,10, 3020 (2003)33rd EPS 2006; T.Windisch et al. : Intermittent radial transport in the linear VINETA device 4 of 4

Intermittent radial transport in the linear VINETA device

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Intermittent radial transport in the linear VINETA device

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