Herrmann, A.

Horton, L.

Konz, C.

Kurzan, B.

Maraschek, M.

Murmann, H.

Müller, H.

Neuhauser, J.

Schmid, A.

English

MPG.PuRe

Thomson scattering analysis of large scale fluctuations in the ASDEXUpgrade edgeB. Kurzan, H. D. Murmann, C. Konz, L. D. Horton, A. Herrmann, M. Maraschek,H. W. Müller, J. Neuhauser, A. Schmid, ASDEX Upgrade TeamMPI für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching, GermanyIntroductionThe expulsion of heat and particles by magnetic field aligned filaments during type-I EdgeLocalized Modes (ELMs) was found at the low field side of ASDEX Upgrade in the steepgradient region and scrape-off layer (SOL) by 2D-Thomson scattering [1], in the far SOL byLangmuir probes [2, 3], and on limiter and divertor structures by 2D-thermography [4, 5]. With2D-Thomson scattering it is now also possible to analyze quantitatively the large scale fluc-tuations of electron density and temperature in the steep gradient region at the plasma edgebetween ELMs. Their toroidal quasi-mode numbers are between 6 and 50 and their fluctuationlevel can reach up to 100%.Experimental Setup 110ICRHantennascatteringvolumesseparatrixflux surfacelimited bydivertor1.40.70.0-0.7-1.40.8 1.7 2.6z [m]R [m] z [cm]2.10 2.18R [m]300-30Figure 1: poloidal cross sectionThe vertical Thomson scattering diagnostic con-sists of a bundle of up to six vertically launched,radially staggered Nd-YAG laser beams. The scat-tered light is observed from the low-field side in16 spatial channels with one optical detection sys-tem per channel only. This arrangement forms avertically stretched (R, z)-matrix of scattering vol-umes in a poloidal cross section. The whole sys-tem was shifted radially to measure low field sideedge plasma profiles and structures in the poloidalplane (Fig. 1). A 2D-snapshot of electron densityand temperature structures is obtained by firing theradially staggered lasers one after the other within2 µs, which defines the ‘exposure time’. The pa-rameters of the discharges #20414 to #20420, which are investigated in this paper, are: toroidalmagnetic field Bt =  2:7 T, plasma current Ip = 1:00 MA, safety factor q95 = (5  5:2), line-averaged density ne = (6:4  8:4) 1019 m 3, heating power by neutral beam co-injectionPNBI = (5  10) MW, and by ion cyclotron resonance PICRH = (1:3  1:9) MW, elongation33rd EPS Conference on Plasma Phys. Rome, 19 - 23 June 2006 ECA Vol.30I, P-2.132  (2006)κ = 1:76 1:84, upper triangularity δu = 0:13 0:18, lower triangularity δl = 0:37 0:38.Resultsx xOOOOa bout inFigure 2: ne and Te contours (a) and fluctuation structures ∆ne, ∆Te (b)with ntor = 13:2 between ELMsThe 2D-structuresof electron densityand temperature, ne,Te, are plotted in (R,z)-space relative toRout, the outermostposition of the sep-aratrix on the lowfield side, as deter-mined by magnetics(Fig. 2a). In plots ofthe fluctuations ∆ne,∆Te, where from asingle snapshot themean over 20 snap-shots is subtracted,small amplitude fluctuations between ELMs come out clearly (Fig. 2b).Figure 3: scaling of ∆ne=hneipol withhneiH5=(dne=dR)Assuming that the observed structures arealigned to the magnetic field, the toroidalquasi-mode number is here with [1] ntor =13:2. The plasma was shifted radially sothat with 2D-Thomson scattering a radial re-gion between 0 and 3 cm inside the sep-aratrix, covering the steep gradient region,could be investigated. The width of thesteep gradient region varies during ELMand inter ELM phases. This is quantified byhneiH5=(dne=dR), where hneiH5 is the elec-tron density line-integral H5 of the DCN in-terferometer, normalized to the integrationpath length inside the separatrix. The H5 line-integral includes the pedestal shoulder. dne=dRis the radial gradient of the electron density, determined by Thomson scattering. The relative33rd EPS 2006; B.Kurzan et al. : Thomson Scattering Analysis of Large Scale Fluctuations In the ASDEX Upgr... 2 of 4density fluctuations ∆ne=hneipol, where hneipol is the poloidally averaged background plasmadensity, determined by Thomson scattering, generally decrease with increasing width of thesteep gradient region both for ELM and inter-ELM phases (Fig. 3). No clear difference be-tween the two phases is found. The data points (symbols ‘+’) are over-plotted here and in thefollowing figures by the contour lines of a histogram function f indicating the frequency ofthe data points. For both ELM and inter-ELM phases the relative density fluctuations decreasea bFigure 4: scaling of ∆ne=hneipol with ntor (a), and of ntor with hneiH5=(dne=dR) (b)with increasing toroidal quasi-mode number (Fig. 4a). For low toroidal quasi-mode numbers,ntor < 10 the relative fluctuation amplitudes approach 1, while for the largest observed modenumbers, ntor = 40  50 the relative fluctuation amplitudes are below 0.25. For a given wavenumber practically all relative fluctuation levels up to an upper boundary are observed, indi-cating that large scale fluctuations in the growing and saturated state are detected. The upperboundary of the observed toroidal mode numbers decreases with increasing widths of the steepgradient region (Fig. 4b). The most frequent mode number is around ntor  12. For widths ofthe steep gradient region, hneiH5=(dne=dR) 3 cm, practically all toroidal wave numbers largerthan ntor = 6, which is the resolution limit of the diagnostic, up to an upper limit are excited.For larger widths of the steep gradient region, hneiH5=(dne=dR)  3 cm, the lower boundaryof the observed ntor increases and the upper boundary decreases. The radial distributions of therelative fluctuations ∆ne=hneipol are overplotted by histogram functions, which are normalizedto 1 for each radial cell (0:5 cm wide), and for each radial cell the mean values of the relativefluctuations were calculated (Fig. 5). For inter-ELM phases the relative fluctuations ∆ne=hneipolextend between 0.2 cm and 2.7 cm inside the separatrix (Fig. 5a). The maximum relative fluc-tuation amplitude of ∆ne=hneipol  0:85 is found at 1 cm inside the separatrix, while the meanvalues of the fluctuations have a broader maximum between 0.5 cm and 1 cm inside the separa-33rd EPS 2006; B.Kurzan et al. : Thomson Scattering Analysis of Large Scale Fluctuations In the ASDEX Upgr... 3 of 4a bFigure 5: radial profile of ∆ne=hneipol between ELMs (a) and around ELMs (b), the red linesare the mean values for each radial cell, which is 0:5 cm widetrix. Note that the experimentally observed single modes are much narrower than the envelopesshown in Fig. 5. For time points around ELMs the maximum relative fluctuation amplitudesreach values of ∆ne=hneipol  1 (Fig. 5b). The corresponding minima with zero density are then‘holes’, already observed earlier [1]. Near to an ELM large relative fluctuation amplitudes arealso observed at the separatrix. They are, however, difficult to catch, because in only 4 out of theinvestigated 300 time points such modes show up. The fluctuations of the electron temperatureshow in principle the same behaviour as the electron density, but with larger error bars.DiscussionA clear identification of the type of large scale mode explaining the large scale fluctuationsobserved with 2D-Thomson scattering is difficult, because no theoretical model exists up tonow, which can explain all the details found: The fluctuation level between ELMs can get veryhigh, ∆ne=hneipol  0:8 1, but no enhanced loss of particles and energy is observed as duringan ELM, as seen in the Dα -signals of Fig. 2. The fluctuation amplitudes get larger with narrowerwidth of the steep gradient region, i.e. steeper gradients (Fig. 3) and have their maximum in thesteep gradient region (Fig. 5) indicating that the observed fluctuations should be gradient driven.References[1] B. Kurzan et al., Phys. Rev. Lett. 95, 145001 (2005)[2] A. Kirk et al., Plasma Phys. Control. Fusion 47, 995 (2005)[3] H. W. Müller et al., Proc. 29th EPS Conf. on Plasma Phys. and Control. Fusion 26B,O-2.06 (2002)[4] A. Herrmann et al., Plasma Phys. Control. Fusion 46, 971 (2004)[5] T. Eich et al., Plasma Phys. Control. Fusion 47, 815 (2005)33rd EPS 2006; B.Kurzan et al. : Thomson Scattering Analysis of Large Scale Fluctuations In the ASDEX Upgr... 4 of 4

Thomson scattering analysis of large scale fluctuations in the ASDEX Upgrade edge

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Thomson scattering analysis of large scale fluctuations in the ASDEX Upgrade edge

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