ASDEX Upgrade Team

Gemisic-Adamov, M.

Horton, L. D.

Kurzan, B.

Meister, H.

Murmann, H.

Neuhauser, J.

Suttrop, W.

English

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Analysis of large and small scale flucuations in the plasma edge of ASDEX Upgrade

Horton, L.

MPG.PuRe

Analysis of large and small scale fluctuations in the plasma edge ofASDEX UpgradeB. Kurzan, M. Gemišić-Adamov, L. D. Horton, H. Meister, H. Murmann, J. Neuhauser,W. Suttrop, ASDEX Upgrade teamMax-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748Garching, GermanyIntroductionLarge and small scale fluctuations of both electron density and temperature are accessible bythe Thomson scattering system on ASDEX Upgrade [1] in the steep edge gradient region of Hmode plasmas: The characteristic structures and spatial distributions of large scale fluctuationswhich exist during and in between Edge Localised Modes (ELMs) were investigated by 2Dsnapshots of the electron density and temperature in the poloidal plane [2, 3].Small scale fluctuations between ELMs are inferred indirectly in the steep gradient region bymeasuring the gradient lengths Lne and LTe of the 1D radial electron density and temperatureprofiles, respectively. The deduced parameter ηe  Lne  LTe was identified as a critical param-eter in theoretical calculations of the fluctuations induced by electrons. Several values for ηewere found on ASDEX Upgrade for different plasma parameters [5].The variation of the mean amplitudes of the large scale fluctuations and of the parameter ηewith plasma parameters for type I ELMy discharges is investigated in this paper.Experimental Setup  z [cm]2.10 2.18R [m]300-30110ICRHantennascatteringvolumesseparatrixflux surfacelimited bydivertor1.40.70.0-0.7-1.40.8 1.7 2.6z [m]R [m]H5Figure 1: poloidal cross sectionThe vertical Thomson scattering diagnostic consistsof a bundle of up to six vertically launched, radiallystaggered Nd-YAG laser beams. The scattered light isobserved from the low-field side in 16 spatial chan-nels. The whole system was shifted radially to mea-sure low field side edge plasma profiles and structuresin the poloidal plane (Fig. 1). The data presented inthis paper were obtained both with the previously usedcharge sensitive analog-to-digital converters resultingin data with larger error bars, and the presently usedtransient recorders and improved data evaluation [4]providing more accurate data.The line-averaged electron density at the plasma34th EPS Conference on Plasma Phys. Warsaw, 2 - 6 July 2007 ECA Vol.31F, P-1.031  (2007)edge including the pedestal shoulder is measured by the DCN interferometer-signal H5.ResultsDischarges with heating powers from neutral beam injection (NI) and ion cyclotron reso-nance heating (ICRH) between 2  5 MW  PNI  PICRH  17 MW and upper and lower triangu-larities of  0  08  δu  0  50, 0  33  δl  0  55, elongations 1  34  κ  1  84, toroidal magneticfields  3 T  Bt  1  9 T and plasma currents 0  8 MA  Ip  1  0 MA were investigated.The probability distribution functions (PDFs) of the large scale fluctuations of both the elec-tron density and temperature between ELMs were investigated in [3] for discharge #20417,which is also included in the above mentioned ensemble: in the middle of the steep gradientregion the PDFs are symmetric and asymmetric both further inwards (more minima), and fur-ther outwards (more maxima). To compare the amplitudes of the large scale fluctuations fordifferent discharge parameters the variance of the normalized electron density, Var  ne 	 ne  ,withne  as the mean value over the time interval of the evaluation, was determined at theradial position in the steep gradient region where its PDF is symmetric. In a plot of the relative2.5 MW ≤ P    + P          ≤ 17 MWNI ICRH0.33 ≤ δ    ≤ 0.55l-0.08 ≤ δ    ≤ 0.50u (a) ∈ [5, 5.5] ∈ [7, 8]∈ [12, 16]NIP     + P          [MW]ICRH0.10 ≤ δ    ≤ 0.39u0.39 ≤ δ    ≤ 0.48l(b)Figure 2: Relative amplitudes of the large scale fluctuations versus normalized edge density fora broad range (a) and a narrow window (b) of heating powers and triangularities.amplitudes of the large scale fluctuations versus the edge line integral H5 normalized to theGreenwald density [6] nGWe  Ip   πa2  with a   0  5 m as the minor radius of the ASDEX Up-grade plasma, a clear increase of the fluctuation amplitudes when approaching the Greenwalddensity is observed (fig. 2a). For a narrow window of heating powers and triangularities a nearlinear increase of Var0  5  ne 	 ne  with the normalized density H5  nGWe is found (fig. 2b).The large scale fluctuations of electron density and temperature are in phase [3]. Thus macro-scopic plasma volumes are apparently moved radially with respect to their positions in the un-perturbed profile by these fluctuations. In a double logarithmic plot of the electron temperaturesversus the electron densities of a plasma edge profile (fig. 3) the radial positions of the measured34th EPS 2007; B.Kurzan et al. : Analysis of large and small scale fluctuations in the plasma edge of ASDEX ... 2 of 4Figure 3: Determination of ηe  Lne  LTe.data are eliminated. In-phase-changes of electrondensity and temperature do not increase the scat-ter in the plot of Te versus ne in contrast to plotsof radial profiles where large scale fluctuationsclearly increase the scatter. Thus the parameterηe  Lne  LTe, which characterizes the small scaleturbulence, can be determined practically indepen-dently of the large scale flucatuations. The ηe pa-rameters were determined in parallel to the ampli-tudes of the large scale fluctuations for the abovementioned set of plasma parameters. No correlation between the ηe data and the amplitudesVar  ne 	 ne  of the large scale0.34 ≤ δ    ≤ 0.51u0 ≤ δ    ≤ 0.17u0.17 ≤ δ    ≤ 0.34u-0.1 ≤ δ    ≤ 0u(a)0.33 ≤ δ   ≤ 0.39l0.39 ≤ δ   ≤ 0.44l0.44 ≤ δ   ≤ 0.50l0.50 ≤ δ   ≤ 0.55l(b)Figure 4: ηe versus δl for several ranges of δu (a), and versus δu for several ranges of δl (b).Figure 5: ηe versus κ .fluctuations or the applied heating power was ob-served up to now. There is however a correlationof ηe with the plasma shape, which is character-ized by the upper and lower triangularities δu andδl and the elongation κ . The dependence of ηe onthese 3 parameters is presented here as projectionsof the parameter space into the 2D planes  ηe  δl (fig. 4a),  ηe  δu  (fig. 4b) and  ηe  κ  (fig. 5). Theerror bars of ηe are indicated for all data points infig. 4 and 5. The data with the larger error bars wereacquired with the charge sensitive digitizers used earlier. Both with increasing lower and uppertriangularities δl , δu the value of ηe is increasing. Note that for δu  0 the parameter ηe has34th EPS 2007; B.Kurzan et al. : Analysis of large and small scale fluctuations in the plasma edge of ASDEX ... 3 of 4a minimum, ηe  1  3 (fig. 4b). At this minimum value also the elongation κ has the smallestvalue in this ensemble of data points (fig. 5). For decreasing negative upper triangularities δuthe parameter ηe is increasing again (fig. 4b).DiscussionThe increase of the amplitudes of the large scale fluctuations in the steep gradient region,Var  ne 	 ne  , with increasing edge density, normalized to the Greenwald density, H5  nGWe ,may indicate that these fluctuations are produced when the plasma edge approaches the densitylimit set by the plasma current. Interestingly there is no threshold value for the first appearenceof the large scale fluctuations near the density limit,  H5  nGWe  th  1, but their amplitudesstart to be visible at H5nGWe  0  3, which is considerably away from the density limit. Theamplitudes of the fluctuations are rising when approaching the density limit.No correlation between the amplitudes of the large scale fluctuations, Var  ne 	 ne  , and theparameter ηe was found so far. This may indicate that the small scale fluctuations parametrizedby ηe are of different origin than the large scale fluctuations. The minimum value ηe  1  3was observed for a plasma shape that in the ensemble of investigated plasma shapes is closestto circular (κ   1  34, δu  0, δl  0  34). With increasing elongation κ , or absolute values ofthe triangularities also increased ηe values are observed. The maximum value of ηe  2  6 isobserved for an elongated triangular plasma shape (κ  1  71, δl  0  53, δu  0  34). For largervalues of the parameters κ , δl , δu the parameter ηe may be constant at around ηe  2  1  0  4.These results are at least qualitatively in agreement with theoretical calculations of toroidalelectron temperature gradient modes [7], where also an increase of ηe with increasing elonga-tion κ and triangularity δ , or its radial gradient dδdr was found, although to a smaller extent.The data base will be extended to further investigate the trends found so far.References[1] H. Murmann et al., Rev. Sci. Instrum. 63 4941 (1992)[2] B. Kurzan et al., Phys. Rev. Lett. 95, 145001 (2005)[3] B. Kurzan et al., Plasma Phys. Control. Fusion 49, 825 (2007)[4] B. Kurzan et al., Plasma Phys. Control. Fusion 46, 299 (2004)[5] J. Neuhauser et al., Plasma Phys. Control. Fusion 44, 855 (2002); A. Kallenbach et al.,Proc. of the 19th IAEA Conference Fusion Energy, Lyon, France, October 2002, paperEX/P4-05; L. D. Horton et al., Nucl. Fusion 45 856 (2005)[6] M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)[7] F. Jenko et al., Phys. Plasmas 8, 4096 (2001)34th EPS 2007; B.Kurzan et al. : Analysis of large and small scale fluctuations in the plasma edge of ASDEX ... 4 of 4

http://hdl.handle.net/21.11116/0000-000A-3AAB-5

Analysis of large and small scale flucuations in the plasma edge of ASDEX Upgrade

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