ASDEX Upgrade Team

De Pena Hempel, S.

Dux, R.

Kallenbach, A.

Meister, H.

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Low-Z-Impurity Transport Coefficients at ASDEX Upgrade

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T401Low-Z-Impurity Transport Coefficients at ASDEX UpgradeS de Pefia Hempel, R Dux, A Kallenbach, H Meister1 ASDEX Upgrade TeamMP1 fl'ir Plasmephysik, EURATOM Association, D—85748 Garching, GermanyIntroductionImpurity transport studies were conducted in ASDEX Upgrade by injecting short puffsof He or N6 gas during an otherwise steady—state phase of a discharge. Charge exchangerecombination spectroscopy (CXRS) measurements were used to monitor the temporalevolution of the radial impurity profiles subsequent to these puffs. Following the tem-poral evolution of the profiles the diffusion coefficient D and the convective velocity 1)were determined. The radial profiles and parametric dependences of the particle transport coeflicients are analysed in different confinement regimes. Furthermore the dynamicbehaviour of the transport during sawtooth—free discharge periods and ELMs is studiedand compared with model predictions.Determination of local transport coefficientsSpatially and temporally resolved impurity densities in the core plasma of the ASDEXUpgrade tokamak are obtained in neutral beam heated discharges from CXRS measurements with a set of 14 Viewing lines of sight. The transport coefficients are determined byfirst inferring the particle flux F from the measured temporal evolution of the impuritydensity profile by inverting the continuity equation in the source free region. The assumption of zero sources is only accurate inward of the region of ionization at the plasmaedge (ppd < 0.8 for Ne and ppol < 0.95 fer He). The impurity fluxes are described by theextended Fick’s Ansatz mm) = "D[r)Vn(r,t) + v(r)7z(r,t) which assumes the particleflux to be made up of a diffusive flux and an unspecified convective flow which incorpc»rates any additional terms not dependent on the density gradient. Thus, the normalizedfiux % at a particular minor radius is an offset linear function of the density scale length2?, consistent with expectations for a trace impurity and time—independent transport co~efiflcients. From such a curve the diffusion coefiicient is determined by the absolute valueof the slope of the linear regression to the data and the convective velocity by the y—intercept. To overcome the instrumental noise in the experimentai data, which gives themain contribution to the uncertainty in the determination of the transport coefficients7smooth functions are used to fit the temporal evolution and radial profile.Dependenees and scaling laws for the impurity transport coefficientsEmpirical scaling laws for the particle transport coefficients were determined by extendingthe previously described flux analysis to 74 different neutral-beam-heated L—, H- andCDH—mode discharges covering the parameter range: 1.89 < BrnrlT] < 2.691 1.99 <Rodd/1W] < 9.131 4.49 < m [1019 :33] < 11.75 und 1.15 < mealtime] < 1.90. Thevariation range in the plasma current was too narrow to examine any dependence onthis parameter and so the evaluation of the scaling was restricted to discharges withIF = 1MA. In addition the effect of possible collinearities with the magnetic field couldbe eluded, Furthermore, only He and Ne pulses during otherwise steady-state phases ofa discharge Were analysed. This was accomplished by allowing a maximal excursion of10% in the time evolution of the line-integrated electron density, neutral injection heatingpower and energy confinement time.1402In the plasma core region a nonlinear regresasion of the diffusion coefficient yieldsDiPp l = 0 2) = 0 293 . Bfi—msaiaoas . Rotivsionze ‘ m—fpsseiomv(l)0 ' . 01' 0 e »irrespective of the discharge confinement mode. Within the uncertainties in the determi-nation of this scaling no significant dependence on the injected impurity species or theelectron density is found. In addition, the difiusion coefiicients are determined to varyroughly as the square root of the total heating power. They exhibit a marked depen—dence on the background plasma species and decrease with the toroidal magnetic fieid.Figure 1a) demonstrates the good reproduction of the measured diiiusion coefficients bythe scaling law of equation 1.1n the outer plasma region the inferred parameter dependencies show the same tendenciesas found for the plasma core, However the errors of the exponents in this region arecomparable to the exponents themselves and are therefore of no sufficient quality todetermine the dependences exactly. Individual regressions for the different confinementregimes lead to similar results. In good agreement with the results of gas puff or gasmodulation experiments in other machines, the diffusion coefficient for stationary plasmaconditions is one or two orders of magnitude larger than the prediction of the neoclassicaltransport theory over the complete radial range and has a hollow radial profile.in m‘ls 12 m inward value“! m min DIIflJsIon maniuam vs mmsmw valp’ - as ' "’ to f” + 9; ‘ 7‘ 9“ /o° r2 vansport dinersncs //7;“ ” hidden In the 4? g I! 'oo] Efi_wnflmmor§’:fi _102‘ > ° ° 3433 r ‘ 3 c4.0-cxno/e l ,>3 + C-u5 o2 ‘ /// meim- 5., p“ "M iu if / '-o in 0.43 0.74 2 .a 2 u 4 0.6 n 2 4 s a 10 u 1: 4 c 4; .m 4:D my vm V [M]Figure l: (1: Measured central diffusion coeflicient versus the prediction of the scaling lawof equation I. b: [Measured pinch velocity versus scaling law at the plasma edge. c: Inwardpinch velocity versus diffusion coeflicient at ppol = 0.8.The inward convective velocity on the contrary only exceeds the neoclassical value at radiilarger than ppol ~ 0.6, where it shows a strong correlation with the diffusion coefficient(figlc), suggesting that a common mechanism gives rise to both transport coefficients.For ppol 2 0.6 the scaling of both transport coefficients exhibits similar dependences ashave been put forward for the diffusion coefficient in the core region (tabular in fig.1c),however, there is a marked influence of the confinement regime 0n the impurity transportbehaviour. This cannot be explained by any parameter dependence and is hidden in theconstant scaling factor. Due to its large relative error no parameter dependences couldbe inferred for the pinch velocity in the central plasma.A comparison of the impurity transport coefficient with published results for the electrontransport {12] points out a good agreement in the absolute magnitude and radial profileshape of the coefficients as well as a clear parallelism concerning the analyzed parametricdependencies. Further on a satisfactory correlation with the scaling laws for the energyand the angular momentum confinement time [3] is found,Instationary accumulation phenomena in sawtooth-free discharge periodsThe impurity transport coeflicients in the previous paragraph were determined in saw—1403toothing discharges and represent sawtooth averaged values. For instationary accumu-lation phenomena during sawtooth-free discharge periods the impurity transport in theplasma center shows a different behaviour. The suppression of sawteeth observed in someradiating boundary CDH-mode discharges is accompanied by the development of densitypeaking [4] and a slightly enhanced energy confinement. Simultaneous measurement andevaluation of the carbon and neon CXRuspectra in addition with Zagrprofile measurementsfrom the deconvolution of the line integrated bremsstrahlung background radiation showsthat although these phases exhibit a central accumulation of neon and a strong peakingof the soft x—ray and Zea-profiles the carbon density profile evolution remains unchanged.The transport analysis in these sawtooth-free discharge periods leads to a diffusion coef—ficient that reduces to neoclassical values for poloidal fluxes smaller than ppnl ~ V2913; so0.65. For larger radii the diffusivity is not affected by the sawtooth collapse. This is con-sistent with the picture that the sawtooth induce a strong convective particle flow insideand outside the q=l—surface. Otherwise, the measured pinch velocity is not significantlyaltered by 'this process.A self consistent modelling of the impurity transport and radiation with the radial im—purity transport code STRAHL from the given electron and impurity densities [5] shovvsthat for discharges without sawteeth the central (ppal < 0.6) transport can be describedby neoclassical terms. This modelling, however, predicts also a small but not observedpeaking of the carbon density. Such an inconsistency may be due to the neglected frictionbetween the impurity species.Dynamic transport behaviour during type I ELM’SDue to the limitations of the detection system regarding its minimal exposure and readouttime, the dynamic ELM-behaviour has to be investigated by a special mapping proce—dure to fasten the effective temporal resolution of the CCD system. This is achieved byanalysing steady state discharge periods and considering each ELM as the consequenceof the repetition of the same event. Using the maxima of the characteristic Ha-Bignal asthe indicator for each ELM a time base can be constructed, which allows the projectionof the density evolution measured in such a phase (up to 200 ELM’S) onto one uniqueevent. To obtain the real temporal behaviour of the density over an ELM the mappedsignal is deconvolved with the CCD~exposure function. The study of the edge transportbehaviour is restricted to helium, because the described analysis is only accurate inwardof the ionisation region of the observed impurity.The ELM’s are described by an enhancement of the particle diffusion coefficient fromD E l 1?; up to Dmax m 6 1"; in combination with an outward directed convective flow witha pinch velocity in the order of +15 1} [fig.2b). However, the contributing of each termto the total particle flow is ambiguous. The rise of the transport coefficients is localizedin a N 10 cm wide layer inside the separatrix. The given error bars include the statisticaluncertainity in the evaluation of the transport coefficients from the F}°"“~8n‘§°"“—plots.The density relaxation after the ELM collapse (dashed lines) is found to be well describedby the same set of transport coefficients as derived from gas puff experiments (dottedlines). The pronounced decrease of v in the region outside ppol = 0.95 is possibly dueto not neglegible source terms in the continuity equation, The Hrs—transport coefficientsderived for this region should therefore be treated with care.The transport coefficients are compared with measurements of the effective electron diffu-sion coefficient and heat conductivity (fig.‘2c). The time evolution, the absolute magnitudeand the localization of the different measurements is found to be in good qualitative and1404n a) dumb" m "as 20 a) madlvu vslodty in me 13mm dflhmkm In mt:D r g . , , ‘ , r"2010cm T]“W” 1-2:m // 15’ 0:3 (naawp’\\ji\:];iT irflL‘NEl-Ml . \ ~ ‘,0 with: 5”6035 D.Mag/meters. ao Wb-1.|1m.n'-T7-1V V],0.6 0.7 0.5 0.5 1.0PM PW p”Figure 2: Measured difi'usion coefficient a) and pinch velocity 6) for an ELM. c) shows acomparison of the eflective He-dzflusivity with values for the electron, diffusion coeficieniand heat conductivity.quantitative agreement (Diff: 73 D? m xfifi), suggesting that a strong link exists betweenthermal, electron and impurity transport during ELMs, The driving mechanism shouldtherefore be of convective and ambipolar nature.The predictions of model calculations based entirely on parallel particle fluxes along stc»chastic magnetic fields during magnetic ELM precursor activity correspond quite well tothe experimental data of COMPASS—D [6] and ASDEX Upgrade [7]. The magnetic perwturbation is assumed to be created by helical perturbation current filaments which aremodelled to match the phase and amplitude of the ELM precursor signal measured bythe Mirnov diagnostic. Subsequent field line tracing shows an ergodization between islandchains and in a radial transport enhancement due to parallel particle motion along theradially excursing field lines. However the achieved good quantitative agreement betweenmodel and measurement has to be treated with care because the calculated heat con—ductivity is based on the transport formula of RechestenRosenbluth [8] which describes aconductive and non ambipolar process. This is inconsistent with the experimental results,which exhibit almost equal thermal and particle transport rates. Nevertheless, an exten—sion of the former model including nonlinear bunching of the electrons [9] shows that theedge electron heat flux can be convective and ambipolar constrained and described by aRechester-Rosenbluth diffusivity, but with the slow ion thermal velocity as the streaming2 '2 r . . 4factor X1 = 01.1%a (77:?) . The determination of the fluctuation phase veloc1ty u = fwill be pOSSible with the new extended magnetic coil arrangement at ASDEX Upgradeand a quantitative comparison of the modelling results to the experiment is expected.References[1] J O’Rourke at 111., Plasma Physics And Controlled Fusion 35:585—594, 1993[2] J—L Lachambre at (11., Plasma Physics And Controlled Fusion 3819434966, 1996[3] H Meister, Diploma Thesis, Universitit Augsburg, 1996[4] A Kallenbach at £51., Plasma Physics And Controlled Fusion 38:2097—2112, 1996[5] R Dux at (21,, Proc. of the 23rd EPS, Kiev, 1: 25—28, 1996[6] R Buttery et at, Proc. of the 22nd BPS, Bournemouth, III: 273—276, 1995[7] H Reimerdes, IPP Report 1/3007 1996[8] A B Rechester and M N Rosenbluth, Physical Review Letters (40)l:38~41,1978[9] P W Terry 8t (11., PhysrPlasmas, 3(5):1999—2005,1996

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Low-Z-Impurity Transport Coefficients at ASDEX Upgrade

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