Ahmad, I.

Barth, C.

Bertschinger, G.

Biel, W.

Dux, R.

Fuchs, G.

Koslowski, H.

Krämer-Flecken, A.

Lehnen, M.

Marchuk, A.

Meiden, H.

Rapp, J.

Unterberg, B.

von Hellermann, M.

English

MPG.PuRe

PROGRESS IN IMPURITY TRANSPORT STUDIES ON TEXTOR USING NEWVUV SPECTROMETERS WITH HIGH TIME RESOLUTIONW. Biel1, I. Ahmad4, C. J. Barth3, G. Bertschinger1, R. Dux2, G. Fuchs1, M. v. Hellermann3,H. R. Koslowski1, A. Krämer-Flecken1, M. Lehnen1, A. Marchuk1, H. J. v. d. Meiden3,J. Rapp1, B. Unterberg1 and the TEXTOR Team1Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM Association,Trilateral Euregio Cluster, D-52425 Jülich, Germany2Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching,Germany3FOM Instituut voor Plasmafysica Rijnhuizen, EURATOM Association, Trilateral EuregioCluster, Postbus 1207, NL-3430 BE Nieuwegein, The Netherlands4Ruhr-Universität Bochum, D-44780 Bochum, GermanyIntroduction and overview. Argon impurity transport in ohmic discharges has been studiedat TEXTOR using new VUV spectrometers with high time resolution (1 ms). In these experi-ments short argon puffs were injected into the discharges and the time evolution of spectros-copic signals from many different ionisation stages was recorded. The time evolution ofnormalised line intensities is used to determine the radial profile of the argon diffusioncoefficient by STRAHL code modelling [1].New VUV spectrometers at TEXTOR. At the limiter tokamak TEXTOR (major radiusR0 = 1.75 m, minor radius a0 = 0.46 m, circular plasma shape) new VUV spectrometers withhigh time resolution have been installed at sightlines located near the horizontal midplane.These systems have been optimised with respect to efficiency and time resolution and each ofthese spectrometers collects 1000 full spectra per second continuously over each TEXTORdischarge. In total a wavelength range of more than one order of magnitude within the VUVrange is covered (see table 1), where all of the first 30 elements of the periodic table havestrong emission lines.VUV spectrometer system wavelength range / nm wavelength resolution / nmSeya-Namioka 57 – 130     0.3SPRED-A 15 – 110     0.4SPRED-B 11 – 33     0.1Table 1: Data of the new VUV spectrometers at TEXTORIn particular, the covered wavelength range comprises intense spectral lines from H-like andHe-like states of low-Z impurities (from He to carbon) as well as strong Mg-like, Na-like, B-like, Be-like and Li-like lines from medium-Z impurities (up to Cu). Thus these spectrometersare well suited to conduct impurity injection experiments.Argon injection experiments. A fast piezo-electric valve was used to inject short argon puffs(FWHM 2 ms) into the flat top phase of a series of OH discharges. Using the new VUVspectrometers, spectral lines from the ionisation stages Ar-VII (58 nm), Ar-VIII (70 nm), Ar-X (17 nm), Ar-XII (15 nm), Ar-XIV (18 nm), Ar-XV (22 nm) and Ar-XVI (35 nm) are mea-sured simultaneously. In the plasma centre, the Ar-XVI and Ar-XVII signals at λ = 0.4 nm arecollected by means of a high-resolution x-ray spectrometer [2] installed in the horizontal mid-plane, while SXR cameras equipped with 100 µm Be foils monitor the central SXR radiationat different sightlines. All spectroscopic signals are recorded with high time resolution(≤ 1 ms). In fig. 1 the measured VUV spectra are shown for different times after an argon puffat t = 3.7 s. In fig. 2 the normalised line intensities are displayed for discharge conditions withmean electron density ne = 3.5⋅1013 cm-3, together with a STRAHL code simulation. In orderto improve the signal-to-noise ratio for the Ar-X, Ar-XII and x-ray (0.4 nm) signals, theexperimental data are averaged over 4 equal discharges (#88709, 10, 27, 28).Fig. 1: VUV spectra measured after argonpuffing in an OH discharge with dischargeconditions IP = 350 kA,  Te(0) = 1.6 keV,  ne= 2.0×1013 cm-3Fig. 2: Measured line intensities (colouredlines) and STRAHL results (dotted lines) forOH discharge conditions IP = 350 kA,  Te(0)= 1.2 keV,  ne = 3.5×1013 cm-3All measured line intensities show a fast increase, followed by a slower decay phase. Thesignals from the higher ionisation stages are delayed with respect to the lower stages by only5 to 50 ms, showing that the bulk of the argon ions moves quite fast from the plasma edgetowards the plasma centre.STRAHL code simulation of the experiments. The experiments are evaluated by simulatingthe gas puff experiments using the predictive impurity transport code STRAHL [1], solvingthe system of time-dependent continuity equations for all ionisation stages of argon in aspatially one-dimensional geometry (flux surface averages). Radial profiles of the time-averaged profiles of ne and Te as measured by interferometry, ECE, Thomson scattering andhelium beam spectroscopy are used as input parameters, while the relevant atomic processesas ionisation, recombination, line excitation and charge exchange between argon ions and thehydrogen neutral particle background are described using data from ADAS. The hydrogenneutral particle density profile nH(r) is taken from a self-consistent RITM-code modelling ofthe discharges. In these calculations we take into account the radial transport contributions byneoclassical diffusion Dneocl and radial drift velocity vneocl as well as the radial profile of theanomalous diffusion coefficient Dan, which is varied iteratively until we find agreementbetween the measured and the calculated spectroscopic signals (see fig. 2). As the techniquedescribed here is not sufficiently sensitive for determining the magnitude of the anomalouspinch velocity van, all results presented below refer to simulations where the anomalous radialdrift velocity is neglected (van = 0).Results. Radial profiles of the emissivities of the argon lines for the same discharge as givenabove are shown in fig. 3. In fig. 4 the radial profiles of D are displayed.Fig. 3: Radial profiles of normalised argonemissivities calculated by STRAHL (samedischarge conditions as in fig. 1) Fig. 4: Radial profiles of argon diffusioncoefficients for OH discharges with differentelectron densitiesThe emissivity shells of the measured VUV lines cover the radial region r = 15...45 cm, whilethe emissivity of the SXR radiation is restricted to the plasma centre (r = 0...15 cm), since the100 µm Be foils practically suppress all argon radiation besides the 0.4 nm resonance lines ofAr-XVII and Ar-XVI. All emissivity shells are significantly broadened compared to thecorona model. The dense coverage of the radial region with emissivity shells allows to derivethe detailed radial profiles of D(r) from the STRAHL modelling, see fig. 4. For the plasmaconditions investigated here we obtain values of Dcore ≈ 0.2...0.5 m2/s for the plasma centre,Dmax ≈ 2...4 m2/s in the radial region near half minor radius (r ≈ 25 cm) and a decrease of D Fig. 5: Density scaling of argon transportpropertiesFig. 6: Predictions for anomalous diffusioncoefficients Dan by theoretical models   towards D ≈ 0.5 m2/s near the plasma edge. The absolute figures exceed the neoclassicalvalues by more than one order of magnitude in both the plasma centre and in the outer plasmaregion. With increasing line-averaged electron density the diffusion coefficient decreases, seefig. 5. We find that both the central value Dcore as well as the mean value Davg show anAlcator-like scaling D ∝  1 / ne.  Simultaneously, both the effective particle confinement timeτP* and the energy confinement time τE increase linearly with density, where the latter relationindicates that the discharges follow the linear ohmic confinement regime (LOC).The results for the anomalous diffusion coefficients are compared to predictions by theoreticalmodels for the dissipative trapped electron (DTE) and the ion temperature gradient driven(ITG) instabilities [3] as well as to a simple gyro-Bohm formula, see fig. 6.  We find that boththe ITG diffusion coefficient as well as the gyro-Bohm value fit to the experimental figuresfor Dmax in the radial region r = 15...25 cm.Summary and conclusions. New VUV spectrometers with high time resolution have beenused in argon injection experiments on TEXTOR to determine the impurity diffusioncoefficient in OH discharges. The simultaneous measurement of spectral lines from manydifferent ionisation stages allows to determine the detailed radial profile of D by STRAHLcode modelling. It is found that D >> Dneocl and Dcore << Dmax (r = 15...25 cm), where bothquantities show an Alcator-like scaling with mean density D ∝  1 / ne.References:[1]: Behringer K, Rep. JET-R(87)08, JET Joint Undertaking, 1987[2]: Weinheimer J et al., Rev Sci Instrum. 72 2566, 2001[3]: Tokar M et al, Poster P3.032, 28th EPS conference, Madeira, 2001E-Mail: w.biel@fz-juelich.de

Progress in Impurity Transport Studies on TEXTOR Using New VUV Spectrometers with High Time Resolution

http://hdl.handle.net/21.11116/0000-0008-470A-E

Progress in Impurity Transport Studies on TEXTOR Using New VUV Spectrometers with High Time Resolution

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