Behringer, K.

Coster, D.

Gafert, J.

Napiontek, B.

Schneider, R.

Thoma, A.

Weinlich, M.

Wenzel, U.

MPG.PuRe

1413Line and Recombination Emission in the ASDEX UpgradeDiver-tor at High DensityB. Napiontek, U. Wenzel‘, K. Behringer, D. Coster7 J. Gafert, R. Schneider, A. Thoma,M. Weinlich and ASDEX UpgradeTeamMaxePlanckJnstitnt fiir Plasmaphysik, EURATOM Association, Garching and *BerlinInt reductionThe most promising divertor regime for ITER is the detached, high density divertor withits high radiation losses. Therefore We focus the spectroscopic investigations on this stateto understand the emission properties, the behavior of the intrinsic impurity sources andthe plasma. parameters.The radiation of carbon and deuterium was investigated by means of a scanning VUVspectrometer. The spatial distributions of the resonance line emission of carbon anddeuterium were integrated to obtain the total radiation power losses. A small temperaturedependent correction to get the total carbon radiation in the divertor was also included.The density dependence of the radiated power was measured in a density ramp with3.5 Ill!W heating power.Detached divertor plasmas are characterized bystrong hydrogen radiation which can be used for 10 \ 12 14 16 lines at sight (16)plasma. diagnostics. By these means Spectroscopiv \ Ical measurements of electron temperature and den—sity in ASDEX. Upgrade Were carried out which arecomparable to the recent experiments at AlcatorC-Mod [1]. We will discuss the properties of cold,dense divertor plasmas and describe a. method forT8 and ne measurements which shall be comparedwith results of plasma modelling using the codesB2-EIRENE.Despite the very low temperature in front of the Figure l: Arrangement of the 16plate carbon emission is high in the detached di- polaidal Sightlmes 0f the divertor spec-vertor. Therefore, we have studied the CD band in _trometer. Another set of 30 Sightiinflé’the visible to estimate the carbon influx via chem- views the plasma parallel to the outertarget plate.I ..laceted lrrnrical erosion.ExperimentExperimental investigations were carried out with two spectroscopic systems. The diver-tor spectrometer (8. fig. 1) observed the plasma along several sightlines in the poloidalcross—section. There are 30 sightlines viewing the plasma parallel to the outer targetplate in a distance of 1...90 mm, another set of 16 sightlines is arranged fan-like. Thecollected light was fed via optical fibers to a. Czerny-Turner or a FastieEbert (Echellegrating) spectrometer with low and high spectral resolution, respectively. Several lines ofsight were detected simultaneously by means of a. fast CCD camera.1414The boundary layer spectrometer (5. fig. 2) scannedthe Whole poloidal cross—section. In this way the innerand outer target plate could be observed. The lightwas analyzed by two spectrometers in the VUV andvisible spectral range.Results: Power losses during a density rampThe divertor radiation was investigated in an L—modedischarge with 3.5 MW heating power. By means ofgas puffing. the line—averaged density in the midplaneincreased from 4.5 - 1019m-3 to 6.5 - 10197724. The ionsaturation current of Langmuir probes in the targetplates first increased, but then dropped by a factorof 4 at 3.15 due to detachment. Spectroscopically,power losses were obtained by summing over the resonance lines of carbon and deuterium in the VUV spec-tral range. In Fig. 3 the results are compared to thebolometer measurements. The power losses rise withFigure 2: Schematic view of theboundary layer spectrometer. Amovable _mirrar allows scanning ofthe whole poloidal cross-sectionincreasing midplane density. The ratio of the carbon to deuterium emission changes from1:1 to 2:1 during the ramp. Main radiator is the CHI ion. The spectroscopically measuredvalues lie 30% below the bolometer values, possibly caused by other impurities (0, F7 Cl,B).The highest radiated power is observed in the de—tached divertor despite the very low temperature ofabout 18V (s. next section)7 Le. there is no drasticreduction of the carbon source. The high inten-sity of the CD band emission even at detachmentsuggests the process of chemical erosion. From themeasured band intensity the methane influx can becalculated. On the other hand, the methane influxcan be calculated using the ion flux to the targetplates measured by Langmuir probes and the yieldfor chemical erosion. In Fig. 4 we compare bothmethods. The absolute values agree well in the at—tached phase. During detachment the influx calcuvHediatedPower[kW]SDD . .#7946 Bolomgiry400 - ‘imam/UV)300 — . .Carbon200 ~ ~100 . Demen'um045 5:0 55Lhasveragad density [16 Elias]Figure 3: Power radiated in the outerdivertor as a function of the midplanedensity: carbon, deuterium and sum oflated from the measured ion flux deviated signifi~ both, compared with total losses mewcantly from the spectroscopic measurements. This sured by bolameters.is probably due to chemical erosion caused by theflux of atoms onto the target plates.Results: Recombining plasmaAt maximum radiation the divertor is already detached. As indicated by the location ofthe emission zones of 0111 and CIV the thermal front position is close to the X~point andthe major part of the divertor is cold.1415Under these conditions we observed the Balmerand Paschen continuum radiation and line radia-tion of the Balmer series up to transitions fromlevel n 14. When the plasma. detaches thereis a. drop in electron temperature and an increasein electron density and therefore strongly increasedrates for radiative and three-body recombination(s. [2]). This leads to a strong increase in hydro-gen radiation. Fig. 5 compares two spectra. at dif-ferent times in an L~mode density limit dischargesimilar to the one discussed above. The ratio ofBalmer to Paschen continuum radiation provides asensitive method to measure the electron temper—ature. During the detachment the electron tem—perature decreased to 0.8 . . . 1.4 eV in the whole di-vertor. The electron density was determined fromthe absolutely calibrated Balmer continuum emis-sion giving maximum values of 1 ‘ “Tum—3. Fromthe spectrum shown in Fig. 5 1.1 10“” m'3 was de—a t$7946Ionnuxli‘]? B004mm»:[61]Time [s]Figure 4: Comparison of the methaneinflux measured spectroscopically(dotted line) and calculated from.the ion fins to the target plate mea-sured by Langmuir probes which isgiven separately as the upper graph.termined which 18 about a factor of 2 higher than the midplane density.In order to investigate the optical thicknessof the divertor plasma we measured simulta— mac [n=7H—m=2)neously LL; and Ha with the boundary layer 15 _ Eigflggtfirllg} ‘spectrometer. At moderate divertor densities :14 ' launched)- y a . a c w c sthe ratio of H,z to La intenslties starts to m— «Em 1952mm)crease with density indicating the effect of op- 310 13 ":2tical thickness. At higher densities and re- ? B (n: 4 )' g 6 Balmeroontlnuumgardmg Lm the effect must be even more pro— .‘g Paschannounced. Thus in the region of high density 4 outplasmas, where the observed continuum radi- 2 l; Aation is preferentially emitted, recombination 360 890into the ground state via excited states is re-duced. Therefore we expect that net recom-bination occurs in the outer zone of the diver-tor plasma and possibly near the target dueto enhanced recombination rates by moleculeencounters.The arrays of lines of sight of the divertorspectrometer allow measurements of spatialT.3 and n, profiles. Spatial resolution is possi-ble in the direction perpendicular to the tar-Wavelength [nm]Figure 5: Spectra amund the Balmsr serieslimit measured with the diuertor spectrom—eter. The midplane density rises from4.1 . lllmm‘8 (at 1.18) to 6.7 - 1019m‘“3(at 2.0 s) leading to fully detachment.The marked regions of continuum radia-tion are used for temperature determination.get plates whereas along the line of sight the emission is integrated (s. Fig. 1). Due todependence of the emissivity on n3 the region with the highest density dominates theintensity. The temperature measurements represent the region with the highest temper—ature along the line of sight because of the steep dependence of the Balmer to Paschen1416continuum ratio for Te S 26V. Therefore We get upper limits for T8 and 115 along theline of sight. This is illustrated in Fig. 6 which shows results of the modelling of the fullydetached divertor plasma using BZ—EIRENE [3] compared with the experiment.Near the outer target plate (within about10 mm) the modelled mean temperaturesdecrease due to the lower temperatures inthe outer divertor. The spectroscopic tem—perature measurements are dominated bythe inner divertor. Thus the measured spatial profiles can only be interpreted as pro-files of mean values of T.s and us in the emis—sion zone of the outer divertor at greaterdistances to the target plate. Thereforequalitative differences between the mod-elled mean parameters and the measurements (5. Fig. 6) especially for TI.3 near thetarget are to be expected. The agreementat greater distances to the target plates andfor the electron density is quite good. Wewill overcome the problem of integratingalong the outer and inner divertor in the‘12 "e10 11029:;752'0 “a TEEeV]1.5“:‘i12 ‘10.8Wo 10 20 50 604OElevation [mm]Figure 6: Modelling (o) and measurements (A,#8620 at 2.7 s) of TS and ‘ne profiles. The riseof measured electron temperature near the targetplate is due to the effect of the inner divertor onhas integrated intensity.new divertor configuration of ASDEX Up-grade where both parts of the divertor will be observed separately.SummaryFully detached divertor plasmas in ASDEX Upgrade are characterized by low temperatures Te < 2 eV, high density n5 ,3 1-1021m"3 and radiative and three-body recombinationin the divertor volume. Despite the low electron temperature, carbon influx is very highcaused by chemical erosion. The detached outer divertor reaches about 0.5 MW radiatedpower due to both the high influx and the high electron density. Main radiators aredeuterium and CHI.The ratio of Balmer to Paschen continuum radiation provides a sensitive method for tem-perature measurements, however, the line integrated values are strongly weighted by thehighest temperatures along sightline. Spatial profiles of electron density and temperatureare in good agreement with first results of modelling recombining divertor plasmas inASDEX Upgrade with BZwEIRENE.References{11 LUMMA, D. TERRY, J. L., and LIPSCHULTZ, 13., PFC/JA—96-33, MIT, 1996[2} BORRAS, K, et al., this conference, P4019[3] COSTER, D.7 et 3.1., this conference, P4.013

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Line and Recombination Emission in the ASDEX Upgrade Divertor at High Density

Line and Recombination Emission in the ASDEX Upgrade Divertor at High Density

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