ASDEX Upgrade Team

Asmussen, K.

Dux, R.

Herrmann, A.

Krieger, K.

Napiontek, B.

Neu, R.

Steinbrink, J.

Thoma, A.

Weinlich, M.

Wenzel, U.

English

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Spectroscopic Measurements of the Tungsten Erosion in the ASDEX Upgrade Divertor

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17409Spectroscopic Measurements of the Tungsten Erosionin the ASDEX Upgrade DivertorA. Thoma, K. Asmussen, R. Dux > K. Krieger, A. Herrmann, B. Napiontek, R. Neu,.l. Steinbrink, M. Weinlich, U. VVenzel, ASDEX Upgrade TeamMax-Planck-lnstitut fiir Plasmaphysik, lPP—EURATOM Association;D~85748 Garching and D—10117 Berlin, GermanyIntroductionAn important design issue for future fusion devices is the surface material of the divertortarget plates. Besides the proper mechanic and thermodynamic qualities the lifetime ofthe target plates should be high [1, 2]. The released impurities should affect the mainplasma as little as possible. In todays devices low-Z materials are uSed to keep theradiated power in the main plasma low. On the other hand high—Z materials like tungstenare expected to exhibit a. much lower net erosion due to the low sputtering yield. Aneffective prompt redeposition in the local magnetic field [3, 4] helps to keep the impuritycontent low. The disadvantage of higt materials is the high radiation power in the coreplasma [5].ExperimentsA tungsten diVertor was mounted in ASDEX Upgrade to investigate its behavior underreactor relevant conditions [6] The tungsten coated tiles [7] covered more than 90% ofthe strike point zone in the divertor. The line of sight of the boundary layer spectrommeter (BLS) at ASDEX Upgrade was swept over the target plates during the dischargesand visible as well as VUV spectra were recorded. The tungsten erosion was measuredby observing a WI spectral emission line at 400.9 nm, which had a typical intensity ofm 10'“2 Win—2y“): The S/XB value was used to compute the influx of neutral tungstenatoms via PM” = S/XB >< l‘phomm S{'XB was derived from a separate laboratory ex—periment for the temperature range 2...18 eV. To obtain values above 20 eV an oven wasinserted in the ASDEX Upgrade divertor to sublime VV’fiCOk at the strikepoint position.Comparing the emission intensities of WI and OI] yields the S/XB ratio.The particle flux to the target plate is derived from thermographic measurements andfrom flush mounted Langmuir probes in the divertor. The presented data were measuredin plasma discharges with divertor electron temperatures in the range from 2 to 60 eVand power loads to the target plates ranging from 2x10“l to 6x105 Wm”.Results and DiscussionFigure 1 shows typical timetraces during an ohmically heated plasma discharge. In thebeginning (1—165) the flux to the target plates isplow but a. high plasma temperature(Taz60 eV) leads to high sputtering values. Later on, the increasing particle flux com-pensates partly. the drop in the electron temperature (Tc=2[l eV). The resulting spatialresolved erosion pattern is plotted in figure 2. One can see a rapid drop of the tungstenerosion at the edge of the tungsten tiles. There is no large radial migration of the releasedtungsten atoms. The tungsten emission is peaked at the maximum of the deuteriumflux and at the maximum of the electron temperature. The radial resolved net erosion,HE‘OS4E+053519329191E+19Demoze+051 E105UE+U045065IE-D?Elev]Wlfa,u.]deuvndiv.toad[s]Q[m’]IpiA}lime [5]Fig. 1: Typical time traces for an ohmicplasma discharge. The rows show the plasmacurrent (17,), the line averaged electron density(77.5) in the midplane, the power load on the tar-get plates, the strikepoint and the observationposition, the WI emission intensity and theelectron temperature (Te) close to the strike-point, As the strikepoint is intersected during1410880008 ._. MMr.Mr.88IlU-l1.: cB n/hmalSIPNPPPouamm-85defleofim angle [’1Fig. 2: Spatial profiles of the WI intensity,the electron temperature (TE) and the particleflux (I‘D) at the outer target plate. The tung-sten emission is radially located at the tung-sten tiles. The doubie peaked emission profilecan be explained as a result of the convolutedeffects of the electron temperature and the deu—terium flux. .the spatial scan tungsten erosion is detected.determined by probe measurements, also reflects the radial flux pattern [8]increasing the power load onto the target plates by raising the additional heating powerto 7.5 MW neutral beam injection increases the WI emission by one order of magnitude.The tungsten erosion drops immediately as the particle flux to the target plates is reducedand shows no measurable dependency on the target surface temperature.During a density limit discharge the divertor electron temperature decreases from60 eV to below ‘2 eV. The tungsten influx drops below the detection limit as the density israised. To quantify the tungsten erosion. a variety of plasma discharges with Very difierentparameters was analyzed, The measured tungsten influx at the strikepoint position wasnormalized to the deuterium/ hydrogen flux. The result is presented in figure 3. About60% of the shown measurements are H-Mode discharges. The lines in figure 3 represent theresults from TRIM sputtering simulations, based on ion beam measurements and takinginto account the energy and anguiar distribution of an isotropic Maxwellian distributionaccelerated in the sheath potential {9, 8]. Comparison of the modeiled data for a puredeuterium plasma with the experimental results exhibits a significantly higher erosion oftungsten than for pure deuterium sputtering. Simulations are also done for plasmas with1% (3+3 or 1% CH. The higher mass of carbon leads to a higher sputtering yield. Thecarbon ions also gain an additional impact energy in the range of Ewan“ #1 Z - 3k?!”ein the sheath potential of m 3kTE. The simulations containing the carbon contributionare in much better agreement with the experimental data. In summary the experimentalresults can be modelled by a divertor plasma with about 0.5% charged light impurities141ito0UIomomammo10“g 10".9>U)Ec .g 10 58 .m- — modelled D'—; Ww. modelled TWEOW—ewwe modelled mg" —> w0 experimental ‘D9 experimental ‘H*me measurements10" 'W1[au]davload[:uwmfiTe[av1New[3u]0 1o 20 an as soT.[eV]Fig. 3: The symbols show the measured sput-tering yield of tungsten. The lines represent re-sults from calculations assuming difierent car—bon impurity charge states and the expectedsputtering yield for pure deuterium sputter-ing [8]. There are also two points of net ero«sion from probe measurements overlayed in thisgraph [8], from which the prompt redepositionfraction of tungsten can be evaluated.time [51 time [s]Fig. 4: 1 MA attached ELMy H-mode plasmadischarges with Neon pufis. #8026 represents atypical discharge With a low electron tempera—ture in the divertor. The tungsten influx is be»low the detection limit. In discharge #8025 thedivertor electron temperature remains above5 eV during the neon puff and the time depen—dence of the tungsten influx correlates clearlywith the time dependence of the Neil] in thecore plasma.which leads to the conclusion that the sputtering of tungsten is clearly dominated by theplasma impurities. A similar increase of the sputtering yield by impurities was found formolybdenum during argon puffing [10].Additional impuritiesThe injection of additional impurities can also reduce the electron temperature by anincreased radiative loss. In ASDEX Upgrade neon is typically used for cooling of theboundary plasma. Figure 4 shows as an example the measurements of two differentattached discharges with neon puffs in the main chamber of AUG. The NeVI signal inthe midplane gives an estimate of the time dependence of the neon concentration.One discharge (#8026) shows a low divertor electron temperature, in the other one(#8025) the neon gas puff did not lower the electron temperature below 5 eV. In the colddivertor plasma of discharge #8026 the additional neon concentration lowers the tungsteninflux. below the detection limit. In discharge #8025 the influx follows clearly the timedependence of the NeVI emission. The tungsten erosion increases with the additionalneon despite of the temperature and divertor power load drop, which is caused by theneon injection. The increased erosion is caused by the additional neon. These observationsare consistent with the rapid drop of the sputtering yield of tungsten impacted by Na”,which reaches a value of 2x10" for plasma temperatures above 20 eV and decreases toless than 10"1 below 4 eV [3]. In discharges with divertor plasma temperatures below2 eV, like the CDH-mode, no tungsten erosion could be measured.1412RedepositionAs neutral tungsten atoms are released from E 100 *ESeVthe surface they will be ionized by the divertor % 80plasma and can be tedepOsited by the gyro— 5motion in the magnetic field. The fraction E 60of the ionization length to the effective gyra— E 40 iii-Celltion radius determines the fraction of prompt g i ' ~~~~~~~~~~~redeposited tungsten atoms within the first :3 20 A .........gyration [3]. Additionally to the spectroscop- E o O 1 2 3 4ically measured yield for the tungsten erosion, 7mm / 1Wfigure 3 shows results for the net erosion, de— , . _. . F1g. 5: Comparison of the measured [*l andtermined by measuring the tungsten loss of _ _ . ’ ‘probes [8]. The difference in these quanti— predicted (- -) redeposmon of tungsten [3].ties can be interpreted as prompt redeposi~tion. For the high temperature point at 406V (112:7K1018m‘3) the net erosion is about50 0/0 of the initially eroded tungsten The prompt redeposition at lower temperatures(TC—=15eV.ne:1:<102°n1“3) 15 at least 95070.10 The difference in the redeposition can be exeplained by the different electron densities of both discharges. The measured values canbe compared with the expected values as shown in figure 5.SummaryThe tungsten erosion in the ASDEX Upgrade divertor has been measured by observingthe WI emission at 400.9 nm. Taking into account the SfXB value the influx is cal»culated for various plasma conditions. The resulting sputtering yield is in the range of10"4 for most divertor plasma conditions. The erosion of tungsten in ASDEX Upgradeis dominated by the intrinsic plasma impurities like carbon. Adding impurities withoutcooling the boundary plasma increases the sputtering yield due to the additional impactenergy gained my the particle in the sheath potential. In general the cooling of the diver-tor plasma reduces the tungsten erosion remarkably, It is experimentally verified that anefficient Iedeposition can be achieved at high electron densities in the divertor plasma.8 KRIEGER K. et al. St. Raphael. France, 1996, J. Nucl. Materi, in press.9 NAUJOKS D. et at, J. Nucl. Mater. 230 (1996) 93.{10 KURZ, C et al. J. Nucl. Mater. 220-222 (1995) 9631References[1] PACHER H et aL, St Raphael. France, 1996, to be published in J. Nucl. Mater.[2] IANDSCHITZJ“ et at, J. Nucl. Mater. 220-222 [1995) 73.[3] NAU]OI\S, D. et al., Nucl Fusion 36 (1996) 671.[4] ROTH, J. etal. ,J1 Nucl.Ma.ter 220~222 (1995} 231.[.5 POST, D et al., At Data Nucl. Data Tables 20 (1977) 397.[6 NBU R et al. Plasma Phys. Controlled Fusion 38 (19.96) A165[7llll] GARClA—ROSALES 1st al.., accepted for pub]. in Nucl. Fusion Technology 1llNl

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Spectroscopic Measurements of the Tungsten Erosion in the ASDEX Upgrade Divertor

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