ASDEX Upgrade Team

Bard, R.

De Pena Hempel, S.

Dux, R.

Fuchs, J. C.

Gude, A.

Kallenbach, A.

Salzmann, H.

Schweinzer, J.

Weinlich, M.

English

Fuchs, J.

MPG.PuRe

1473Radiative Cooling and Improved Confinementin ASDEX UpgradeA. Kallenbach. R. Dux, A. Bard. S. de Pefia Hempel, J. C. Fuchs. A. Gude.H. Salzmann. J. Schweinzer, M. Weinlich and the ASDEX Upgrade TeamMPl fiir Plasmaphysik. EURATOM Association, 13—8-5748 Garching, GermanyIntroductionThe typical experimental domain of radiatively cooled scenarios is characterized bystrong gas puffing and cold (livertor conditions. While almost complete power removalby radiation could be demonstrated on ASDEX Upgrade with neon cooling [1], onlymoderate energy confinement improvement above the L-inode level (HITERSQF m 1.6) isobtained. On the other hand. very good confinement (HITERBQP > '2) can be achievedwith very low neutral flux levels in the main chamber which is synonymous for a lowSOL electron density. In this case, the confinement improvement is attributed to anedge barrier, leading to a pressure pedestal. But, divertor electron temperatures arehigh for these conditions. In this paper. such a discharge with edge pedestal andneon radiative cooling is compared with the standard cold divertor, type—ill ELM)!CDH~scenario, where confinement gain is attributed to core density profile peaking.The radiative efficiency P326Pmd/JZEU turns out to be degraded for the hot divertorconditions. The origin of this degradation is investigated by impurity tranSport modelling.Radiative cooling for hot divertor conditionsFig. 1 shows time traces of a discharge (#8197) with slightly increased trianegularity and plasma volume and a low SOL electron density (see Table 1).A short neon puff at t:3 3 leadsto a moderate increase 0f the core Ne valve flux Zeff Prad mainchamberradiation and a weak reduction of N615 ‘ l: ’ ' ‘ l ' ‘the target power load The ELM 3320’ f". HM fizzfrequency is reduced from 60 Hz 2mm» 1' E i '2to 14 Hz, resulting in high peak ‘E"2°"“”"""" I T ””””{.3Erik:tar et ower loads after the neon ‘ , H I ' . l. l _ ' .pug. EVhile the generally lower softk-ra em1ss1on Dim.” Prad dlygt‘ljr i WELM frequency is attributed to : :3000 " "I;the higher triangularity, it can - . :2000: "2not be distinguished whether the : :1000 W 1improved confinement is caused J ‘ 1 I 1 I I I . -by the plasma shaping directly smefiw‘iV , . M1 9‘2‘““’”1°ad Wor by the low neutral flux level :WMW 0‘6 ' l i » 4‘/ SOL electron density. The C 20,4 .. i I:long ELM events which can be I :02 _ ’ ' v- ’ -lseen in Fig. 1 after the neon ' . . . . . ' , . . , ;putt consist of short bursts of 2'9 tili1'e1(s) 3 2-9 mil“; (s) 33individual ELMs: The quiescentH* phases are terminated by atype—I ELM which is succeededby ‘24 type III ELMs within10 milliseconds. The sawtoothFigure 1: Time traces of a discharge with hot divertor.good energy confinement {HITEF‘SQ’E 2) and moderate m»diative cooling with neon. PM”: 5 MW .00 A D+ NB]...i‘.ll1474frequency is 14 Hz throughout the discharge, after the neon puff, the sawteeth appearright after the ELM bursts. The improved confinement scenario exhibits a. very thin andhot SOL and divertor piasme with an electron temperature TE :1 40 eV at the outer target.Radiative efficiency transport modellingThe temporal developmentof impurity profiles is mod- Laur-elled with the impurity transi ,5— l ' -------hifsffiémaport code STRAHL [2], 113— E- mma mama“. ‘ing measured electron den- ° “No.9city and temperature profiles.The STRAHL output inter— nface allows direct compar— :0 m mppooi”ison with bolometry, spec— 5; m”troscopy, soft—X measure— ; mafia-2;;-ments and charge-exchange E M0 'recombination spectroscopy 3 “"1” “.9732(fully stripped impurity den— i ”“7"sity profiles). The overall 3.60 mu can onconsistency of STRAHL out— ”PMput and measurements is typ— Figure 2: Edge electron and neon density profiles and trans-ically within j: 30 % when 51] port parameters for modelling of the discharge shown in Fig.the diagnOStics mentioned are 1. Parameters of a ’standard’ type-III ELMy discharge withadjusted simultaneously. moderate gas pufling are given in dotted lines for comparison.197 ai-E sum as mInJl‘In Mum a.“ 2mm LEW m a.|m:-. WWI“us.“ If /‘ r / “39' ' ‘ 5w [I 1/ 11mmE“ 21am 35*“ lasso! ' \ > 5‘1“’ \ scamsg “5"" ”m I lkyfh 3.2mm \\. J' U WV‘ ‘ \ewe , r 8197 «M uj Leewt # 76615 imm-( I I I Am ’ l on. AI.” am 33D 320 $30 2 n 309 5.") 3.20 33"! i L30 2.30 2.70 3 W 3.50 ‘80“B? W: .5.500% R-C IMM IL 1/ 2‘“ Mile“ BXRCIM MB. 51 \QJII'IJJJ ‘fa" ' m sown: \\94 4290-66E /l/ / 1.60906 mm; \/ \2.5: a!3 ’ _,-\/ Lama «was \\mum some 2mmgin-ma i . . . an- m . i 5‘ .230 3.00 I.” 3.20 3.30 2. D 3.00 3J0 3.$ 330 1.30 2.50 270 3.1D 3.50 LN 2.30 2.70 5.10 3.50urns“) Ilme (5] “III. (5) Mia (I)Figure 3: Time traces of soft-X emissivity and Nev” VUV line emissivity for the dischargeshown in Fig. I and the reference discharge for comparison. (—- model, - - - can)Transport coefficients and density profiles in the outer plasma for the H* phase of Fig.1 are shown in Fig. 2, values for a standard type~III ELM}r discharge with neon coolingare given for comparison. Inside ppo, = 0.8, transport coefficients (D, v) are taken‘fromdynamic pulse analysis using charge exchange recombination spectroscopy [3]. The transport coefficients for ,07, > 0.8 are adapted for diflerent discharge scenarios by matchingthe temporal evolution of SXR emissivities for various viewing lines and different filterthicknesses (8 and 100 cm Be). While with the 100 um filters only continuum radiationis measured. with the 8 ,um filters also line radiation of highly ionized species contributes.1475The radiation profiles before the neon injection are simulated assuming carbon as singleimpurity (C concentration m 2 %) and matching its influx. After the neon puff. the carboninflux is kept on its initial value. the transport coefficients (Dy) are taken identical tothose of neon. The ELMs are simulated by increasing the diffusivity to D: 12 mz/s over.5 cm inside the separatrix, the radiation during ELMs is not modelled due to the lack offast enough Tait)» n50) diagnOStics. Sawtooth are treated using an analytical model forthe profile reorganization. Experiment and modelling are compared in Fig. 3. It shouldbe noted, that the transport model shown in Fig. 1 represents a possible, but not theunique solution for the observed fluxes and profile shapes if the uncertainties of the mea-surements are taken into account (see, e.g., [4]) In fact, simultaneously doubling D andv for 3% 8197 over 3 cm inside the separatrix results in a prediction of the experimentaldata with a consistency which is similar to the results shovvn in Fig. 3. Fortunately, theratio v/D is more robust than the individual values. The question arises, whether thestrong neon inward driftduring the H“ phase is due : neoclassicairieon drifts . Ito neoclassical effects. Fig. 50} 44 shows the various contri- 703‘ l 1')butions to Vnem the den— got" id "7"-sity gradient driven Pfirsch— > i edge barrier .. \\7Schliiter term dominating '50 :sggwégm atthe VT term. In combina— “o i if:§‘""§i.'§f§;‘flffi'§f§l _tion with an anomalous D i ”gang-$313113: ,of 1 1112/5 (which is higher .150lW1?7 T We" ‘ ‘ — ~40 #766‘5 . . .than what one would exe 0'70 ”0 pm 0‘50 "0° ”*7“ 05° F,” 03° 1-00pent wmhm a transport barv Figure 4: Neoclassical drifts of neon for the discharge shown inricr), the neoclassrcal V’W Fig. 1. t: 3.28 s and # 7666' for comparison.would fit the experimentaldata. However. there areuncertainties connected to vmo since the C—Ne collisions and the influence of the ra~dial electric field E are not considered in its calculation. Further. an anomalous inwardpinch has to be used to reproduce the steep deuteron profile shown in Fig. 2 with simple1-d modelling including the effect of charge exchange.We arrive at the following picture of the impurity transport: In the H—mode, D and vpeak around PM = 0.8-0.9, and a transport barrier near the separatrix occurs during theH“ phases. Between ELMS, anomalous diffusion is reduced within the barrier and a stronginward drift is seen just inside the separatrix. This inward drift leads to a. fast impuritybuild—up close inside the separatrix, its magnitude is similar to the neoclassical level. Theclearly anomalous inward drift around pp01=0.85 does not lead to substantial impuritybuildup since it is compensated by the large values of D. No distinct differences are seenin the radiative efficiencies of radiative type-1H ELMy H4 and L—modes, where the possi-ble contribution of a neoclassical inward drift is masked by anomalous transport or ELMs.Origin of the poor radiative efficiency with edge pedestalThe main reason for the poor radiative characteristics of the discharge with edge pedestalis the peaking of the impurity density profile with respect to its edge value. If thetransport is characterized by diffusivity D and drift v. the relative profile shape is givenby the peaking factor Fmt = emp(— f; v/Ddr’). High values of ps at radial pesitionswith low specific emissivity means plasma pollution Without radiative pay—off and1476# type HITERSQP (3' ”git/I h—SOL Tdiu P" Pp<1 soée. e e a I For:[1019 m— ] [6V] [MW] [MW]8197 edge pedestal 2 0.12 1.5 0.6 40 1.8 1.4 4.57666 type—III ELMs 1.6 0.09 10 1.0 7 3.5 2 1.1Table 1: Comparison of radiative characteristics of two improved confinement scenarios withradiative cooling. 5 refers to triangulari’ty. TS” is the peak temperature from target Langmuirprobes1 P: is total neon main chamber radiation per core neon Zcff increase. 1?: 1 MA, (495:4.therefore low values of Pa, The situation for the discharges in comparison is illustrated inFig. 5; A high drift veloeity is only efiective to produce profile peaking when connectedto a region with low D.ConclusionsThe major drawback for radiative cool—ing of a discharge with edge pressure A s __ #8197pedestal1 expressed by the i w radia— 35 ~--- weestive efficiency rifled/6&2”, originates E 5 ~from the large. values of Vin/1D close “into the separatrix. Significant values of f; 4 .V511 are obtained from neoclassical ef~ E 3 V “8‘ Dan “12/5facts, however, an anomalous contribu~ L ”a“ transa modeltion to Vl‘n may be present. During Lug- 2 (_ experimeptypalll ELMy or L-rnode phases, theneoclassical drifts are masked by othermechanisms and strong profile peakingnear the edge is not observed. To obwtain efficient radiative cooling, a radia-tor should be chosen which exhibits itspeak emissivity just inside the trans-port barrier. Other means to increasethe radiative efficiency would be den—sity/ temperature profile control to sup—port neoclassical drift terms with out-1 . . . .0.70 0.90 1.00polFigure 5: Impurity profile peaking factor Freak forthe anomalous transport model of Fig. 2 (far. lines)and for neoclassical drifts according to Fig. 4 andD: I mZ/s. The normalized neon radiation profilefor #819711: obtained by multiplication of PM” withthe specific emissivity (shaded curve}0.80ward direction. Since the power bursts connected to ELMs succeeding long quiescent H’“phases will contradict lifetime requirements of the divertor in a reactor. rapid pellet injec~tion could be used to trigger ELMS with controlled frequency which flatten the impurityprofile shape over the. pedestal region. The necessary frequency depends on the time Scalefor buildup of the impurity pedestal and on the width A of the pedestal f o< v/A. Fordesigning a reactor scenario with ”edge pedestal’, better estimates for the edge transportcoefficients, including the neoclassical ones, are required to allow for calculation of thetradeoff between confinement degradation and more efficient radiative cooling as well ashelium exhaust. To achieve this, refinement of the edge diagnostics is highly desirable.References[1] A. Kallenbach et al., Nucl. Fus. 35 (1995) 1231. [3] S. de Pena Hempel et 31., this conference,[2] R. Du): et a1., PPCF 38 (1996) 989. [4] R. Giannellaet 5.1., Nucl. F‘us. 34 (1994) 1185.

Radiative Cooling and Improved Confinement in ASDEX Upgrade

Max Planck Society eDoc Server

http://hdl.handle.net/21.11116/0000-000A-8113-E

Radiative Cooling and Improved Confinement in ASDEX Upgrade

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe

Max Planck Society eDoc Server