Power exhaust by SOL and pedestal radiation at ASDEX Upgrade and JET

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JET

M. Bernertâ^,^∗,M. Wischmeierâ,A. Huber^b,F. Reimold^b, B. Lipschultz^d, C. Lowry^c, S. Brezinsek^b, R. Duxâ, T.Eichâ,A. Kallenbachâ, A. Lebschyâ,C. Maggiê, R.McDermottâ, T. Pütterichâ, S. Wiesen^b,JET contributors^f^,¹,the EUROfusion MST1 team²,the ASDEX Upgrade Team

a Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany

b Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung - Plasmaphysik, 52425 Jülich, Germany

c European Commission, B-1049 Brussels, Belgium

d University of York, York Plasma Institute, Heslington, York, YO10 5DD, United Kingdom

e CCFE, Culham Science Centre, Abingdon OX14 3DB, UK

f EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK

a rt i c l e i n f o

Article history:

Received 15 July 2016 Revised 27 November 2016 Accepted 22 December 2016 Available online 27 January 2017

a b s t r a c t

Futurefusionreactorsrequireasafe,steadystatedivertoroperation.Apossiblesolution forthepower exhaustchallengeisthedetacheddivertoroperationinscenarioswithhighradiatedpowerfractions.The radiationcanbeincreasedbyseedingimpurities,suchasNfordominantscrape-off-layerradiation, Ne orArforSOLandpedestalradiationandKrfordominantcoreradiation.

Recentexperimentsontwooftheall-metaltokamaks,ASDEXUpgrade(AUG)andJET,demonstrateop- erationwithhighradiatedpowerfractionsandafully-detacheddivertorbyN,NeorKrseedingwitha conventionaldivertorinaverticaltargetgeometry.Forbothdevicessimilarobservationscanbemade.

Inthescenarios withthe highest radiatedpower fraction,thedominant radiationoriginates fromthe conﬁnedregion,inthecaseofNandNeseedingconcentratedinaregionclosetotheX-point.

Applyingtheseseedimpuritiesforhighlyradiativescenariosimpactslocalplasmaparametersandalters theimpuritytransportinthepedestalregion.Thus,plasmaconﬁnementandstabilitycanbeaffected.A properunderstandingoftheeffectsbytheseimpuritiesisrequiredinordertopredicttheapplicabilityof suchscenariosforfuturedevices.

ThisisanopenaccessarticleundertheCCBY-NC-NDlicense.

(http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

Futurefusion reactors requireplasmascenarios withhighdis- sipated powerfractionsinorder toreduce thepowerﬂux onthe divertortargetplates.ForITER,about85–90%oftheexhaustpower needs tobe dissipatedandforapossibleDEMOreactora fraction ofmorethan95%isrequiredtomeetthemateriallimits[1,2].With

∗ Corresponding author.

E-mail address: matthias.bernert@ipp.mpg.de (M. Bernert).

1 See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia

2 See http://www.euro-fusionscipub.org/mst1 .

asufficientreductionofthepowerfluxtothedivertor,theelectron temperatureinthedivertorisreduceddowntoseveraleV,where atomicprocessessetinandvolumetricrecombinationbecomesef- ficient.In this detached state the particle and power flux to the targetsreducessignificantly.

The power dissipation consists of perpendicular transport, chargeexchangelossesandradiationlosses,thelatterusuallybe- ingthebiggestcontribution.Theradiatedpowercanbeincreased by introducing seed impurities. These can be chosen to increase primarilythecore(e.g.Kr,W)orscrape-off-layerradiation(e.g.N, Ne).InordertoretaintheenhancedenergyconﬁnementoftheH- mode,thepowerﬂuxacrosstheseparatrixhastostayabovetheL- Hthreshold[3].Therefore,atITERadditionalradiationinthecore isnotviable,whileforDEMOtheexpectedexcessofalphaheating http://dx.doi.org/10.1016/j.nme.2016.12.029

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Fig. 1. Radiation distribution of nitrogen seeded discharges at AUG (#30506) and JET (JPN 85067) in the detached state.

abovetheH-modethresholdallows(andrequires)signiﬁcantcore radiation[4].

High radiation scenarios with different seed impurities were tested at two of the all-metal tokamaks, ASDEX Upgrade (AUG) andJET. Theseexperiments aimed todemonstratepower exhaust athighestheatfluxes.Thedominantparameterdeterminingthese heatfluxesis theratioof thepowerflux over theseparatrix P_sep andthe majorradius oftheplasmaR [5]. ForITER,thisisin the rangeofPsep/R≈ 15MW/mand forDEMOexpected tobe in the range of 15–20 MW/m [1]. At AUG, values of up to Psep/R=12 MW/mwere achieved, while at JET experimentswere limitedto about5MW/m.

This publication compares the observations at both machines andtheimpactofthevariousseedimpurities ontheplasmasce- narios.It isordered by thedifferent seedimpurities used,show- ing forboth devices nitrogenseeding in Section 2, neonseeding inSection3andkryptonseedinginSection4.Section5compares theobservations at both devicesand Section 6gives a summary andshortoutlook.

2. Nitrogenseeding

The use of nitrogen fordivertor cooling is a well established technique[6]. It is commonlyobserved that the energy conﬁne- menttimeoftheplasmaisincreasedwiththeinjectionofnitrogen inan all-metaldevice[7].At thesametime,theNseedingisob- servedto alterthe ELMcharacteristics,increasing their frequency anddecreasingtherelativeenergyloss[8].

Fig. 1a shows a tomographic reconstruction of the radiation distribution in AUG discharge #30506, where 18MW of heating power are applied at a constant N seeding rate. The normalized confinementfollowingthe ITERphysics basescaling [9]isaround H₉₈=0.9 atupto 95% oftheGreenwald density.In betweenthe type-IIIELMs,thedivertorisinapronounced detachedstate[10], wheretheparticlefluxontothetargetissignificantlyreducedfor about10cmalong the target. The radiatedpower fraction(f_rad= P_rad/Pheat)inthisdischargeisaround75%,whileindischargeswith lowerheatingpower(e.g.AUG #29383)up to90% oftheheating powerisradiatedinthedetachedstate.

In Fig. 1b a tomographic reconstruction of JET pulse 85067 is shown, where also 18MW of heating power are applied. The plasmaisinaELM-lessH-mode,theconﬁnementisaroundH₉₈= 0.7at90%oftheGreenwalddensity.Thedivertorisinafullyde-

Fig. 2. Evolution of radiation around X-point, line of sight measurements (top), cen- tral nitrogen concentration (middle) and temporal evolution of tomographic reconstruction (bottom).

tached state and the radiated power fraction at about 75%, the maximumobservedatJET[11].Note,thatinthisconditionnosig- nificantheat flux ismeasured atthedivertor [12,13].At ahigher heatingpower(JPN87201,P_heat=27MW),theplasmaisinatype- IELMyH-modebutshowssimilarvaluesofconfinementandradi- atedpowerfractionasat18MW.

The comparison in Fig. 1 shows that for both devices, in N seededdetacheddischarges,thedominantradiationisemitted by a small region inside the conﬁned region, above the X-point. In both cases, about 5MW are radiated from this region, which is about40%ofthetotalradiationforJETandAUG.

ForAUG,thisso-calledX-pointradiatorisconsistentlyobserved in the N seeded detached condition, not only at highest heating powers [14]. Such a local radiatorinside the conﬁned region canalsobereproducedbymodelingwiththeSOLPScodepackage [15,16].

Fig. 2shows the temporal evolution ofthe location of the X- pointradiatorindischargeAUG#30506,whereaconstantNseed- ingrateisapplied.Thisconstantseeding leadsto aslowincrease oftheNconcentrationintheconfinedplasmaduetotheresidence timeofnitrogeninthevacuumchamber.TheX-pointradiatorde- velopsintheregionoftheX-pointandwithincreasing Nconcen- trationmovesupwardsinsidetheconfinedregion.Itappearstobe very localizedandnot significantly elongated alongthe magnetic fieldlines.

Theintenseradiationinthisregionindicatesastrongreduction ofthelocaltemperatureto valuesof10–100eV,whereNradiates efficiently.ElectrontemperaturesofonlyafeweVareindicatedby theSOLPSmodelingandbythe observationofdeuteriumlinera- diation in the region below the X-point radiator. However, there is nodirect measurementof theelectron temperaturein thisre- gionavailableyet.Suchalocalreductionoftheelectrontempera- turemightrepresentaso-calledradiationcondensation,wherethe characteristics of the impurity radiation lead to a strong cooling downtothetemperatureofthemostefficientradiation.Thisleads toa densityincreaseinthisregion anda furtheramplificationof theradiationlosses.ThisindicatesasimilarityoftheX-pointradi- atortotheMARFEphenomenon[17].However,theoperationwith sucha X-pointradiatordoesnot leadtoanunstableplasma, asit isusuallyobservedwithMARFEsintokamakswithcarbonasfirst wallmaterial.AsshowninFig.2,thisradiatorcanexistforseveral secondsandmodulationsbyELMsandheatingpowertripsdonot leadtoanimmediateendofthedischarge.

Such strong poloidal asymmetries of radiation and electron temperaturemight be unexpected for the conﬁnedregion of the plasma. However, the X-point region has a high ﬂux expansion

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Fig. 3. Vertical position of the radiator relative to the X-Point in AUG #32273 with the modulation of heating power and N seeding (top).

and, thus,insidetheconfined region,alongconnection lengthto the midplane. Therefore, poloidaltemperature gradients resultin ratherlowparalleltemperaturegradients.Thistemperaturegradi- entdrivesapowerfluxintotheradiatingregion,whichcanbesus- tainedby theradial powerflux intothespecific fluxsurface, and whichstabilizesthetemperatureinthisregion.Astheconnection lengthisstronglychangingattheX-point(ifmovingverticallyup- wards),theregion ofintenseradiationmightbe determinedby a local equilibriumbetweenpowerflux driven intothe region(de- termined by the parallel temperature gradient)and the radiated power. This can also be an explanation of the observed vertical movementoftheX-pointradiator.

UsingthelinesofsightoftheAXUV diodediagnostic [18],the verticallocationandextendoftheX-pointradiatorcanbetracked.

Fig. 3 showsthe vertical position relative to theX-point fordis- chargeAUG#32273.Byareductionofheatingpower,thisradiator movesfurther insidethe conﬁnedregion.The innermostposition showninFig.3correspondstoρpol ≈ 0.9975.Withanincreaseof the heating power, the equilibration point of the radiator moves closer to the X-point. The increase ofthe N seeding levels leads againtoaninwardmovement.Iftheradiatormovestoofarinside theconﬁnedregion,adisruptionistriggered(e.g.inAUG#32274).

SimilartoobservationsofMARFEsatthetwodeviceswithcarbon as ﬁrst wall material [19,20], the radiator moves upwards along thehigh-ﬁeldsideandmostlikelytriggers adisruptive2/1-island throughtheshrinkingofthecurrentchannelbytheedgecooling.

The X-point radiatoris observed in both devices, atAUG and JET, indicating that it isa general operational regime fordevices withafull-metalwall.Forbothdevicestheradiatoris,indetached conditions,insidetheconfinedregionanddissipatingasignificant fractionoftheinjectedpowerbyradiation.Itstillneedstobeeval- uated,inwhichwaythisradiationimpactsthepowerfluxoverthe separatrix, whichdetermines theaccessto H-mode,andhowthe plasmaconfinementofsuchascenarioisaffected.

3. Neonseeding

Neonisapromisingcandidateasanedgeradiatingimpurityfor futurereactorscenarios.Itradiatesdominantlyattemperaturesof about50eV,whichexistintheSOL.Itdoesnottraptritiuminthe deviceaspredictedforN,whichpotentiallyformstritiatedammo- nia [21].Therefore,highradiationscenarios withNeinsteadofN seedingweretestedatbothdevices,AUGandJET.

Fig. 4. Time traces of Ne seeded discharge AUG #32272. The gray lines indicate when the conﬁnement starts to increase and when the ELM frequency drops, re- spectively.

3.1. NeseedingatAUG

Fig.4 shows time traces ofa Ne seeded discharge#32272 at AUG. The Ne seeding level is increased in three steps. The ﬁrst two levels lead to a slight increase of the central Ne concentra- tionandtheoverallradiationlevel.However,thedivertorradiation (Fig.4(d))isonlymarginallyaffected bytheinjectedneon, show- ing that Ne does not increase the divertor radiation to required levels foreﬃcient power exhaust(compared to Nin Fig. 2). The presenteddischarge is ended by a central accumulation oftung- sten, which results from changes in pedestal transport and ELM frequencycausedbytheNeseeding,asdescribedbelow.

The injection of Ne leads to an improvement of the confinement,hereseenat4.1s,similartotheobservationsmadewithni- trogen. Unlike for N, where the pedestal top temperature is increased and, thus, leads to the confinement improvement [22], withNe the pedestal top densityis increased. Edge kinetic pro- filesbefore andaftertheconfinementimprovementareshownin Fig.5(a).

The ratio of the neoclassical radial drift and diffusion coeﬃ- cients for impurities, shown in Fig. 5(b), is calculated with the NEOARTcode[23].Theneoclassicalinward drift,whichdominates the pedestal impurity transport in between ELMs [24], increases signiﬁcantlyforNeaswellasfortungstenwiththeincreasedden- sity gradient. This leads to an increase of the peaking factor of tungstenatthepedestal

FW=n_W(ρpol=0.97) nW(ρpol=1) ⁼

0.97 1.0

v

Ddr [25]from5to35.

Furthermore,theELMfrequencyissteadilyreducingduringthe Neseeding (Fig.4(e)).Thisisthetypicalbehaviouroftype-IELMs duetothereductionoftheheatingpower[26].Inthepresentcase, thecentralradiationofNereducestheactualpowerﬂuxoverthe separatrix and thus leads to the reduction of ELM frequency. By

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Fig. 5. Top: Edge proﬁles of electron density and temperature before and after the conﬁnement improvement. Bottom: NEOART calculation of the neoclassical impurity transport at the pedestal.

0.00 MW/m³ 0.60 MW/m³ 1.20 MW/m³ 1.80 MW/m³ 2.40 MW/m³ 3.00 MW/m³

(a) JPN 85441 (b)

53.37s

JPN 85441 53.47s

Fig. 6. Radiation distribution of the Ne seeded discharge JPN 85441. (a): H-mode, detached state; (b) L-mode, attached state.

thedecreaseintheELMfrequency,theeffectofimpurityﬂushing byELMs[24]issigniﬁcantlyreduced.

Botheffectstogether,namelytheincreasedneoclassicalinward transport and the reduced ELM ﬂushing, lead to a strong, self- amplifyingincrease oftheimpurity concentrationintheconﬁned plasmaand,thus,thecollapseofthedischarge.

The lackofeﬃcientdivertorcoolingandtheabovementioned effects of impurity transport make it impossibleto studypower exhaustwithNeseeding atthe testedAUG parameters.In future devicesthepedestalimpuritytransportisexpectedtobeverydif- ferentfromcurrentexperiments duetothe higherpedestal temperature gradients and lower density gradients. The temperature screeningofimpurities increasesand thedensitygradient driven inwardpinch reduces[27].Therefore,the applicationofNeseed- ingforpowerexhaustinsuchdevicesmightstillbepossible.

3.2.NeseedingatJET

AtJET,Neseedingtohighradiatedpowerfractions(f_rad>50%) leadstoaditheringbetweenELM-less H-modesandL-modesand aconﬁnementofH₉₈ ≤ 0.8[11].Inthesecasesthedominantradi- ationisemittedfromtheregioninsidetheX-point,similar tothe observationswithNseeding. Withthedithering betweenH-and L-mode,the divertorgoesfroma detached (H-mode)to attached (L-mode)state.InH-modetheradiationisconcentratedatthere- gionjustinsidetheX-pointwhileinL-modetheradiationismore diffuseinthedivertorandincreasedintheSOL(seeFig.6).

Fig. 7. Time traces of Kr seeded discharge AUG #33256.

Thetransitions betweenH-andL-mode indicatethat thecen- tral radiationlosses by Ne reduce the powerflux over the sepa- ratrixbelowtheL-H powerthreshold.In L-mode,thepower flux over the separatrix is about 1MW higher than in H-mode. This difference might determine the change between H-and L-mode, however,thepowerfluxinH-modeiswith9.5MWstillabovethe L-Hthresholdscalingofabout8.7MW.

The ditheringbetweenH-andL-mode makes Neseeding also not applicable for power exhaust scenarios at JET for the tested heatingpowerofabout19MW.Withan externalheatingofmore than 22MW,the plasma mightstay in H-mode and Necould be furthertestedasaSOLanddivertorradiator.However, suchheat- ingpowerswerenotavailablefortheseexperiments.

4. Kryptonseeding

Kryptonhasahighradiationeﬃciencyatelectrontemperatures ofabout300eV. Thus, itis expectedtocreatestrong radiationin the pedestalregion inside theseparatrix, asit isrequired forfu- turefusionreactors. Additionally,at15eV thecoolingfactorofKr hasanotherpeak,therefore,alsoincreasedradiationinadetached divertorcanbeexpected.

4.1. KrseedingatAUG

Fig. 7 showstime tracesof discharge AUG #33256, where Kr seedingisapplied.Verylow seedingratesofKrare required(Kr

<210²¹l/s),therefore,agaspuff modulationschemeisappliedin ordertoachievereliableparticleﬂuxesfromthegasvalves.There isnoeffectseenbythismodulation,indicatingthat thetransport timescaleofKrisabove30ms.Inthepresenteddischarge,aNpuff wasappliedbyarealtimefeedback[6]tokeeptheelectrontem- peratureattheouter divertortargetaround 5eV.Theadditionally injectedKrisstepwisereplacingtheNpuff.

Withthe ﬁrststep,there isonly asmallreduction on theav- eraged N seeding ratein order to keep the divertortemperature

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Fig. 8. Radiation distribution (ELM averaged) with dominant Kr radiation for AUG

#33256. The radiated power fraction is up to 90% and a radiating ring evolves at the pedestal. Such a radially narrow structure, which is most likely poloidally sym- metric, cannot fully be reconstructed by the tomographic inversion (see Fig. 9 for details). The radiation in the divertor legs is dominated by ELM induced radiation.

Fig. 9. Left: Time traces of AXUV diode measurements at the pedestal region for AUG #33256. Right: Viewing volume of AXUV channels.

constant. The ELM frequency reduces slightly as the power ﬂux over the separatrix is reduced by an increased central radiation.

The drop inplasmastored energyat 3s iscaused by an internal 3/2NTM, whichdevelopsatthehighbetanormalizedof2.7.This reducesthe storedenergyby 10%,which remains stablelateron.

Otherplasmaparametersdonotchangesigniﬁcantly.

WiththesecondlevelofKrseeding,thedivertorgoesintothe detached state (T_div ≈ 0eV) andno further Nseeding isrequired fordivertorcooling.WithKrasdominantradiator,aradiatingring is forming around the pedestal region (see Fig. 8). The Kr radiation inside the confined region dissipates sufficient power to re- ducethepowerfluxesontothetargetplatestoreachdetachment.

Theradiatedpowerfractionisupto90%.ThisisobservedwithKr concentrationsoflessthan5· 10⁻⁴(Fig.10,[28]).Withtheonsetof detachment,theplasmadensityincreasesbymorethan30%.

Fig.9showsthelineintegratedmeasurements ofthepedestal radiation by AXUV diodes. The dominant radiation is emitted in theregionofchannelDHC44,whichisdominantlymeasuringbe- tween ρpol=0.95− 1. This is identiﬁed by the strong signal in- creasefromDHC45to DHC44,butno furtherincrease to DHC43.

Withthestrongradiationinsidetheconﬁnedregion,theELMfre- quencyreducesto10–20Hz.

In Fig. 9 the strong modulation of the pedestal radiation by ELMs can also be seen. In between ELMs, the pedestal radiation increasesrapidly,whilethepedestalisstronglyerodedbyanELM.

This can be explained by a stronginward transport of Krin between ELMs, created by an increased densitypedestal as shown inSection3.1.Kriscoolingthepedestalregionand, thus,leading to afurther increaseofthe density.Thisisa self-amplifying pro- cessandleadstransientlytohollowdensityproﬁles.TheKrinthe

Fig. 10. Kinetic proﬁles (top) and radiation proﬁles (bottom, inverted from bolom- etry measurements and modeled by STRAHL [29] ) for two discharges before and after the radiation is dominated by Kr. Adapted from [28] .

pedestalregionisﬂushedoutby ELMsandthepedestal radiation isdecreased.AfteranELM,Krisagaintransportedinside.

Athigh ELMfrequencies, the balance ofinward transport and ELMﬂushing ofKr could potentially lead to a quasi steadystate scenario.However,asKrincreasesthecentralradiationandbythis reducesthepower ﬂuxover theseparatrix, theELM frequencyis inherentlyreducedandthenetinward transport ofKris increasing. This resultsin a very low ELMfrequency andcan lead to a (minor)radiativecollapseoftheplasma,seeninFig.7at4.8s.At- temptstokeepahighELMfrequencye.g.byadditionalNinjection showedthattheeffectofKrontheELMfrequencyisstilldominat- ing.Therefore,othermeansofELMfrequencycontrolarerequired andnotyettested.

TheimpactoftheKrradiationontheconfinementdependson theradial location ofthe radiation.Fig. 10compares kinetic pro- filesandradiationprofiles fortwo Krseededdischargeswithdif- ferentheating powers.In the dischargewithhighheating power (P_heat=19MW),thepedestaltoptemperatureisreducedbytheKr radiationfrom1.2keVto800eV.Theregionofmaximumemissivity ofKrstaysinsidethepedestalregionandthestoredenergyofthe plasmaisobservedtonotbeaffected.FortheSTRAHLsimulation,a Krconcentrationof5· 10⁻⁴wasassumed.Thisiswellreproducing theradiatedpowermeasurements, eventhough noother impuri- tieswereincludedinthecalculation,and, thus,indicates aupper thresholdfortheKrconcentration[28].

In the case with lower heating power (P_heat=10.5MW), Kr leadsto a reduction of the pedestal top temperature from more than 500eV to lessthan 400eV. In these cases the dominant Kr radiation shifts further inside the conﬁned region and the con-