Characterization and origin of large size dust particles produced in the Alcator C-Mod tokamak

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Characterization and origin of large size dust particles

produced in the Alcator C-Mod tokamak

Cécile Arnas, James Irby, Sébastien Celli, Gregory de Temmerman, Younes

Addab, Lenaic Couëdel, Christian Grisolia, Yijun Lin, Céline Martin, Cédric

Pardanaud, et al.

To cite this version:

Cécile Arnas, James Irby, Sébastien Celli, Gregory de Temmerman, Younes Addab, et al..

Charac-terization and origin of large size dust particles produced in the Alcator C-Mod tokamak. Nuclear

Materials and Energy, Elsevier, 2017, �10.1016/j.nme.2017.02.027�. �hal-01499487�

ContentslistsavailableatScienceDirect

Nuclear

Materials

and

Energy

journalhomepage:www.elsevier.com/locate/nme

Characterization

and

origin

of

large

size

dust

particles

produced

in

the

Alcator

C-Mod

tokamak

C.

Arnas

a,∗

_,

_J.

_Irby

b

_,

_S.

_Celli

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_G.

_De

_Temmerman

c

_,

_Y.

_Addab

a

_,

_L.

_Couëdel

a

_,

_C.

_Grisolia

d

_,

Y.

Lin

b

_,

_C.

_Martin

a

_,

_C.

_Pardanaud

a

_,

_S.

_Pierson

b

a CNRS, Aix-Marseille université, Laboratoire PIIM, campus St Jérôme, 13397 Marseille, France b M.I.T. Plasma Science and Fusion Center, 175 Albany Street, Cambridge, MA 02139, USA c ITER Organization, Route de Vinon-sur-Verdon, 13067 St Paul Lez Durance, France d IRFM/CEA Cadarache, 13108 St Paul Lez Durance, France

a

r

t

i

c

l

e

i

n

f

o

Article history: Received 17 January 2017 Revised 22 February 2017 Accepted 23 February 2017 Available online xxx Keywords: Dust formation Molybdenum Tungsten Boron Alloy

a

b

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t

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PostmortemanalysesofdustcollectedinAlcatorC-Modhavehighlightedaproductionoflargesizedust particles.Thequantitiesofsuchlargeparticlesarehigherthaninanyothertokamak.Theyaredivided intwoclassesasafunctionoftheirshapeandconsequently,theirorigin.Roundeddustparticlessuchas spheresandsplashesconstitutethefirstclass.Theseparticlesaretheresultofhighheatloadsonvarious leading edges ofplasmafacing componentsand possibly,theirmelting during plasmaoperation. The heatedoralreadymoltenmaterialcanbedestabilizedduringdisruptionsanddropletsareemittedacross thevacuumchamber.Aftersolidification,theresultingroundedparticlesareeitherinpureelementsor inalloys.Flake-likedustparticles,whicharemainlyduetolightmaterialcoatingdelamination,constitute thesecondclassofdustparticles.

© 2017TheAuthors.PublishedbyElsevierLtd. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense. (http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

Althoughthepresenceofdustincurrenttokamaksisnot con-sideredasafetyissue,theFrenchnuclearsafetyauthorityhas im-posed a limit on the dust inventory in ITER to avoid a possible disseminationduringaLOVA(LossOfVacuumAccident)andaICE (IngressofCoolantEvent).Thepossibilityofdustmobilization dur-ing these potential accidents depends on the produced quantity and theadhesion forces, whichmaintain them onPFCs. In addi-tion tosafetyissues, themobilizationoftungsten dust (ITERwill useafull-Wdivertor)couldbeasourceofhigh-Zcontaminationof thecoreplasma.Thesepotentialharmfulsituationshavemotivated many studies ondust production intokamaks withgraphite and metal PFCs:origins [1–6],characterizationby post-mortem analy-ses [7–9],adhesionforces [10–12],transport [13–18],removal tech-niques [19,20]andinfluenceontheperformanceoffusiondevices

[21–23].However,amongthewidevarietyofdustparticles gener-ated by plasma-wall interaction,none ofthe post-mortem inves-tigations allowed a clear identification of those produced during instabilities (ELMs) and off-normal events such as VDEs or

dis-∗ _{Corresponding author.}

E-mail address: cecile.arnas@univ-amu.fr (C. Arnas).

ruptions. Onlyfastcamerasallowed a direct visualizationof dust particles withsize larger than 5–10μm [24,25]. Thelack of pre-cisemeasurementsinsuch conditionsleadstolargeuncertainties onthesizeandkindofdustparticlesproducedduringoff normal events incurrent tokamaks. These uncertainties make extrapola-tionsdifficultforITER [26].

Post-mortemanalyses presented in this paper allowed study-ing the shapes, compositions and sizes of dust particles gener-atedinthefull-metaltokamak,AlcatorC-Mod. Elemental compo-sitions established by energydispersiveX-ray spectroscopy(EDS) allowedselectingmolybdenum(Mo),boron (B),tungsten (W)and thecorrespondingmixedmaterialparticles,whichwereproduced byplasma-wallinteraction.Sizedistributionsestablished by scan-ning electron microscopy (SEM) revealed unexpected large aver-age sizes (tens of μm), larger than those already established in other tokamakswithgraphite andmetal PFCs [2–5].Twogeneral dustshapeswere identified,defining twodifferentorigins. Spher-icalandsplash-likeparticlesinpure Mo,W andBwere foundas wellasinMo-BandMo-Walloys.Cameravideoshaveshownthat theiroriginisduetoanoverheating ofvarious PFCleadingedges duringplasmaoperations-possiblytheir melting-followedby an emissionofmoltenmaterialdropletsduringdisruptions [27,28].B flake-likedustparticles producedby thedelamination ofBlayers

http://dx.doi.org/10.1016/j.nme.2017.02.027

2352-1791/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

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Fig. 1. Schematic of the Alcator C-Mod horizontal mid-plane showing 10 toroidal sectors corresponding to 10 lower outer divertor modules. Sectors are identified by the names: AB, BC… JK, KA. On the left side, the blue part shows where dust was collected in a given sector. (For interpretation of the references to color in this fig- ure legend, the reader is referred to the web version of this article.)

Fig. 2. View of the outer divertor, on the right side. A toroidal row of W tiles was inserted in 2007 in the strike point region and removed in 2010. An arrow indicates the W tile row position.

orB multi-layers were also identified and constitute the second classofparticlesinAlcatorC-Mod.

2. Methodstoselectdustproducedbyplasma-wallinteraction

Alcator C-Mod isa molybdenum–tokamakof0.68mmajor ra-diusand0.22mminorradius,operatingwithatoroidal magnetic fieldof3–8Tandaplasmacurrentof0.4–2MA.Itisdividedin10 toroidalsectorscorrespondingto10lowerouterdivertormodules. Thesectorsareidentifiedbythenames:AB,BC… JK,KA asshown intheschematicofAlcatorC-Modhorizontalsection(Fig.1).

A toroidalrowofW tileswasinsertedin2007 (Fig.2), inthe strike point region of the outer divertor and removed in 2010. Beforethe 2015 plasmacampaign, the10 outer divertormodules wereslightlyrotatedby1° toavoiddamagesonMotileedges.

Dustparticleswerecollectedattheendof2007and2015 cam-paignstoidentifydifferences.Theywerecollectedinalmostallthe sectors,usingavacuumcleaner(Atrix,modelVACEXP-04E)loaded withHEPAfiltersofefficiencygreaterthan0.3μm.Foreach inves-tigatedsector (∼1m2_{),the vacuumedareaswerethe outer}

diver-tormodulesurface(∼0.25m2_{),thefloorbetweentheouterand}

in-nerdivertors,theareabehindtheouterdivertormodulewhenthis PFC wasremovedandanyother horizontal areasmoving out to-wardthewall,includingports.Foreachplasmacampaign, around 10HEPAfilterswereusedandidentifiedbythenameofthe corre-spondingsector(AB,BC… JK,KA).Then,dustsampleswere trans-ferredinglassboxesforcharacterization.

Foragivensector,sievesof500,400,300,200,100and50μm openings were used to separate dust in samples of decreasing sizes. Sievesof400/500μm openingshaveeliminated the biggest pieces and debris coming fromlimiters, antennas anddiagnostic environments (pieces ofcables, insulators, welding, screws…) in-stalled along the wall as well as dust coming from outside the vacuum chamber.EDS analyses were systematically done in par-allel to SEM to identifyMo, B andW elementsdirectly on sam-ples. Another method consisted in transferring dust particles on thesurfaceoffreshresin.Aftersolidification,thisresinwassanded andpolishedtoobtaincross-sectionsofdustparticlesallowingthe studyoftheirinternalcomposition.

Several samples were particularly contaminated with glass fibers,silver,ceramics,stainless steeldebrisandmicasheetswith a broad size range.In a first study, only the least polluted sam-plescomingfromDEandEFsectors(RFantennasectorsasshown

Fig.1)wereconsidered.

Size distributions were established from SEM measurements with the Saisam–Microvision software. It provides known con-tours (sphere, ellipse and other) to easily characterize spherical andelongated particles.The contourofirregular shaped particles ishand-drawn.Then, thesoftwareprovidestheequivalent diame-terofthesmallestcircleinwhichtheprojectionofalltheparticle contourscanbecircumscribed.

3. Originofdustcollectedintokamakswithgraphiteand metalPFCs

Earlierpost-mortemanalysesofdustcollectedincurrent toka-maks have shown that their origin depends on PFC materials, graphite or metal. However, in collected dust samplesthere was so far noclear identificationof dustproduced during off normal eventssuchasdisruptions.

IntokamakswithgraphitePFCs,thedustproductionismainly relatedtoPFCchemicalandphysicalerosion.Apartoftheeroded materialgoesback toPFCs toformco-depositedlayers. Their de-laminationorflakingproducedustparticlesusuallycalled,“flakes”

[2,29]. Aggregatesof sphericalnanoparticleswere also foundand have two possible origins [29,30]: i) they can be initiated by molecular ions growing in the coldest plasma regions through specific collisions ofhydrocarbon or carbon radicals, ii)they can alsobegeneratedbycondensationofoversaturatedcarbonvapour when large thermal loads on PFCs produce graphite sublimation

[2].

Laboratory experiments on disruption simulations have pro-ducedanotherdustcategory [31].Itwasshownthatduring ther-mal loads delivering a power density much larger than that of graphite sublimation,graphitebrittle destruction occursand pro-duces dust of 10s of μm. However, brittle destruction was not clearlyidentifiedingraphitetokamak [32,33]whileevidenceswere found in samples coming from the Extrap reverse field pinch

[33,34].

Post-mortemanalysesallowedconcludingthatingraphite toka-maks, the layerflaking duringnormaloperations isthe principal dustsourceandsizedistributionsareusuallylognormal.The aver-agesizefound fromall thedustdistributions was(2.8± 2.4) μm

[2].The presenceoflarge isolated flake-like dust(100s μm) was alsomentioned [1,2].

Fig. 3. SEM image of dust particles collected on DE sector (2015) with a size range of 50–100 μm. EDS mappings provide the sample composition: B, Mo, Al, O and Mg element positions are presented successively.

RegardingtokamakswithmetalPFCs,analyseswere performed to characterize dust samples, collected in comparable positions after several plasma campaigns in ASDEX Upgrade (AUG). Since AUGoperateswithWPFCs,nothickdepositedlayerwasobserved

[3].W-dominatedparticleswereseparatedintwoclasses:spheres composed mainly of pure W or coated with light material and irregular-shaped particles.The latter were identified as conglom-erateswithvaryingW fractioninB-Cmatrices.Bothclasseswere attributedtoarcingontilesurfaces [35,36].Theirsizedistributions establishedovermanyplasmacampaignswerefittedbylognormal functions. The sphere average size was∼2μm andthe irregular-shaped dust average size was _∼1μm [35]. The rare presence of large particles(10sμm)wasalsomentionedandattributedtooff normalevents [3].

IntheFTUtokamakequippedwithaliquidlithiumlimiter(LLL), threedustcategorieswerefound [4]:spheroids,smasheddroplets, bothgeneratedbymeltedmaterialduetoarcingandmorelikelyto higherheatloadsprocessesonPFCS,andflakes.Spheroids, consist-ingofalmostexclusivelyMoorstainlesssteelhaddiameters span-ningfrom hundreds nmto μm.The final count median diameter was1.87μmandthegeometricstandarddeviationwas2.09μm.On theother hand,lithium-baseddusts,withsizesextendingup toa fewmmwerealsofoundinsidetheductwheretheLLLisinstalled. InJET-ILW,afterthefirstshutdown,atiny numberofdust par-ticleswasanalysed.Theseparticleswerefoundontheapronofthe so-calledtile1,locatedintheinnerdivertor [5].Analyticalstudies were performedforinstance ona Beflake with50× 70 μmsize, a5μmBedropletanda∼20μmW-basedagglomerateoriginating fromW coatingoncarbontiles. Interestingly,emptyW spheroids with∼5μmsizeswerealsocollected.

Dedicated meltexperimentsofW surfaceswerecarried-outin TEXTORbyusingthemovablesamplemanipulator,whichwas in-troduced inside theLCFS [37]. Melt layer motionand ejectionof dust into the plasma were studied using a 2D camera system. A spray offine particles (μm range) wasobserved together with somerandomlargedropletslookingassplashes(10–100μm).Post mortem analysesofredeposited dropletsprovidedanaveragesize of 4μm. Some particles with a 10–20μm size range were also foundwhilelargestdropletswerefoundassingleoccurrencesand thesmallestones(spray)weredifficulttofind.

Fig. 4. Rounded-shape dust with a size range of 10 0–20 0 μm, collected in EF sector (2007). Arrows show hemispheres. Flat faces indicate a contact with PFC surfaces.

4. CompositionsandmorphologiesofdustcollectedinAlcator C-ModDE-EFsectors

4.1. DirectanalysesofdustsampleswithSEMandEDS

SEMimage of Fig.3 showsdustparticles withasize rangeof 50–100 μm (DEsector-2015). EDS mappings give the spatial dis-tributionof dominantelements:B, Mo,Al, OandMg. TheB and Momappingsubtractionindicates ina firstapproach that notall ofMoparticleshaveaB-coatingduetowallboronizationandthat pure B dust also exists. This result will be developed further in the text. The presence ofAl and Oindicates thepresence of ce-ramic debris most likely coming fromalumina insulators (Al203)

on divertor components.Glass fibers (Si, O, Mg, Ca, Na, K) most likelycomingfromtheinsulationusedaroundinstrumentation ca-bles are alsopresentbut onlyO andMgelements oftheir com-positionare shown. IsolatedMg-dominated dust alsoappears.Cu andsteel (Fe, Ni, Cr, Mo) isolated dusts were also identified but not shown. The fractionofcontaminant dust was28%. Generally, tracesofallmaterialscomingfromtheequipmentinstalledinthe vacuumchamberappearinEDSspectrabutonlyelementsshowing sufficienthighX-raylineswereconsidered.

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Fig. 5. Dust particles with 42 μm average size (DE sector-2007) mainly composed with Mo spheres and some W spheres. Steel spheres and irregular Ag dusts were also identified but not shown by EDS.

Fig. 6. Sample mainly composed of B-flakes with a size range of 20 0–30 0 μm (DE sector-2015).

Fig.4showsanexampleofrounded-shapeparticles(EF sector-2007) with a size range of 100–200 μm. Some of them have spheroidshapes,otherarehalf-spheres.Theflatfaces(redarrows) indicateacontactwithPFCsurfaces.Althoughcompositionsare es-tablishedwithEDS,itshouldbenotedthatundertheSEMelectron beam,the higherthe atomicnumberof an element,the brighter itappears. Thus, the whiteparticle atthe top ofthe image con-sistsmainlyofMowhileneighboringspheres(leftandrightofthe whitesphere) consist ofB andMo mixture (dark andwhite, re-spectively)aswell asthetwohalf-spheresatthebottomleftside andthetwoquasi-spheresatthebottomrightside.Thedark elon-gatedparticleatthetoprightsideismadeofB-dominated mate-rial.

Fig.5showsdustparticleswithasizerangeof0–50μmcoming fromtheDEsector-2007.Around50%ofalltheparticlescollected inthissector with a 50μm sieveopening are spheres of42 μm averagesize.In thesampleshowedin Fig.5,73% ofparticlesare spheres.80% ofthesespheresare inMo,8.5%in W,8.5% insteel (notshownin Fig.5)and3%inAg(notshownin Fig.5).

Fig.6showsaseconddustcategory.Theymainlyconsistinflat piecesofBcoatings,lookinglikeflakeswitha sizerangeof200– 300μm(DEsector-2015).Oneofthem atthe topoftheimage is broken.Atthebottom,anotheroneof∼30μmthicknessisalmost perpendicularto thesubstrate. Piecesofglass fibersappear over-saturatedsincetheycannotconductcorrectlythechargesfromthe microscopeelectronbeam.

4.2.Dustcross-sectionanalysesbySEMandEDS

Since EDS mappingcannot give elemental compositionsalong thicknessdeeperthan 10s μm,dust cross-sectionswere also an-alyzed(preparationdescribedinChap.2). Fig 7showsthe cross-sectionsofa∼300μmMoparticle(DEsector-2007),a∼180μmB sphere(DEsector-2007)anda∼120μmWsphere(EFsector-2007),

withtherespectivecompositionsgivenbyEDSspot-modespectra. ThisinvestigationhasconfirmedtheproductionofpureB,Moand Wroundedparticleswithsizeslargerthan100μm.

Fig. 8-a) shows the cross-section of a rounded dust of B-Mo mixedmaterial(DEsector-2007).Onecanroughlydeduce informa-tionon thisalloyformation fromthematerialstructure. Fig.8-b) showsthe existence of a heterogeneous binaryalloywith a lim-itedsolubilityofB andMo.Itiscomposed oflargeB grains(dark grains)anda structuredB-Mo mixture.Alternatebrightanddark layers (lamellar microstructure) in various areas are characteris-tic of eutectic mixture. At the eutectic point, as for a pure ele-ment,theliquidtransformssimultaneouslyintwosolid phasesat auniquetemperature(eutectictemperature),lowerthanthe melt-ingtemperatureofeachcomponent.LetusrecallthattheMoand Bmeltingtemperaturesare 2623°Cand2075°C,respectively.The presence oflarge B grainsindicates that in theinitial mixture, B was dominant and started the solidification at 2075°C. This re-sultedinthepresenceofgrowingBgrainsinaB-Momelted mix-ture during the cooling from the B melting temperature to the lowereutectictemperature. Inotherwords,thesolidification ofB grainscontinueduntilenoughMoatomswereremovedsothatthe remaining liquid was of eutectic composition and the solidifica-tionoftheresultingmixtureappearedattheeutectictemperature. Generally, all the analyzed B-Mo dust particles presenta similar microstructure.

Fig.9-a)showsthecross-sectionofaparticle,whichconsistsof W(white)andB(dark)grains(EFsector-2007).Themicrostructure highlightedinthezoomedimage(Fig.9-b)showsaheterogeneous binaryalloywithalimitedsolubilityofB andW.The grain sizes andarrangementsvarygreatlyintothe dust.Inparticular,Wand Bgrainsizesintheleftpartof Fig.9-b)arelargerthanintheright part.No obvious organizedmicrostructure,characteristicof a eu-tecticcompositionwasobservedinthisdust.Unfortunately,onlya fewsuch particleswere foundsothata moredetailed characteri-zationwasnotpossible.

5. Sizedistributions

Using EDS, size distributions were only established with dust in Mo,B andMo-B alloy. Fig.10-a)gives the size distributionof dust comingfrom 2007-DEand 2007-EFsectors. Foreach sector, sizemeasurementswereperformedonsamplesobtainedwith suc-cessivesieveopenings.Then,theobtainedhistogramswereadded to provide thefinal size distribution. The sizesof 1697particles, whichconstitutejustapartofthedustquantityproducedinthese sectors, were measured inthis way. Fig.10-b)gives thesize dis-tribution ofa dust particlepart comingfrom the2015-DE sector (1065particles).Inthiscase,thesizesofallparticlessievedwitha 50μmopening,weremeasured.Withabinwidthof20μm,inboth cases, the most probablesize is ∼50μm and the maximum size is ∼450μm. In both cases, the largest sizes are mostly provided

Fig. 7. Cross-sections of a ∼300 μm Mo dust (DE sector-2007), a ∼180 μm B sphere (DE sector-2007) and a ∼120 μm W sphere (EF sector-2007). EDS spot-mode spectra give the respective compositions.

Fig. 8. a) Cross-section of a ∼125 μm dust in B-Mo alloy; b) magnification showing two solid phases: B grains (dark) are mixed with a B-Mo mixture of lamellar microstruc- ture, characteristic of an eutectic composition.

Fig. 9. a) Cross-section of a ∼255 μm dust in W-B alloy; b) magnification of a heterogeneous solid. The B-dominated phase (dark) and W-dominated phase (white) have different proportions on the left and right parts of the image.

by thinsplash-likeandthinflake-likeparticleswhilethesmallest onesare mostlyroundthick particles.Inthe finalsize histogram, 80% (84%) ofthe 2007 (2015) particles contain Mo,i.e. pure Mo andMo-Balloy,theremainingdustbeingmadeofpureB.

Itisimportanttoaddthatsmallerdustparticlesthanthose pre-sentedinthedistributionsof Fig.10existveryprobably.Themain propertyofHEPAfiltersbeingtotrapdustparticlesinsideamatof randomlyarrangedfibres,noinformationcanbeobtainedonthese smallsizes.

6. DiscussiononthedustorigininalcatorC-Mod

The videosnapshots (visiblecamera)of Fig. 11showa typical exampleof amolten materialdropletemission aftera major dis-ruptiongoing upwardduringthe2016plasmacampaign.The dis-ruptionoccurredbetweenthesnapshotsof Fig.11-a)and Fig.11-b). Afterthedisruption(Fig11-b)andc)),dropletsofmoltenmaterial wereejectedfromradialtileedges,whichwereglowingduringthe plasmaphase,beforethedisruption(topof Fig11-a)-arrow show-ing an example).Themajorityofthesedroplets were transported

inthe vacuumchamber around thecentral columnfrom theleft to the rightside. The plasma began witha lower single null di-vertedthatwasmovedtoanuppersinglenulldiverted.ICRF heat-ing(3.5MW)wasturnedonduringthelaterconfiguration.

Fig.12-a)showsapartoftheupperdivertor(leftside)andthe radialslots(centerpart)leadingtothecryopumps [38].The addi-tionofICRFheatingleadstoan over-heatingoftheseslotsedges, whichcanbegintomelt(Fig.11-a)).Somevideosindeedshowthat somemoltenmaterialdropletscanalreadybeexpelledfromthese glowing regions duringplasmaoperation. After disruptionsgoing upward,thetileedgesfromwhichdropletsareemittedstillremain glowingforsometime asshownbyarrowsin Fig11-c). Fig.12-b) showsan example of the resulting damage. Mo meltingappears onthepartsofthetileleadingedgeandsurface,whicharein con-tactwiththe upperdivertortarget(leftpartofthe image).Three longirregular, redepositedmelts arealso presentonthe tile sur-face(centralsquareof Fig.12-b)).

Fig. 13 shows melt damage on a sector leading edge of the outerdivertor.Theinitialmisalignmentofthissectorisatthe ori-gin of the melting of the tile edges receiving high parallel heat

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Fig. 10. a) Size distribution of 1697 particles coming from 2007-DE and 2007-EF sectors; b) size distribution of 1065 particles coming from 2015-DE sector. In both cases, the most probable size is ∼50 μm and the maximum is ∼450 μm.

fluxesinthe strike point region (C-Modhas a highparallel heat fluxowingtoitshighmagneticfield andsmallsize) [39].To sup-press melting damage, instead of toroidaly aligning the divertor sectors,aslightrotationaroundthemiddleofeachsectorwas per-formed atthe beginning of the 2015 campaign. The goal wasto shadoweach module leading edge by the upstream neighboring sector.Asexpected, avisual inspectionafterthechamber venting assuredthat fewdamage wereproduced duringthe plasma cam-paigneventhoughdisruptionsgoingdownwardandupwardwere stillproduced.Moreover,the quantityofdusttransferredfromall theHEPAfiltercartridges(afilterforalmosteachsector)towards boxesforsamplingwassmallerin2015thanin2007.Severalmass measurementswerethen performedandhaveprovidedrough es-timationsof the produced quantities. In particular, it is impossi-bletoestimate the dust quantity,whichcan be trapped inHEPA filters; there wasa highadherence of smallest dust particles on the walls of canisters and on several plastic boxes where they were transferred for further sampling; there wasa general con-taminationofB,MoandWdustparticlesbydust-debrisofvarious composition and in the sieve samples of dust coming from DE-,EF-2007 and DE-2015 sectors, contaminant dust appeared with various percentages.Within theselimits, the mass ofdust parti-cles coming fromEF-2007 sector andtransferred from HEPA fil-tercanisters(∼194.5mg)wasonaverage∼3timeslargerthanthe

Fig. 12. a) Part of the upper divertor (left side) and radial slots (center part) leading to the neutral cryopumps. Damages on the slot tiles are produced radially. Whitish and colored coatings on components, which are far from the strike point region result from many boronization sequences; b) magnification showing that Mo melt- ing takes place on the tile surfaces and leading edges, which are in contact with the upper divertor target. Three long Mo redeposited melts are present on the tile surface (into the central square).

mass ofdust transferred fromtheEF-2015 filtercanister; the ra-tioofthe transferred dustmasseswas∼4in thecaseofDE sec-tor (2007/2015) and ∼6 in the case of FG sector (∼430mg was transferredfromtheFG-2007 filtercanister).Theseresults,added tothefact thattheaverageenergyinjected in2280dischargesin 2015 was0.66MJ/dischargeagainst2026dischargesin2007with 0.45MJ/discharge, may indicate that less dust was produced in 2015.Theseresultsalsosuggest thatthedustproductionwasnot toroidallyhomogeneous.

Regarding W dust, less than 25 particles were found in the samples of DE-EF sectors(2007). Theirrounded shapes(spheres, splashes-like)associatedwithlargesizes(40–150μm)indicatethat they were produced during severe heat loads on theW tilerow

Fig. 11. a) Video snapshot showing the glowing of radial tile edges, located in the upper divertor region as well as the glowing of the surrounding plasma (1.35 s). These light emissions appear during ICRF heating. The image was taken just before a disruption going upward (1.47 s); b) and c), after disruption (1.52 s, 1.54 s respectively), droplets of molten material are ejected from the tile edges, which were glowing before disruption. Droplets that appear as hot spots around the central column are transported from the left to the right side in the vacuum chamber (image coordinate).

Fig. 13. Detail of the leading edge of an outer divertor module. The tiles of the leading edge were melted repeatedly during the 2007 plasma campaign due to their misalignment.

thatwastoroidalyinsertedintheouterdivertorstrikepointregion (noavailableimage ofWdamageduringthe2007campaign).The lownumberofWparticlescouldsuggestthattheywereproduced from other sectors and then transported towards DE-EF sectors. Note that even if the W melting temperature(3425°C) is larger than the one of Mo,W and Modust sizesare in the same size range

Fig. 12-a)also showsthe presence ofa whitish coating (right sideoftheimage)oncomponentsurfacesfarfromthestrikepoint region, resulting from frequent boronization sequences [40]. The presenceofcoloredlayersinother locationscouldbe duetolight interferencesorheatingvariationsonthincoatings.Conversely,the presenceofBflake-likeparticlesisexplainedbythedelamination ofbadlyadherentthickerlayersormultilayers.Roundededgesofa largepartofthemalsosuggestdelaminationunderhighheatloads.

7. Conclusion

Post-mortem analyses of dust collected in Alcator C-Mod af-ter the 2007 campaign (installationof a toroidal rowof W tiles in the outer divertor and removed in 2010) and the 2015 cam-paign (slight rotation of the outer divertor modules) have high-lighted a productionof large size dust, in higherquantities than inanyothertokamak.Sizedistributionsshowthatforbothplasma campaigns,themostprobablesizeis∼50μmandthelargestsizes can reach 450μm.B flake-like dusts, which mainly compose the largestparticles,comefromthedelaminationofBlayersor multi-layers.Roundeddustparticlessuchasspheresandsplash-like par-ticles inpure Mo,B, W andinmixedmaterials are coming from anemissionofthecorrespondingmoltenmaterials.Cameravideos ofplasmaoperationsaswell asvisualinspectionsofthe PFCs af-terplasmacampaignsallowedfindingtheirorigins.Theslotedges of thecryopumpsandthe leadingedges ofthe 10outer divertor modules whentheselater aremisalignedareoverheatedandcan meltduringplasmaoperations.Largesizedropletsofmolten mate-rialarethenemittedrepeatedlyfromtheseregionsduring disrup-tions, in additionto some droplets, which could be emitted dur-ingtheplasma. Aslightrotationofalltheouterdivertormodules wasperformedin2015,inordertoshadowtheirleadingedgesby theupstreamneighboringsector.Asexpected,muchlessdamages were observed at theend ofthe plasmacampaign andlessdust particleswerecollectedalthoughdisruptionsgoingdownwardand upwardwerestillproduced.

Acknowledgments

AuthorswishtothankG.GiacomettifromAix-Marseille univer-sité/laboratoire PIIM for his technical assistance during the dust cross-section procedure andM. Cabié fromAix-Marseille

Univer-sité/CP2Mforheradvises onSEM.Thiswork wassupported by a grantcomingfromtheFrenchFédérationde RechercheFusionpar ConfinementMagnétique.Theviewsandopinionsexpressedherein donotnecessarilyreflectthoseoftheITEROrganization.

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