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Shin Kajita 1 , Noriyasu Ohno 1 , Shuichi Takamura 2

19 th PSI conf. in San Diego (May 25, 2010). Tungsten blow-off in response to the ignition of arcing: revival of arcing issue in future fusion devices. Shin Kajita 1 , Noriyasu Ohno 1 , Shuichi Takamura 2 Nagoya University 1 , Aichi Institute of Technology 2.

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Shin Kajita 1 , Noriyasu Ohno 1 , Shuichi Takamura 2

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  1. 19thPSIconf. inSanDiego(May 25, 2010) Tungsten blow-off in response to the ignition of arcing: revival of arcing issue in future fusion devices Shin Kajita1,Noriyasu Ohno1,Shuichi Takamura2 Nagoya University1, Aichi Institute of Technology2 Acknowledgementtothecontributionof N.Yoshida(Kyushuuniv.)

  2. overview • Introduction • Heliumirradiationeffects • Ignitionofarcing • Nature ofarctrail • Conclusion

  3. Arcingissueinfusiondevices -longstandingPSIissue- • Arcinghasbeenextensivelyinvestigatedin1980sintokamaks. • Afterward,ithasbeenthoughtofasaminorissue,becauseitcouldbeeliminatedbyadvanceddischargecontrol,andthemainimpuritysourceis though to besputtering. • However,revivalofarcing couldbebroughtaboutfromnewaspects: Luminescencefromarcspotonlimiter R.Clausingetal.J.Nucl.Mater(1981) ArctrackintheDITEtokamak. McCrackenetal.Nucl.Fusion(1978) • PulsedheatloadaccompaniedwithELMs(EdgelocalizedModes) • Surfacemorphologychangeduetoplasmairradiation

  4. Effectofpulsedheatload • InITER,theheatloadsofELMs(EdgeLocalizedModes)anddisruptionare1MJm-2(0.1-1ms),and10MJm-2(1-10ms),respectively. • Inadditiontothematerialdamage(evaporation,formingcracksanddroplets,MHDmotionofthemeltinglayer),arcingcouldbetriggeredbythetransientheatload. Microstructure of tungsten surface irradiated by plasmastream. Arkhipov,FED(2000) Images of deformed tungsten M1 at four different applied power densities (single shots): (A)0.33GWm−2; (B)0.44GWm−2; (C) 0.55GWm−2; (D) 0.88GWm−2. Pintsuk,FED(2007)

  5. Morphologychangebyplasma irradiation IrradiationwithHe IrradiationwithH, D 0.25 keV Ionbeam H. Iwakiri, JNM(2000) Nanometersizedbubbles M. Ye, JNM(2003) Blistering D. Nishijima, JNM(2004) Formationofmicrometersizedbubblesat>1500K. blacking (nanostructure) S. Takamura, PFR(2006) S. Kajita, NF(2007)

  6. ExperimentsinthedivertorsimulatorNAGDIS-II DemonstrationoftheeffectoftransientheatloadonheliumirradiatedW • Pre-irradiationofHelium • ⇒formationofnanostructure • Rubylaserirradiation • (0.6ms,5MJm-2） • Similarasthetype-IELMsinITER ne>1018-1019m-3 Te~5-15eV

  7. Formation process of the nanostructure • Formation of pin-holes • Rod-like structure is formed • Digging and swelling processes may occur. Hefluence:(a) 6×1024 m−2,(b) 1.1×1025 m−2,(c) 1.8×1025 m−2,(d) 2.4 × 1025 m−2 (1) Initial surface (4) (2) (3) By Prof. Yoshida S.Kajita,etal.NuclearFusion(2009)

  8. Heliumbubblelayerexistunderprotrusions 100nm TEMsamplewaspreparedbyFIBmilling(t=50nm) 1400K,50eV-He plasma,6x1024He+/m2 • Thesubsurfacelayer(~100nminthickness)andprotrusionsarepackedwithbubbles.Thesizeofthebubblesarelessthan50nmindiameter. Obs.by Prof.Yoshida (Kyushuuniv.)

  9. ArcignitedonnanostructuredWsurface B Frombottom 50000fps(1frame20ms) • Arcspotmovesfreelyinretrograde(-jxB)direction. 30000fps(1frame33ms) Fromback The amount of Wejection is significantly increased! Backsideofthesurface

  10. Features of arcing : Voltage & current jumps, strong emission. voltage • After arcing is ignited, voltage and current jump, and strong emission is observed. • The current during arcing is limited by the power supply in the present experiment. current emission

  11. IgnitionconditionI:Morphologychange ・Triggeringofarcingrequiresonlyaslightmorphologychange.(muchlessthan1025m-2seemssufficient.) ・Laser position is changed shot-by-shot. ・Arcdurationincreaseswithheliumfluence,andlongerthanthepulsewidthwhenfluenceis>3x1025m-2. Arcing is triggered He fluence :3.3 x 1024m-2 He fluence :1.4 x 1024m-2

  12. 200nm Arcingmaybetriggeredbybubble bursting • Incident ion energy~ 50 eV • Surface temperature ~1400 K • He fluence: 6x1024 m-2 TEMObs.By Prof.Yoshida (Kyushuuniv.) Protrusions Highly pressurized He bubbles • ・Increaseofthetemperatureinthelidofbubblescouldresultintheburstingofbubbles. • Theburstingmaymakeitmucheasiertoignitearcing. Schematicofplasmaassistedlaserablation. S.Kajita,etal.APL(2007)

  13. IgnitionconditionII:TargetbiasingisimportantfactortotriggerarcingIgnitionconditionII:Targetbiasingisimportantfactortotriggerarcing Arcing is triggered • Arcingisnevertriggeredwhenthetargetvoltageishigherthan-55V,butconstantlytriggeredwhenthebiasingvoltageissufficientlylow(here,-60V,whichissufficientlylowerthanthefloatingpotentialof-18V!). • Ithasnotyetunderstoodthemechanismofthisbiasingdependence. No Arcing

  14. Demonstrationofunipolar arc(UA) ・Arcingignitedevenatthefloatingpotential.(TherehasbeennoreportofUAinsteadystateplasmainlaboratoryexperiments.) ・ThesituationisveryclosetotheUAmodelproposedbyRobsonandThoneman50yearsago. ・ELMscouldtriggertheunipolararcingwitheaseforheliumirradiatedW. Splitofarcspotcanbeseen. S.Kajitaetal.Nucl.Fusion(Letter)(2009)

  15. Analysis of arc trail :arcspotgrouping TrailonthenanostructuredWissimilarastheman’sfootprintsinthesnow • Arcspotmovesalongwithretrogradedirection+acuteanglerule. • Arcspotof~10mmmoveswithforminggroup. S.Kajitaetal.PhysLetterA(2009)

  16. Fractalityoftrailundermagnetizedcondition -self-affinefractal(scaledependsondirection)- DigitizedSEMmicrographsofarctrail. • Fromthedistributionofthedotsinradiusr,thenumberofdotsrepresentsfractalitylocally,butnotglobally. • self-affinefractality • Locally:randommotion • Globally:linearmotionduetomagneticfield B=0.1T r S.Kajitaetal.J.Phys.Soc.Jpn.(2010)

  17. conclusion • Synergistic irradiation effects of helium plasma and laser pulses were investigated experimentally. • On the heliumirradiatedW surface, unipolar arcing/arcing is ignited promptly in response to the laser pulse irradiation. • Theheliumfluenceof~3x1024m-2seemssufficientlyaltertheignitionconditionofarcing,probablybecauseoftheburstingofheliumbubbles. • The arcing significantly enhances W blow-off. This result may urge re-consideration of arcing issue in nuclear fusion research. • Fromthedetailedinvestigation,followingfeatureofarcinghasbeenrevealed • <Nature of Arcing> • From the arc trail, arc spot moves with forming group. • The arc spot moves randomly, but globally moves in some direction that is determined by the axial and parallel magnetic field.The arc trail has self-affine fractality.

  18. Morphologychangebyheliumirradiation Kyushuuniv.(JA) (obs.Dr.Iwakiri) formation conditionofthe fiberformnanostructure(fuzz) Temperaturerange,1000K <T<2000K Incidentionenergy，>20eV S.Kajita,etal.Nucl.Fusion(2007,2009)

  19. ArcignitedonnanostructuredWsurface Frombottom 50000fps(1frame20ms) Emissioncouldcontinueformuchlongertimethanthepulsewidth.

  20. Arcingobservedfrombackside • Arcspotmovesfreelyinretrograde(-jxB)direction. B Fromback 30000fps (1frame33ms) Backsideofthesurface  Arctrailwasrecordedclearlyonthesurface Notethattheelectrodeisbiasedinthiscase.

  21. Filterspectroscopy:VisualizationofreleasedW NAGDIS-II W filter (400.9 nm) Laserinvacuum HeirradiatedW 0.8 J/cm2 He filter (707 nm) Laserinplasma HeirradiatedW 1.4 J/cm2 WandHearereleasedinresponsetothelaserpulse. Schematicofthefilterspectroscopysystem (b) W I(400.9 nm), (c) He I(706.5 nm)

  22. Laser fluence Plasmaassistedlaserablation w/oHeirradiation：>1 J/cm2 (i)Fluence 7.7x1025m-2：<0.5 J/cm2 (ii)Fluence 3.8x1026m-2：<0.2 J/cm2 Reductioninablationthreshold • HiglypressurizedHe(>MPa). • Heatingofthelidofhole • ⇒thestressexertedonthelidexceedthetensilestress • ⇒Burstingofholes • ⇒Ablationwithlowlaserfluence S. Kajitaetal.,Appl. Phys. Lett. (2007)

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