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Ignition of unipolar arcing on nanostructured tungsten

International Workshop on Breakdown Science and High Gradient Technology (April 19, 2012 in KEK). Ignition of unipolar arcing on nanostructured tungsten. Shin Kajita, Nagoya university Acknowledgement Noriyasu Ohno, Nagoya university Shuichi Takamura, Aichi Institute of Technology

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Ignition of unipolar arcing on nanostructured tungsten

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  1. International Workshop on Breakdown Science and High Gradient Technology (April19, 2012inKEK) Ignition of unipolar arcing on nanostructured tungsten Shin Kajita, Nagoyauniversity Acknowledgement Noriyasu Ohno, Nagoyauniversity Shuichi Takamura, AichiInstituteofTechnology Masayuki Tokitani, Suguru Masuzaki, NIFS Naoaki Yoshida, KyusyuUniv.

  2. NuclearFusionExperiments:ITER Divertorregion Divertorcassette • Materialinfusionreactorare (tungsten)willbesubjectedtoahighheatload,~10MW/m2. • Andalsoexposedtothetransientheatload.InITER,ELMs (EdgeLocalizedModes)heatloadisexpectedtobe0.5MJ/m2for0.1-1ms. • France,Cadarache • EU,India,Japan,Korea,Russia,US • Firstplasmawillbeproducedin2019.

  3. Arcingissueinfusiondevices -longstandingPSIissue- ASDEX-U • Arcinghasbeenextensivelyinvestigatedin1980sintokamaks. • Mechanism:unipolararcing  Rohde,19thPSIconference,2010,SanDiego • Although,afterward,arcingwasthoughttobeaminorissue,revivalofarcing couldbebroughtupfromnewaspects: • -PulsedheatloadaccompaniedwithELMs • -Surfacemorphologychangebyplasmairradiation Schwirzke,IEEETrans.PlasmaSci.(1991) • Anodeandcathodeexistonaplate. • Electronreleasefromcathodespot • Currentloopisformedwithinoneplate

  4. Theprobleminfusiondevice: Morphologychangebyfusionproducthelium D-Tnuclearfusionprocess D+THe(3.5MeV)+n(14.1MeV) Concentrationwillbeupto10%indivertor. formation conditionofthefiberformnanostructure(fuzz) Temperature:1000K <T<2000K Incidentionenergy:>20eV • Bythenanostructureformation • Fieldelectronemissionisenhanced. • Thermaldiffusivityissignificantlydecreasednearthesurfaceanomaloussurfacetemperatureincreaseinresponsetotransientheatload. S.Kajita,etal.Nucl.Fusion47(2007)1358. S.Kajita,etal.Nucl.Fusion49(2009)095005. S.Kajita,Appl.Phys.Exp.(2010)

  5. PulsedheatloadandplasmairradiationtoW Damagedbytheplasmairradiation Transientheatload ＝ ?? ＋ WeperformedlaserirradiationexperimentsbyusingWexposed to heliumplasma. • Pre-irradiationofHelium • ⇒formationofnanostructure • Rubylaserirradiation • (0.6ms,5MJm-2） • Similarasthetype-IELMsinITER Divertorsimulator NAGDIS-II ne>1018-1019m-3 Te~5-15eV

  6. An arcingobservedfrombackside • Arcspotmovesfreelyinretrograde(-jxB)direction. B Fromback 30000fps (1frame33ms) Backsideofthesurface  Arctrailwasrecordedclearlyonthesurface Notethattheelectrodeisbiasedinthiscase.

  7. Observedfrom frontside(laserirradiatedside) Arcing(biased).Framerate:1000000fps

  8. Criticalevidenceofunipolar arc(UA) ・DemonstrationofELMsonnanostructuredWusinglaser. ・UA is confirmed from the jumpofthefloatingpotential. S.Kajitaetal.Nucl.Fusion(Letter)(2009)

  9. Arc spot motion in oblique magnetic field: thearcspotsrotatearoundtheelectrode • Arcspotmovesgloballytothe directiondeterminedbytheaxialandparallelmagneticfields.

  10. Ectonmechanismofunipolararcing TheunipolararcingonthenanostructuredWwasexplainedusingEctonmechanism(Explosiveelectronemissionprocess).

  11. arcspotsform a group and move together • Arcspotmovesalongwithretrogradedirection+acuteanglerule. • Arcspotof~10mmmoveswithforminggroup. S.Kajitaetal.PhysLetterA(2009)

  12. 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)

  13. FractalitydecreaseswithB D=1.46 ±0.10 D=2.07 ±0.18 • LocalfractaldimensionDwas2.07±0.18atB=0.02T,butdecreasesto1.46±0.10at0.2T. B=0.2T B=0.02T S.Kajitaetal.PlasmaPhys.Cotrol.Fusion (2011)

  14. IgnitionconditionI:HeFluencedependence Pulseenergy~0.035MJm-2 • Laser position is changed shot-by-shot. • Currentjumpdurationincreaseswithheliumfluence,andarcwasinitiatedwhen>3x1025m-2. Currentjumpduration[s] Fromadditionalexp:necessaryfluencedecreasedasincreasingthelaserpulseenergy.

  15. IgnitionconditionII:TargetpotentialisimportantfactortotriggerarcingIgnitionconditionII:Targetpotentialisimportantfactortotriggerarcing Arcing is triggered • Arcingisnevertriggeredwhenthetargetvoltageishigherthan-55V,butconstantlytriggeredwhenthebiasingvoltageissufficientlylow(here,-60V,whichissufficientlylowerthanthefloatingpotentialof-18V!). • Arcing might be suppressed if we could control the target potential. Pulseenergy~0.7MJm-2 Currentjumpduration[ms] No Arcing

  16. Ignition condition III : laser power dependence Threshold is VERY LOW on nanostructured W Currentjumpduration[ms] Nanostructure can melt even at 0.1 MJm-2 because the thermal diffusivity significantly decreased. (Kajita, NF(2007)) ・When the nanostructure is formed on the surface, arcing is initiated with very a low power pulse. ・Thethresholdpoweris~0.01-0.02MJm-2,whichismuchlowerthanthetypicalTYPE-IELMsinITER(~1MJm-2). S. Kajita, et al., Plasma Phys. Control. Fusion 54 (2012) 035009.

  17. Fuzz-WexposedtotheLHDplasma T：1460K G：1.2×1022 /m2s Fluence：2.2×1025 /m2 Energy：57eV • HeIrradiationinNAGDIS-IIandinstalledinLHD. LHD:LargeHelicalDevice (@Gifu,Japan)

  18. Arctrailanalysis:Brownianlike motionofarcspotswasobserved • ExposedtotheLHDplasmafor2s. • brightemissionwasobserved. • Thisresultsstronglysuggestthatarcingcanbe easilyinitiatedwhenthenanostructureisformedonthesurfaceevenwithouttransients. • Nanostructuredisappearedinsomepart.Arctrailwasclearlyrecodedonthesurface. M.Tokitanietal.Nucl.Fusion51(2011)102001.

  19. conclusion • UnipolararcwasinitiatedonthenanostructuredWsurfaceinsteadystateplasmaenvironment. • Fromfundamentalarcexperiments,itisfoundthatarcingcanbeinitiatedunderthefusionrelevantconditionswhenthesurfaceiscoveredwithnanostructures.Theignitionconditionswereinvestigatedintermsoftheheliumfluence,laserpower,(plasmadensity,targetpotential). • TheinitiationofarcingonthenanostructuredWhasbeendemonstratedinLHD.Arcingwasinitiatedwithouttransients. • Arcingcouldbeanimportantissueinfuturefusiondevices.Itisimportanttorevealtheinitiationprocessandmechanismandfindavoidanceormitigationstrategies.

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