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Ignition of Unipolar Arcing on Nanostructured Tungsten in Fusion Devices

This workshop delves into the ignition of unipolar arcing on nanostructured tungsten in fusion devices, addressing issues in high-gradient technology. The research explores the impact of pulsating heat loads and plasma radiation on tungsten surfaces and their resistance to transient heat loads. With a focus on arc formation and morphology changes caused by helium fusion products, the study investigates the anomalous surface temperature increase and electron emission enhancements on nanostructured surfaces. The presentation highlights critical evidence of unipolar arcing and the role of laser irradiation experiments in simulating ITER-like conditions. Ecton mechanisms and fractality of arc trails under magnetized conditions are also discussed, shedding light on the behavior of arcing spots on nanostructured surfaces. The findings provide valuable insights into triggering and controlling arcing in fusion devices to enhance safety and efficiency.

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Ignition of Unipolar Arcing on Nanostructured Tungsten in Fusion Devices

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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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