1. 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 19 th PSI conf. in San Diego (May 25, 2010) Acknowledgement to the contribution of N. Yoshida (Kyushu univ.)

2. overview Introduction Helium irradiation effects Ignition of arcing Nature of arc trail Conclusion

3. Arcing issue in fusion devices - longstanding PSI issue - Arcing has been extensively investigated in 1980s in tokamaks. Afterward, it has been thought of as a minor issue, because it could be eliminated by advanced discharge control, and the main impurity source is though to be sputtering. However, revival of arcing could be brought about from new aspects: Arc track in the DITE tokamak. McCracken et al. Nucl. Fusion (1978) Luminescence from arc spot on limiter R. Clausing et al. J. Nucl. Mater (1981) -Pulsed heat load accompanied with ELMs (Edge localized Modes) -Surface morphology change due to plasma irradiation

4. Effect of pulsed heat load Microstructure of tungsten surface irradiated by plasma stream. 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) In ITER, the heat loads of ELMs(Edge Localized Modes) and disruption are 1 MJm -2 (0.1-1 ms), and 10 MJm -2 (1-10 ms), respectively. In addition to the material damage (evaporation, forming cracks and droplets, MHD motion of the melting layer), arcing could be triggered by the transient heat load.

5. Morphology change by plasma irradiation S. Takamura, PFR(2006) S. Kajita, NF(2007) Blistering blacking (nanostructure) H. Iwakiri, JNM(2000) 0.25 keV Ion beam M. Ye, JNM(2003) Formation of micrometer sized bubbles at >1500 K. D. Nishijima, JNM(2004) Irradiation with H, D Irradiation with He Nanometer sized bubbles

6. Experiments in the divertor simulator NAGDIS-II n e >10 18 -10 19 m -3 T e ~5-15 eV Pre-irradiation of Helium ⇒ formation of nanostructure Ruby laser irradiation (0.6 ms, 5 MJm -2 ） Similar as the type-I ELMs in ITER Demonstration of the effect of transient heat load on helium irradiated W

7. Formation process of the nanostructure Formation of pin-holes Rod-like structure is formed Digging and swelling processes may occur. S. Kajita, et al. Nuclear Fusion (2009) He fluence: (a) 6×10 24 m −2, (b) 1.1×10 25 m −2, (c) 1.8×10 25 m −2, (d) 2.4 × 10 25 m −2 By Prof. Yoshida (3) (2) (1) (4) Initial surface

8. Helium bubble layer exist under protrusions 100nm Obs. by Prof. Yoshida (Kyushu univ.) TEM sample was prepared by FIB milling (t=50nm) 1400K, 50eV-He plasma, 6x10 24 He + /m 2 The subsurface layer (~100 nm in thickness) and protrusions are packed with bubbles. The size of the bubbles are less than 50 nm in diameter.

9. Arc spot moves freely in retrograde (-jxB) direction. Arc ignited on nanostructured W surface From bottom From back 50 000 fps (1 frame 20  s) 30 000 fps (1 frame 33  s) Backside of the surface B The amount of W ejection is significantly increased!

10. Features of arcing : Voltage & current jumps, strong emission. voltage current emission 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.

11. Ignition condition I: Morphology change ・ Triggering of arcing requires only a slight morphology change. (much less than 10 25 m -2 seems sufficient.) ・ Laser position is changed shot-by-shot. ・ Arc duration increases with helium fluence, and longer than the pulse width when fluence is >3x10 25 m -2. Arcing is triggered He fluence :1.4 x 10 24 m -2 He fluence :3.3 x 10 24 m -2

12. Arcing may be triggered by bubble bursting 200nm ・ Increase of the temperature in the lid of bubbles could result in the bursting of bubbles. The bursting may make it much easier to ignite arcing. Schematic of plasma assisted laser ablation. S. Kajita, et al. APL (2007) TEM Obs. By Prof. Yoshida (Kyushu univ.) Incident ion energy~ 50 eV Surface temperature ~1400 K He fluence: 6x10 24 m -2 Protrusions Highly pressurized He bubbles

13. Ignition condition II: Target biasing is important factor to trigger arcing Arcing is never triggered when the target voltage is higher than -55 V, but constantly triggered when the biasing voltage is sufficiently low (here, - 60 V, which is sufficiently lower than the floating potential of -18 V!). It has not yet understood the mechanism of this biasing dependence. Arcing is triggered No Arcing

14. Demonstration of unipolar arc(UA) S. Kajita et al. Nucl. Fusion (Letter) (2009) Split of arc spot can be seen. ・ Arcing ignited even at the floating potential. (There has been no report of UA in steady state plasma in laboratory experiments.) ・ The situation is very close to the UA model proposed by Robson and Thoneman 50 years ago. ・ ELMs could trigger the unipolar arcing with ease for helium irradiated W.

15. Analysis of arc trail : arc spot grouping Arc spot moves along with retrograde direction + acute angle rule. Arc spot of ~10  m moves with forming group. S. Kajita et al. Phys Letter A (2009) Trail on the nanostructured W is similar as the man’s footprints in the snow

16. Fractality of trail under magnetized condition - self-affine fractal (scale depends on direction) - From the distribution of the dots in radius r, the number of dots represents fractality locally, but not globally.  self-affine fractality Locally: random motion Globally: linear motion due to magnetic field r Digitized SEM micrographs of arc trail. S. Kajita et al. J. Phys. Soc. Jpn. (2010) B=0.1 T

17. conclusion Synergistic irradiation effects of helium plasma and laser pulses were investigated experimentally. On the helium irradiated W surface, unipolar arcing/arcing is ignited promptly in response to the laser pulse irradiation. The helium fluence of ~3x10 24 m -2 seems sufficiently alter the ignition condition of arcing, probably because of the bursting of helium bubbles. The arcing significantly enhances W blow-off. This result may urge re- consideration of arcing issue in nuclear fusion research. From the detailed investigation, following feature of arcing has been revealed 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. Morphology change by helium irradiation formation condition of the fiberform nanostructure (fuzz) Temperature range, 1000 K < T < 2000 K Incident ion energy ， >20 eV Kyushu univ. (JA) (obs. Dr. Iwakiri) S. Kajita, et al. Nucl. Fusion (2007, 2009)

19. Arc ignited on nanostructured W surface From bottom 50 000 fps (1 frame 20  s) Emission could continue for much longer time than the pulse width.

20. B Arcing observed from backside From back 30 000 fps (1 frame 33  s) Arc trail was recorded clearly on the surface Arc spot moves freely in retrograde (-jxB) direction. Backside of the surface  Note that the electrode is biased in this case.

21. Filter spectroscopy: Visualization of released W W filter (400.9 nm) Laser in vacuum He filter (707 nm) Laser in plasma W and He are released in response to the laser pulse. He irradiated W 0.8 J/cm 2 He irradiated W 1.4 J/cm 2 Schematic of the filter spectroscopy system (b) W I (400.9 nm), (c) He I (706.5 nm) NAGDIS-II

22. Plasma assisted laser ablation Reduction in ablation threshold w/o He irradiation ： >1 J/cm 2 (i) Fluence 7.7x10 25 m -2 ： <0.5 J/cm 2 (ii) Fluence 3.8x10 26 m -2 ： <0.2 J/cm 2 Higly pressurized He (>MPa). Heating of the lid of hole ⇒ the stress exerted on the lid exceed the tensile stress ⇒ Bursting of holes ⇒ Ablation with low laser fluence Laser fluence S. Kajita et al., Appl. Phys. Lett. (2007)