Exfoliation of the tungsten fibreform nanostructure by unipolar arcing in the LHD divertor plasma

The tungsten nanostructure (W-fuzz) created in the linear divertor simulator (NAGDIS) was exposed to the Large Helical Device (LHD) divertor plasma for only 2 s (1 shot) to study exfoliation/erosion and microscopic modifications due to the high heat/particle loading under high magnetic field conditions. Very fine and randomly moved unipolar arc trails were clearly observed on about half of the W-fuzz area (6 × 10 mm2). The fuzzy surface was exfoliated by continuously moving arc spots even for the very short exposure time. This is the first observation of unipolar arcing and exfoliation of some areas of the W-fuzz structure itself in a large plasma confinement device with a high magnetic field. The typical width and depth of each arc trail were about 8 µm and 1 µm, respectively, and the arc spots moved randomly on the micrometre scale. The fractality of the arc trails was analysed using a box-counting method, and the fractal dimension (D) of the arc trails was estimated to be D ≈ 1.922. This value indicated that the arc spots moved in Brownian motion, and were scarcely influenced by the magnetic field. One should note that such a large scale exfoliation due to unipolar arcing may enhance the surface erosion of the tungsten armour and act as a serious impurity source for fusion plasmas.

[1]  R. Doerner,et al.  Helium induced nanoscopic morphology on tungsten under fusion relevant plasma conditions , 2008 .

[2]  H. Yamada,et al.  Characterization of Surface Modifications of Plasma-Facing Components in LHD , 2010 .

[3]  Masaki Osakabe,et al.  The divertor plasma characteristics in the Large Helical Device , 2002 .

[4]  G. Mesyats,et al.  The ecton mechanism of unipolar arcing in magnetic confinement fusion devices , 2010 .

[5]  Y. Tsuji,et al.  Self-Affine Fractality of Bifurcating Arc Trail in Magnetized Plasma , 2010 .

[6]  N. Yoshida,et al.  Surface Modification and Correlated Internal Damage in Tungsten Irradiated with Low Energy Helium Ions at 1273 K , 2005 .

[7]  H. Yamada,et al.  Edge Plasma Transport in the Helical Divertor Configuration in LHD , 2010 .

[8]  S. Masuzaki,et al.  Ion temperature measurement using an ion sensitive probe in the LHD divertor plasma , 2003 .

[9]  S. Takamura,et al.  Prompt ignition of a unipolar arc on helium irradiated tungsten , 2009 .

[10]  B. Juttner,et al.  Influence of residual gases on cathode spot behavior , 1991 .

[11]  S. Takamura,et al.  Formation of Nanostructured Tungsten with Arborescent Shape due to Helium Plasma Irradiation , 2006 .

[12]  N. Yoshida,et al.  Microstructure evolution in tungsten during low-energy helium ion irradiation , 2000 .

[13]  S. Takamura,et al.  Motion of unipolar arc spots ignited on a nanostructured tungsten surface , 2011 .

[14]  H. Kurishita,et al.  Observations of suppressed retention and blistering for tungsten exposed to deuterium–helium mixture plasmas , 2009 .

[15]  A. Iwamae,et al.  Nanostructured Black Metal: Novel Fabrication Method by Use of Self-Growing Helium Bubbles , 2010 .

[16]  Wataru Sakaguchi,et al.  Formation process of tungsten nanostructure by the exposure to helium plasma under fusion relevant plasma conditions , 2009 .

[17]  Y. Tsuji,et al.  Direct observation of cathode spot grouping using nanostructured electrode , 2009 .

[18]  N. Yoshida,et al.  Effects of helium bombardment on the deuterium behavior in tungsten , 2002 .

[19]  K. Tokunaga,et al.  Impact of low energy helium irradiation on plasma facing metals , 2005 .