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Experiments on erosion and dust formation on graphite materials have been performed using high power induction plasmas containing high atomic hydrogen flux (∼10 24 m −2 s −1 ). Chemical sputtering by atomic hydrogen irradiation with an incident energy below 1 eV eroded the graphite targets significantly, and the sputtering yield was roughly estimated to be 0.002-0.005, which is as high as that obtained by ion beam experiments. The transport of the released hydrocarbon along the gas flow results in carbon dust formation on the eroded graphite target and also on the silicon and graphite targets located at the remote position. The dust structure strongly depends on the target surface temperature, and the graphite dust turns into diamond crystals when the surface temperature rises to 1100 K. Carbon materials, such as isotropic graphite, and carbon fiber reinforced composites (CFC) are superior plasma facing components, which are used in a fusion reactor because of their high thermal conductivity and tensile strength. Divertor graphite tiles, however, are eroded significantly by irradiation with high particle and heat flux divertor plasmas. Sputtering erosion and dust formation of carbon materials have significant effects on reactor performance, such as tritium retention, impurity release, degradation of vacuum sealing, and electrical isolation, etc. So far several methods to reduce the divertor plasma heat load have been tested and it is recognized that the detached divertor plasma operation is the most probable one in the fusion reactor. Although the detached divertor works well for heat load reduction, the graphite erosion by irradiation of low energy hydrogen ions and atoms in the detached divertor plasma is not yet understood.In this paper, high power inductively coupled plasmas (ICPs) [1] with a power level of ∼20 kW are used to study plasma surface interactions and dust formation mechanism. High power ICPs have characteristic features, such as a high particle flux (ion flux: 10 20 -10 21 m −2 s −1 , atomic hydrogen flux: 10 23 -10 24 m −2 s −1 ), high heat flux (∼1 MW/m 2 ), and low temperature (∼ 1 eV) under experimental conditions, which include an input power of 15 kW, argon gas flow rate of 60 slpm and hydrogen gas flow rate of 2 slpm. High power ICPs are the irradiation source for both low energy and dominant atomic hydrogen. Although the working gas pressure is high (P ∼ 5 kPa), these features author's e-mail: 1027take@ee.t.kanazawa-u.ac.jp are very helpful in studying the fundamental mechanism of carbon erosion and dust formation in detached plasmas.Experimental results of argon-hydrogen mixture (Ar: 60 slpm, H 2 : 0, 2, 8 slpm) plasma irradiation onto graphite targets are reported. Figure 1 shows a schematic diagram of a plasma irradiation system. One graphite target (IG-430U fabricated by TOYO Tanso Co.) is located under the plasma torch and five other graphite or silicon crystal targets, with diameters of 15 mm, are placed at different positions to determine how the irradiation condition affects grap...

Experiments on erosion and dust formation on graphite materials have been performed using high power induction plasmas containing high atomic hydrogen flux (~10 24 m -2 s -1 ). Chemical sputtering by atomic hydrogen irradiation with incident energy below 1 eV eroded the graphite targets significantly, and the sputtering yield was roughly estimated to be 0.002-0.005, which is as high as that obtained by ion beam and fusion plasma experiments. The transport of the released hydrocarbon along the gas flow, interacting with low temperature plasmas, results in carbon dust formation on the eroded graphite target and also on the silicon and graphite samples located at the remote position. The dust size and density observed on the samples decreases with distance from the graphite target. The dust shape strongly depends on the target surface temperature, and the graphite dust turns into polyhedral particle like diamond when the surface temperature rises to 1100 K.

Chemical erosion of carbon materials and dust formation in low-temperature and neutral particle-dominated plasmas were investigated using high-pressure inductively coupled plasmas. Experiments were performed with Ar/H 2 /N 2 mixture plasma irradiation to graphite targets. The addition of just a few percent of nitrogen gas to hydrogen led to significant suppression of carbon dust formation on the graphite target. From optical emission spectroscopy, CN band spectra were observed strongly in Ar/H 2 /N 2 plasmas with a decrease of CH and C 2 band emission intensity. These results showed that CN bond formation, which caused chemical erosion of carbon by producing volatile CN, HCN, and C 2 N 2 particles, might have been a key suppression mechanism of the carbon particle aggregation. Carbon materials are used for the plasma-facing components (PFC) in fusion devices because of their superior thermomechanical properties. However, dust particles are formed by plasma-surface interactions in fusion experimental devices [1][2][3][4][5]. Because carbon dust retains large amounts of hydrogen isotopes, the dust particles in fusion reactors cause safety problems mainly concerning the tritium inventory. The suppression of dust formation is an important issue in future fusion reactors. In this study, we report on experiments conducted to investigate the influence of nitrogen injection into argon/hydrogen plasmas on carbon dust formation by using high-power inductively coupled plasmas (ICP) [6,7]. Nitrogen injection has been considered and tested as one of the methods for tritium and co-deposits removal in carbon PFC [8,9]. Carbon dust formation in addition to tritium removal efficiency should be studied in nitrogen containing plasmas.Experiments have been performed in Ar/H 2 /N 2 mixture plasma irradiation to an isotropic graphite target (IG-430U, Toyo Tanso Co. Ltd.) at a surface temperature of ∼1020 K. The argon gas flow rate is 101 Pa m 3 /s, and the flow rates of hydrogen and nitrogen gas into the argon plasma are 3.4-5.1 Pa m 3 /s and 0-0.51 Pa m 3 /s, respectively. The working gas pressure is ∼4 kPa. The electron temperature is ∼1 eV. vertor plasma conditions. The irradiation time is set to 180 minutes. The surface temperature is measured through a quartz window with a radiation thermometer. A Scanning Electron Microscope (SEM) was used to observe the generated dust particles on the targets. The dominant erosion process of the graphite target is due to chemical sputtering by low-energy hydrogen atoms under Ar/H 2 plasma irradiation. Many carbon dust particles are observed on the graphite target eroded by chemical erosion [7]. Figure 1 shows the number density and size of the carbon dust particles, target weight loss, and optical emission intensity of CH (431.4 nm), C 2 (516.5 nm), CN (388.3 nm), and NH (336 nm) band spectra normalized to Ar I (750.4 nm) emission as a function of N 2 injection ratio into hydrogen, where the N 2 injection ratio is defined as the nitrogen gas flow rate normalized by the sum of nitrogen and hydr...

We experimentally investigate chemical erosion of polycrystalline graphite targets coated with boron-doped diamond (BDD) using an induction plasma containing low-energy, high-atomic-hydrogen flux. Chemical erosion is drastically suppressed by diamond coating the graphite target. The chemical sputtering yield for the BDD layer is about two orders of magnitude lower than that for the graphite target. After exposure in low-temperature hydrogen plasmas, however, the surface morphology of the BDD target is significantly modified. The polycrystalline diamond is eroded near the grain boundary, and many pits with diamond-like shapes are observed on the crystal surface. X-ray photoelectron spectroscopy and Raman spectroscopy reveal that the hydrogen atoms penetrate into the BDD target to a depth of at least ∼20 nm. In using graphite materials as plasma-facing components (PFCs) in fusion devices such as divertor tiles, it is recognized that the high chemical erosion rate due to the presence of hydrogen fuel and the related carbon dust formation will cause severe performance degradation in future fusion reactors. Diamond, which is a typical carbon crystal and has an sp 3 electronic structure, offers several features including an extremely high thermal conductivity, a low chemical reactivity, [1] and an increase of electrical conductivity that is possible through boron doping, making it superior for use as a PFC in fusion reactors. To date, several experimental efforts have focused on diamond erosion by energetic-ion irradiation [2, 3], but little effort has been devoted to low-energy atomic hydrogen. In the present research, boron-doped diamond (BDD) is tested for erosion by low-temperature (∼1 eV), high-flux hydrogen neutrals to check its suitability for divertor tiles used in a detached divertor.The experiments were performed in an Ar/H 2 mixture plasma generated by an inductively coupled plasma (ICP), which has the characteristic features of high neutral particle flux Γ H ∼ 10 23 -10 24 m −2 · s −1 , low ion flux Γ i ∼ 10 19 -10 20 m −2 · s −1 , and low electron temperature T e ∼ 1 eV [4]. The working gas pressure is ∼4 kPa. An approximately 30-nm thick polycrystalline BDD layer covers an isotropic graphite, where the boron doping rate is ∼0.5%. We deposit a BDD coating on an isotropic graphite target using hot-filament chemical vapor deposition (CVD). We vary irradiation time from 60 to 1200 minutes to examine the author's e-mail: 1027take@ee.t.kanazawa-u.ac.jp effect of target erosion due to hydrogen fluence into the target. The surface temperature is measured thorough a quartz window using a radiation thermometer. Figure 1 shows the weight loss of the BDD and the graphite targets as a function of atomic hydrogen fluence. Here, atomic hydrogen fluence is estimated using the results of an electromagnetic fluid simulation under local thermodynamic equilibrium conditions [5]. The erosion of both the graphite and the BDD target increases linearly with increasing hydrogen fluence. The chemical sputtering yield (Y) is estim...