A. Kohyama

H. Kishimoto

Y. Asakura

National Institute for Fusion Science (NIFS-Repository)

  Development of real time dust monitoring methods for fusion devise is crucial to realize long-term safely operation because dust particles pose safety issues due to their chemical reactivity, tritium retention, and radioactive content. They may cause operational issues such as deterioration of plasma confinement. For these reasons, the maximum quantity of dust particles in the ITER vessel is limited below 6 kg of carbon or 11 kg of beryllium and 77 kg of tungsten.1) We aim to develop a dust monitoring method using quartz crystal microbalances (QCMs) equipped with a dust eliminating filter.2-4) The method offers simultaneous measurements of mass deposition rate of precursors responsible to film deposition and dust particles. Here we report results of a first operation of QCMs in a divertor simulator. Dust particles were produced due to interactions between a graphite target and helicon H2 plasmas in the divertor simulator. The QCMs were set on the reactor wall at 100 mm below the graphite target. The QCM sensors are cooled by water at 20 oC using a water cooling system. Gas of pure H2 was supplied at a pressure of 5 mTorr. The gas was supplied at 13 sccm in flow rate at pumping port to reduce effects of gas flow on dust transport. H2 plasmas were generated by applying 13.56 MHz pulsed RF voltage to a helicon antenna. The discharging period was 0.25 s, and the interval was 1.0 s to avoid overheating the quartz discharge tube. The QCM sensor has three channels. Channel 1 was used to measure deposition rate due to radicals and dust particles. Channel 2 was covered by a dust eliminating filter to measure the deposition rate due to radicals. For the filter, most of dust particles are deposited on the filter plate, while a part of deposition precursors such as CH3 can pass through the filter because they are reflected on the filter plate. Channel 2 was covered by a SUS board to monitor effects of pressure and temperature on signals of QCMs. A resonance frequency f of QCM is decreased by mass of deposits on the QCM. The deposition rate is deduced from the frequency shift ('f) using the Saubrey equation given by   'f = - 2f02 A-1 (PqUq)-1/2 'm,   (1)  where 'f is frequency shift due to mass change on QCM, f0 the fundamental resonance frequency of crystals, A area of QCM, Pq the shear modulus of the quartz and Uq the density of crystal. Figure 1 shows time evolution of 'f of three QCMs. For ch. 1 and 2, 'f slightly increase just after turning on the discharge due to temperature and/or pressure change, then monotonically decrease. The rate of the frequency shift for ch.2 is much slower than that for ch.1. 'f for ch. 3 is nearly constant. Using eq. (1), deduced mass deposition rates for ch.1 (DRw/o filter) and ch. 2 (DRw/ filter) are 27.1 and 1.29 Pg m-2 s-1, respectively.  After discharges, carbon films were sampled from the filter surfaces and observed with the transmission electron microscope (TEM). Figure 2 shows TEM images of the films. Spherical grains below 20 nm in size are observed in the films sampled from the surface faced to discharges. They might be small particles deposited into films. On the other hands, a few grains are appeared in the films sampled from QCM under the filter. The volume of spherical dust in unit area for the surface of the filter and QCM under the filter are 2.0910-9 and 1.2210-10 m3 m-2 , respectively. The eliminating ratio of dust particles in volume is 94.2 %. Thus, DRw/ filter is deposition rate predominantly due to radicals. Thus the mass deposition rate of dust particles accumulated on vessel wall can be measured using the QCMs.  Fig. 1. Time evolution of resonance frequency shift of QCMs.               Fig. 2. TEM images of carbon films (a) without the dust eliminating filter, (b) with the filter.   1) Roth, J. et al.: J. Nucl. Mater. 390-391 (2009) 1. 2) Nishiyama, K. et al.: J. Nucl. Mater. 438 (2013) S788. 3) Kim, Y. et al.: Jpn. J. Appl. Phys. 52 (2013) 01AD01. 4) Koga, K. et al.: Rev. Sci. Instrum. 76 (2005) 113501.    (a)㻌 (b)㻌  R&D and technology integration of high performance materials for plasma facing components (PFCs) and first wall are inevitable for the early realization of fusion reactor. Tungsten (W) is becoming a prime candidate as armor material for protecting high heat flux and suppressing plasma contamination. Although ITER is preparing to use W divertor, many issues are still remaining. The application of SiC/SiC composites to divertor as a substrate to W will be a potential solution to current W divertor concepts.  SiC/SiC fabricated by Nano-Infiltration and Transient Eutectic phase (NITE) method has various advantages (low cost, high shape and size flexibility, etc.). In addition, small mismatch in coefficient of thermal expansion between W and SiC is a great advantage in configuring the divertor system. This report provides the high performance HHFC materials R & D status and the plasma exposure test results from LHD. Figure 1. Test samples before Plasma Exposure   W-SiC/SiC dual plate was fabricated by hot pressing, where sample was W plate bonded on NITE-SiC/SiC plate. The plate for the plasma exposure test, with the dimension of 30 x 20 x 2 mm3 was shown in Figure 1. The thickness of W and SiC/SiC was the same with 1.0 mm. The test samples fabricated were mounted on a carbon stand by molybdenum plate and bolt. The test port for plasma exposure was the port number 10.5L of LHD. The shot number was #121947. The plasma exposure data are; the maximum applied heat flux ≈ 3 MW/m2, neutral beam injection (NBI) ≈ 4.9 MW, plasma density ≈ 1.5 x 1018 ions/m3, the peak plasma stored energy ≈ 760 KJ, and shot length = 7 seconds.  Figure 2 shows the results of the  plasma exposure test. Many clear  traces of run-away electrons and melted regions on the W surface (plasma exposure side)                        can be identified along the plasma strike point indicated by the red dashed lines. In addition, from the side views Figure 2. Surface Morphology after Plasma Exposure  of the plates, the melted regions on W surface are recognized to be through thickness melting with gap formation at W-SiC/SiC. However, no significant damage was observed in SiC/SiC side, and the bonding between W and SiC/SiC was damaged only near the melted zone and no crack propagation along interface was observed.  The result of ultrasonic test is shown in Figure 3. The blue area indicates damaged region of bonding interface and yellow area is with no significant damage along bonding interface by the plasma exposure. The blue area with the width of about 14mm is expanding from the plasma exposure area.  Figure 3. The Result of Ultrasonic Test  The result from the divertor material performance evaluation test in LHD with 3 MW/m2 heat load plasma exposure test without cooling the W-SiC/SiC was confirmed good resistance. Mechanical property measurement and microstructure analysis are on-going. As the conclusion, excellent potentiality and attractiveness of W-SiC/SiC plate as PFC material was presented by the plasma exposure test at LHD. Additional plasma exposure tests with cooling and appropriate model configuration of divertor with higher heat flux and longer exposure time should be done systematically for the next step. This results will be analyzed with the previous plasma exposure test with 10MW/m2 . 87§23. Stability of Advanced Tungsten - SiC/SiC Plate under Plasma Exposure in LHDKohyama, A., Asakura, Y., Kishimoto, H. (Muroran Institute of Technology)

Stability of Advanced Tungsten - SiC/SiC Plate under Plasma Exposure in LHD

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Stability of Advanced Tungsten - SiC/SiC Plate under Plasma Exposure in LHD

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