M. Tokitani

S. Masuzaki

T. Tanabe

National Institute for Fusion Science (NIFS-Repository)

    The armor materials of the first wall/blanket and the divertor will be subjected by hydrogen isotopes including tritium, neutron and helium produced by DT burning in fusion devices. In particular, behavior of implantation, diffusion and permeation of tritium of the armor materials of the first wall/blanket and the divertor is very important because of the control of fuel, tritium retention, limit of tritium discharge experiment. In addition, beam irradiation experiments have showed that helium irradiation influences hydrogen isotope retention of the plasma facing materials such as tungsten. In the present study, material exposure experiments in LHD have been performed to investigate the synergistic effects simultaneous implantation of hydrogen and helium on hydrogen retention and microstructure changes of materials in high temperature plasma confinement device.  A probe head were transferred to the vacuum chamber position by using the retractable material-probe system equipped with the LHD, and then exposed to hydrogen or hydrogen helium mixture discharges with magnetic axis of 3.6 m and 3.53 m. Three sets of tungsten (W) and SUS316L were mounted on the probe head equipped with a rotatable shatter.  The two sets were fixed on the bottom part of the probe head. The one set was exposed 18 hydrogen discharges (H/(H+He)=0.9, ratio of spectroscopic measurement ) of 18 s in total, and the other one was exposed by 9 hydrogen/helium mixture plasma discharges (H/(H+He)=0.4, ratio of spectroscopic measurement) of 40s in total.  On the other hands, the one set, which was fixed on the surface of the shatter, was exposed by the all discharges after the transfer to the vacuum chamber position. The temperature of the probe head was monitored by thermocouples.  After the exposure, surface morphology was examined by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS),  respectively. In addition, chemical composition including in hydrogen and helium has been examined by Rutherford Backscattering Spectrometry (RBS) and  Elastic Recoil Detection(ERD) . Fig. 1 shows the results of ERD analyses of tungsten using 4.0MeV16O4+ probe. Helium in addition to hydrogen was detected on the tungsten, which was exposed by the all plasma discharges as shown in fig. 1(b). Hydrogen was detected on tungsten which was exposed by the hydrogen discharges. On the other hand, a small amount of helium was detected on tungsten exposed hydrogen/helium mixture plasma in addition to large amount of hydrogen. However, there were little differences between the amounts of hydrogen, which was irradiated or unirrradated tungsten as shown in fig.1 (a). The hydrogen on tungsten unirradiated is expected due to absorption by exposure of atmosphere. Further experiments, which high fluence of hydrogen/helium is exposed on the samples, will be required.                                                  Fig.1  ERD analyses of W (a) unirradiated W, (b) W irradiated by all discharges. (c)W irradiated by hydrogen discharges (H/(H+He)=0.9, ratio of spectroscopic measurement), (d) W irradiated by hydrogen and helium mixture discharges (H/(H+He)=0.4) 200 400 600 800100101102103  CountsChannel number(a)H200 400 600 800100101102103HeH  CountsChannel number(b)200 400 600 800100101102103H  CountsChannel number(c)200 400 600 800100101102103HeH  CountsChannel number(d)    Tungsten (W) is a candidate of plasma facing material (PFM) in the next generation fusion devices such as DEMO from the viewpoints of tritium retention and dust generation in vacuum vessel. One of the problems of W as PFM is surface modification by melting 1). Melting also causes impurity injection to core plasma with splash or spray phenomena 2). So it is necessary to predict the motion of the melt layer, and to investigate the transport of droplets of W in the next generation fusion devices for safety and steady operation. In LHD, small W-rods were exposed to divertor plasma by using movable material probe system to investigate melting layer behavior and droplets transport. Figure 1(a) shows the material probe setup. Three W-rods with different lengths (5×5.6×1mm, 5×20.2×1mm, 5×15.1×1mm) were on the graphite stage. The graphite stage was for collecting W ejected from the rods. Incident angle of magnetic field lines was ~70 degree to the rods surfaces, and parallel to the graphite stage surface. This material probe was installed from a bottom port (4.5L port) in LHD. The rods were exposed to NBI heated hydrogen discharges, and total exposure time was ~ 4 s. We could not observe spray, splash and droplet motion during experiment. The surface analyses of the rods and the graphite stage were conducted after the exposure. Figure 1(b) shows the rods after the exposure. Two rods’ edges melted, but the shortest rod did not melt. That means the high heat flux area was narrow as shown in Fig. 1(b) like a sheet-plasma. It was consistent with the previous observation of the divertor plasma profile with Langmuir probes 3). Forces to melt layer are gravity, Lorentz-force (J×B) and plasma pressure. J is current between plasma and rods caused by electron thermal emission. The melt layers did not move in the gravity direction, but they seemed to move plasma flow direction. In this experiment, Lorentz-force was considered to be small because the angle between J and B was small. Figure 2(a) shows the surface of the graphite stage after the exposure. Droplets were not observed on the stage. Around the melted rods, the surface color was changed by deposits. EDX (Energy Dispersive X-ray analysis) was conducted, and the dominant deposit was confirmed to be W. Figure 2(b) shows the two dimensional distribution of the W composition ratio around the top rod. The extension of the distribution at the plasma upstream side (front) was smaller than that at the plasma downstream side (back). Figure 2(c) shows the comparison between measured W composition ratios profiles at the front and back sides and simple cosine distribution. At the back side, they were similar though the composition ratio profile was much narrower than cosine distribution in the front side. From these results, it is considered that W sputtered or evaporated from the rods deposit on the graphite stage, and W atoms were ionized in the divertor plasma in the front side. The mean-free-path of sputtered W for ionization is around 10mm.   1)  Roth, J. et al.: Journal of Nuclear Materials  390-391 (2009) 1. 2)  Coenen, J.W. et al.: Journal of Nuclear Materials, in press. 3)  Ezumi, N. et al.: Journal of Nuclear Materials 313-316 (2003) 696. W rodGraphite stageSUS316B (~1T)(a)(b) Fig. 1. (a) Experimental setup. (b) W-rods after plasma irradiation. yB (~1T)Plasma flow10 mm(a)W-rod position Distance along field lines (mm)y (mm) (%)(b) 00.51051015202530distance (mm)00.510 5 10 15 20 25 30distance (mm)cosine distribution frontback(c) Fig. 2. (a) W deposited graphite stage. (b) 2D distribution of W composition ratio on the stage. (c) W composition ratio profiles normalized by the max value along y=0 line at front and back sides, respectively. The composition ratio was measured by EDX (Energy Dispersive X-ray analysis).57§32. Induced W-rod Melting Experiment in the LHD DivertorTanabe, T. (Kyushu Univ.),  Masuzaki, S., Tokitani, M.

Induced W-rod Melting Experiment in the LHD Divertor

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Induced W-rod Melting Experiment in the LHD Divertor

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