Yoshitaka Gotoh

High resolution electron microscopy of graphite defect structures after KeV hydrogen ion bombardment

Abstract Structure changes of graphite due to keV-energy hydrogen ion bombardment are studied by means of high resolution electron microscopy (HREM). Highly graphitized vapor grown graphite fibers of diameters around 700 nm are irradiated side on with 1 keV H + ion beam to various fluences ranging from 1 × 10 14 to 1 × 10 18 H + /cm 2 . The (002) lattice fringe images from the fiber side edges give “edge-on” projections of several tens of nanometers deep layers of the H + implanted graphite basal face, thus enabling direct observation of defect profiles in the surface layer. In a 1 keV H + ion bombardment at ambient temperature, interlayer spacing, d 002 , within 12 nm from the graphite surface increases from the initial value of 0.336 nm to larger values ranging from 0.40 to 0.53 nm at an ion fluence of 1 × 10 17 H + /cm 2 . The d 002 value ranges from 0.45 to 0.58 nm at an ion fluence of 1 × 10 18 H + /cm 2 . Mean crystallite size, L a , on the other hand, decreases with fluence from the initial value above 100 nm to an equilibrium at 0.5 nm after a 1 keV H + irradiation to a fluence of1 × 10 17 H + /cm 2 . The increase in the interlayer spacing is attributed at least to an increased van der Waals radius of the hexagonal carbon networks due to chemical bonding between lattice carbon atoms and hydrogen atoms at interlayer positions.

Sputtering characteristics of B4C-overlaid graphite for keV energy deuterium ion irradiation

Two types of B 4 C-overlaid graphite (CFC), conversion and CVD B 4 C, together with bare CFC (PCC-2S) and/or HP B 4 C, were investigated with respect to erosion yields for 1 keV D + , D 2 /CD 4 TDS after 1 keV D + implantation, and thermal diffusivity/conductivity, in a temperature range from 300 to 1400 K. The erosion yields of both conversion and CVD B 4 C were found to be much lower than that of the bare CFC (PCC-2S), in both chemical sputtering (600–1100 K) and RES (1200–1400 K) temperature regions. The D 2 TDS peak of the conversion B 4 C was found to be located at nearly 200 K lower temperature than that of the bare CFC (PCC-2S), indicating much lower activation energy for detrapping/recombination of trapped D in the conversion B 4 C and in the CFC. The CD 4 TDS peak of the conversion B 4 C was found to be much weaker in intensity than that of the bare CFC (PCC-2S), in agreement with the present erosion yield results. Thermal diffusivities and conductivities of both the conversion B 4 C/PCC-2S and the CVD B 4 C, were measured to be nearly 1/10 of that of the bare CFC (PCC-2S), and to decrease with increasing temperatures.

Thermal desorption spectroscopy of boron/carbon films after keV deuterium irradiation

Abstract Thermal desorption spectroscopy (TDS) of D 2 and CD 4 was done on boron/carbon films (B/(B + C) = 0–74%), after 3 keV D + 3 irradiation to 4.5 × 10 17 D/cm 2 at 473 K. The D 2 desorption peaks were observed at 1050, 850 and 650 K. For a sputter B/C film (0%), only the 1050 K peak was observed. With increasing boron concentration to 3%, a sharp peak appeared at 850 K, the intensity of which was found to increase with increasing boron concentration to 23%, and then to decrease at 74%. The 650 K shoulder, which was observed for high boron concentration specimens, was speculated to be deuterium trapped by boron atoms in the boron clusters. The relative amount of CD 4 desorption was found to decrease with increasing boron concentration, which was attributed to the decrease in the trapped deuterium concentration in the implantation layer at temperatures at which CD 4 desorption proceeds.

Interlayer structure changes of graphite after hydrogen ion irradiation

Changes in graphite interlayer orderings due to keV energy hydrogen irradiation were investigated as a basic step for studies of plasma-wall interactions in fusion reactors. Distribution of interplanar spacings within a 0–45 nm deep implantation layer was measured using Fourier transforms of the (002) lattice fringe images from HRTEM taken at side facets of 700 nm diameter graphite whiskers after 1 keV H+ irradiation. Discrete increases of the interplanar spacing from 0.34 nm to 0.38, 0.41 and 0.43 nm were observed, with other indications at 0.47, 0.56, 0.62 and 0.69 nm. A sub-peak formation at 0.376, and at 0.402 nm was also observed in (002) spectra of X-ray diffraction (XRD) of 200 nm diameter whiskers after 1–6 keV H+ irradiations at 673 K, and at 623 K, respectively. The increases to 0.38 and 0.41–0.43 nm were explained through sp3 type CH bond formation in hexagonal carbon networks at the ‘low’ trapped H densities (H/C 0.4), respectively, while those at 0.47–0.69 nm were attributed to interlayer, axial CH3-bond formations and CxHy molecule insertions.

Studies on properties of low-Z ceramics as limiter materials —electron beam and textor limiter tests

Abstract Several types of low- Z ceramics are investigated with regard to their applicability as high heat-flux component materials for nuclear fusion reactors. Their thermomechanical behaviours under realistic plasma conditions are studied through both out-of-pile, laboratory test and in-pile, TEXTOR test. Thermal shock resistivities of several types of SiC. nitrides, graphite and coated systems are tested by bombardment with an electron beam at energy densities between 1 and 500 MJ/m 2 (out-of-pile test). The maximum tolerable pulse length at different power densities for total fractures of various qualities of SiC is determined. Candidate-material segments are “sandwiched” between Inconel 600 reference segments. Main damage observed on the test limiter surface after exposure to the TEXTOR plasma is due to unipolar arcing. Difference of both arcing behaviour and redeposited-metal behaviour on the test-limiter segments are discussed.

FT-IR studies of graphite after keV-energy hydrogen ion irradiation

Abstract Fourier-transform infrared absorption spectroscopy (FT-IR) studies were made on vapor-grown carbon fiber (VGCF) after successive irradiations of 6, 3 and 1 keV-H+ to saturation at 373–923 K, and after the irradiations at 623 K followed by a heat-treatment at 893–1150 K. Reference hydrocarbons, cholesterol (C27H45OH) and menthol (C10H19OH), were also measured for C–H stretch band frequencies and relative integrated intensity factors, κ−−CHx. For the irradiated VGCF, a band was found to be centered at 2892 cm−1, in between a –CH3 symmetric (2873 cm−1) and a >CH2 asymmetric (2924 cm−1) stretch band, which was assigned to a >CH– stretch band. Relative densities of the CHx groups, assuming κCH−−:κ−−CH3:κ>CH2:κ>CH−− =0.12:2.2:1.1:1.0, showed that >CH2 decreases in density with increasing the irradiation temperature beyond 1000 K, while >CH– reaches maximum at around 823 K and then decreases. The –CH3 group decreases to a minimum from 623 to 823 K, and increases at above 823 K, indicating that methane forms at around 800 K through abstraction of H from >CH– by free CH3. The density ratio of >CH– to >CH2 reached a maximum at 0.4 at around 800 K, indicating that, in the keV-H+ implantation layer, the implanted H atoms are trapped mainly at >CH2, and subsidiarily at >CH–, and –CH3, at defects, below 900 K.

New composite composed of boron carbide and carbon fiber with high thermal conductivity for first wall

Abstract A new composite was created from B 4 C powder and carbon fiber by hot-pressing at 1700°C or more. The composite sintered at 1700°C with 20–35 vol% B 4 C shows a thermal conductivity of 250 W/m·K at 25°C which is slightly lower than the felt type C/C, but its value becomes higher than the C/C at temperatures above 400°C. The composite with 40 at% B shows more controllable recycling properties than B 4 C. The erosion yield for the composite is about half the yield for graphite at 800 K. After electron beam irradiation in order to test heat resistance no cracks were detected up to 22–23 MW/m 2 leading to a surface temperature of 2500°C.

Erosion characteristics of B4C-converted CFC composite

Erosion characteristics of B 4 C converted CFC composite (conv-B 4 C) and bare CFC were investigated for 3 keV D 3 + , O + and Ne + irradiations in the 400-1400 K range. In both D + and O + irradiation cases, chemical sputtering observed for bare CFC was suppressed through B 4 C conversion. For D + irradiation to conv-B 4 C, the suppression of chemical sputtering was explained through a decrease in the trapped D density within the implantation layer around 700 K at which CD 4 desorption could take place. While as to O + irradiation to conv-B 4 C below 800 K, the suppression was explained through self-sputtering of trapped O atoms in place of target boron and carbon atoms

High heat load tests of a graphite armor first wall with a water cooling jacket

Heat load tests are performed on actively cooled graphite/stainless steel laminated structures, for the simulation of steady-state first wall operations in nuclear fusion reactors. Graphite armor tiles (40 mm × 40 mm × 10 mm) are brazed to stainless steel substrates (1 mm thick, 68 mm diameter) with the insertion of copper-carbon fiber composite (Cu-C) compliant layers (1.7 mm thick). Ag-28 wt%Cu-5wt%Ti solder and Ag-28wt%Cu solder are used for brazing the graphite/ Cu-C interface and Cu-C/stainless steel interface, respectively. Fine grain isotropic graphite and carbon fiber felt reinforced carbon composite (felt C-C) are chosen for armor tile materials. Thermal conductivities of these graphite materials, being measured through a laser flashing method, are 109 and 170 W/mK at room temperature, respectively. Heat load tests are carried out by using a 40 kW DC power source (maximum current: 1000 A). Pulsed thermal loads at heat fluxes ranging from 1 to 6 MW/m2 are realized either through resistance heating of a carbon felt heater on test specimens (1–4 MW/m2) or through electric arcing between a cathode graphite rod and graphite tiles of test specimens (> 4 MW/m2). Surface temperatures of graphite tiles are found to reach equilibrium temperatures at around 20 s after the start of heating. The equilibrium temperatures are higher by 30–40% for isotropic graphite tiles as compared to felt C-C tiles: 1800 °C for isotropic graphite and 1300 °C for felt C-C, at 6 MW/m2. The equilibrium temperature at the Cu-C layer for a 6 MW/m2 heat flux is measured to be around 600 °C.

Development and material testing of OF-Cu /DS-Cu/OF-Cu triplex tube (dispersion strengthened copper clad with oxygen free-copper) and trial fabrication of a vertical target mock-up for ITER divertor

Abstract For the divertor target of the International Thermonuclear Experimental Reactor (ITER), an OF-Cu/DS-Cu/OF-Cu triplex-structured cooling tube has been newly fabricated through powder metallurgy and drawing. The triplex structure comprised an aluminium oxide (0.5 mass%) - dispersion strengthened copper core (DS-Cu) clad with oxygen free copper (OF-Cu), a compliant layer for joining to the carbon fiber composite (CFC) tiles, and with an inner skin which tightly grasps a twisted INCONEL tape to assist heat transfer. Physical and mechanical properties of the DS-Cu core after heat treatment at 850°C for 600 s were investigated. Also, CFC brazability, fabricability and feasibility of the triplex tube for cooling channels for the divertor target were studied: A large scale vertical target mock-up of a 1500 mm length, 35 mm width and 3000 mm radius curved front face, has been fabricated with nearly 50 pieces of “saddle”-shaped one-dimensional (1D)-CFC tiles were brazed on to 1500 mm long triplex tubes set in grooves of OF-Cu heat sink blocks joined to a stainless-steel back plate. The mock-up was tested under 20 MW/m 2 for 15 s for 1000 cycle thermal loadings, which simulated transient heat loadings of a vertical target of an ITER divertor.