R. Jimbou

Initial Boronization of JT-60U Tokamak Using Decaborane

A decaborane-based boronization system has been installed in the JT-60U tokamak in order to reduce the influx of impurities during plasma discharges. Boronization has been performed under a glow discharge using a helium-decaborane gas mixture. The properties of the boron films deposited through boronization and the effects of boronization on the tokamak discharges were investigated. It was found that the deposition of a boron layer with high purity was achieved with few impurities other than hydrogen through boronization, and that the present boronization deposited toroidally nonuniform boron film. It was also found that the decaborane-based boronization resulted in good plasma performance similar to that of conventional boronization.

High heat flux experiment on B4C-overlaid C/C composites for plasma facing materials of JT-60U

High heat flux experiments (5–40 MW/m 2 , 5 s and 550 MW/m 2 , 5–10 ms) in the JAERI electron beam irradiation stand (JEBIS) have been carried out on three kinds (conversion, CVD and LLPS) of B 4 C-overlaid C/C composites, on which B 4 C is overlaid with a thickness of 100–250 μm. Measurements were made with respect to the weight loss, changes of the surface morphology and of the surface atomic composition, and the surface temperature. As a result of these experiments, it is found that B 4 C layers of all samples have no damages except small weight losses up to 12 MW/m 2 heat loads, which are estimated at the divertor tiles of JT-60U in normal plasma operation, and that the conversion method is the best of the three methods applied in the present tests, since no exfoliation has occurred even under the disruption conditions.

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.

Retention of deuterium implanted into B4C-overlaid isotropic graphites and hot-pressed B4C

Retention characteristics of two kinds of B4C-overlaid graphites and hot-pressed B4C were investigated. An ion beam of 3 keV D2+ was implanted into the specimens at room temperature. The amount of retained deuteriums was measured as function of the implantation fluence and temperature by elastic recoil detection analysis. Thermal release behavior of implanted deuteriums was also measured by isochronal annealing. The concentration of retained deuterium reaches saturation similarly in three kinds of B4C-overlaid specimens at the fluences over 1018 D+/cm2 as in isotropic graphite. The release temperature, at which the number of retained deuterium decreases to one half in isochronal annealing, are about 250 K lower for three kinds of B4C specimens than for graphite. The release temperature of deuterium from unsaturated hot-pressed B4C in isochronal annealing is about 500 K higher than that from saturated one.

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.

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.

Thermal conductivity and retention characteristics of composites made of boron carbide and carbon fibers with extremely high thermal conductivity for first wall armour

Abstract The thermal conductivity of the composite hot-pressed at 2100°C including B 4 C and carbon fibers with a thermal conductivity of 1100 W / m · K was nearly the same as that of the composite including carbon fibers with a thermal conductivity of 600 W / m · K . This resulted from the higher amount of B diffused into the carbon fibers through the larger interface. The B 4 C content in the composite can be reduced from 35 to 20 vol% which resulted from the more uniform distribution of B 4 C by stacking the flat cloth woven of carbon fibers (carbon fiber plain fabrics) than in the composite with 35 vol% B 4 C including curled carbon fiber plain fabrics. The decrease in the B 4 C content does not result in the degradation of D (deuterium)-retention characteristics or D-recycling property, but will bring about the decreased amount of the surface layer to be melted under the bombardment of high energy hydrogen ions such as disruptions because of higher thermal conduction of the composite.

OPERATION EXPERIENCES WITH JT-60U PLASMA FACING COMPONENTS AND EVALUATION TESTS OF B4C-OVERLAID CFC/GRAPHITES

Erosion of carbon fiber composite divertor tiles of JT-60U has been reduced significantly by the precise alignment and insitu taper-shaping of tile edges. The divertor tiles are coated with redeposited carbon films. None of graphite first wall tiles has been broken. Evaluation tests of B4C-converted and -coated CFC/graphite have been performed from viewpoints of high heat fluxdurability, thermal shock, deuterium retention, and erosion yields. JT-60U in-pile test has also been carried out. The results exhibit satisfactory performance for the divertor plate and first wall of JT-60U.

Surface compositional change of B4C under D2+ implantation in high and low vacua and its effect on hydrogen retention

Abstract The relation between retention characteristics and compositional change in the surface layer of B 4 C was investigated. The release temperature of implanted deuterium was 100°C lower for hot-pressed B 4 C and 90°C lower for CVD boron carbide that were implanted with base pressure of higher vacuum, than of lower vacuum. This temperature shift would be owing to the decrease in the amount of boron oxide formed in the surface layer of B 4 C. This decrease of boron oxide in the surface layer is ascribed to the improvement of base pressure in a chamber for implanting deuterium ion from 3 × 10 −6 to 9 × 10 −8 Pa. On the contrary, the deuterium release temperature of graphite varied hardly with base pressure in implantation.

The reaction of H2O, O2 and energetic O2+ on boron carbide

Reaction of polycrystalline B4C with energetic O2+ was investigated mainly using Rutherford backscattering spectroscopy (RBS). For 5 keV O2+ irradiation, all of the irradiated oxygen was retained up to the fluence of 2 × 1017 O/cm2 in a temperature range from RT to 600°C. The areal density of saturated retention was ∼ 3 × 1017 O/cm2. The release of the implanted oxygen begins above 600°C and almost all the oxygen desorbs at ∼ 1000°C. The depth profile of oxygen retained at RT had a big maximum around 8 nm, while the depth profile at 600°C had a broad peak near 8 nm. In contrast to the above results by RBS, retained oxygen was hardly measured by Auger electron spectroscopy and X-ray photoelectron spectroscopy with Ar+ sputtering. This implies that there are at least two types of trapped states: one is the physically trapped state of a gaseous form (CO or O2) and the other is a chemically bound state (BO bond). It was also found that boron oxide is formed even at RT using simultaneous electron/He+ irradiation during H2O exposure, while the oxygen molecule scarcely reacts with the B4C surface under the simultaneous irradiation of electron/He+.