K. Sone

Measurement of chemical sputtering yields of various types of carbon

Abstract Measurements of the chemical sputtering during the bombardment of pyrolytic graphite, isotropic carbon and glassy carbon with 0.1–6 keV hydrogen ions have been made in the temperature rarige of room temperature to 700°C. The maximum production rate occurs at 1 keV for the incident energy and 525°C for the target temperature in all types of carbon. Energy and temperature dependences of chemical sputtering of carbon are not affected by the structures of the carbon. The reason is ascribed to radiation damage of the surface of the carbon. The dose dependence of the methane production rate was influenced by the hydrogen concentration in a target prior to bombardment, but the steady rate was obtained after the target was bombarded with protons at a dose of more than 1 × 10 18 H + /cm 2 .

Low Energy Selfsputtering Yields of Molybdenum and Tungsten

Selfsputtering yields of molybdenum and tungsten have been measured by the weight change method in the energy range between 0.1 keV and 10 keV at normal incidence. Measured yields are compared with two different universal empirical formulae proposed to predict the energy dependence of the sputtering yield and it is found that measured yields agree with the predicted ones from these two formulae within a factor of three for both molybdenum and tungsten. The results are discussed in view of their importance to the plasma-wall interaction in fusion devices.

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Abstract Surface erosion in pyrolytic graphite by 100 keV 4 He + and 200 keV H + 2 ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10 17 to 5 × 10 18 particles/cm 2 . Although the surface is eroded at 1 × 10 17 particles/cm 2 in helium bombardment, it is not eroded so heavily even at 5 × 10 17 particles/cm 2 in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10 18 particles/cm 2 , and a cone structure appears at 5 × 10 18 particles/cm 2 , which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10 18 particles/cm 2 , many circular blisters are formed which are sputtered off at 5 × 10 18 particles/cm 2 . The surface roughness of the target also affects the erosion.

Reduction of erosion by blistering in molybdenum surfaces with a multi-groove microstructure

Abstract Polycrystalline molybdenum samples with electropolished surfaces and with microscopically roughened surfaces have been bombarded with helium ions of 100 and 400 keV energies. The target temperatures during bombardment ranged from room temperature to 850°C. The roughened surface has a multi-grooved structure which was prepared by the well-known microfabrication processes using chemical dry etching. The structure consists of a number of rectangular parallel grooves, the width and depth of which are 3–7 and 2–3 μm, respectively. It was found by scanning electron microscopy that surface erosion can be effectively reduced on the multi-grooved surface. A criterion for the reduction of surface erosion by blistering was verified experimentally. The helium re-emission rate during 100 keV helium ion bombardment was also measured in the same temperature range and was discussed in relation to the observed surface erosion.

Sputtering of silicon carbide coatings by low-energy hydrogen ions

Abstract Various types of silicon carbide coatings made by reactive ion-plating have been bombarded with a 3.0 keV H+3 ion beam at temperatures around 500°C. The sputtering yield in stoichiometric samples (i.e. Si : C = 1 : 1) at 500°C was 1.15 × 10 −2 atoms/H+. As the stoichiometry deviates from this point, the sputtering yield has larger values. The temperature dependence of the sputtering yield in stoichiometric samples was negligible below 600°C. No surface topography changes occurred in stoichiometric samples even at a high fluence of 2 × 10 20 H + /cm 2 , while severe erosion took place in non-stoichiometric samples. By Auger electron spectroscopy (AES), carbon exists on the surface in the form of carbide in stoichiometric SiC before and after bombardment, while it exists in the form of graphite in carbon rich samples, which suggests that the bound state of carbon in the form of carbide should correspond to the low sputtering yield in stoichiometric SiC coatings. The surface stoichiometry changes due to hydrogen bombardment were observed by AES, where the carbon population increases in stoichiometric SiC, while it decreases in carbon rich samples, which was supported as well by the results from electron probe X-ray microanalysis.

The effect of wall materials on hydrogen recycling in JT-60

Abstract A model calculation on hydrogen recycling at the wall in JT-60 has been made for three different wall materials (molybdenum, Inconel 625 and titanium carbide) for the case when the plasma is heated by neutral beam injection. It has been assumed that only charge exchange neutrals hit the wall. Values of the recycling coefficient required to keep constant plasma densities for two different flux densities 1 × 1016 and 3 × 1016 H/cm2·s onto the wall have been estimated to be 0.75 and 0.92, respectively. By assuming Maxwellian energy distributions (200 and 400 eV), the recycling coefficients have been calculated as a function of time and wall temperature by taking account of particle re-emission due to backscattering, diffusion and saturation overflow. The wall temperature regions which satisfy the above criteria for 5 and 10 s discharges have been discussed for the three materials.

Continuous observation of surface erosion of polycrystalline molybdenum implanted with helium ions with different energies

Abstract Surface erosion of polycrystalline molybdenum caused by helium ion bombardment, with various irradiation modes of different energies ranged from 60 to 200 keV, was studied by continuous observation of the surfaces under helium ion bombardment. The development processes of the surface deformation are presented as a function of irradiation dose and the critical fluence for surface deformation was precisely determined. The critical helium concentrations for blistering were obtained from the calculated profiles of implated helium at the critical fluence and it was found that the critical concentrations for both 100 and 200 keV single implantation were almost the same, whereas the critical concentrations for sequential irradiation were distributed around that for a single implantation. In addition, it was observed that the average thickness of the exfoliated layer lay nearly at the peak in the depth profile.

Surface microstructural effects on angular distribution of molybdenum particles sputtered with low energy Ne+ ions

Abstract Angular distributions of sputtered Mo atoms have been measured. Annealed and non-annealed polycrystalline Mo targets with grain sizes of ~50 and ~5 μm, respectively were bombarded by 0.6 and 1.5 keV Ne + ions to investigate surface microstructural effects on the distributions of the sputtered atoms. The angles of incidence were chosen to be 0 and 45° with respect to the surface normal. For the 45° incidence, the more forward emission was observed for the lower incident energy ions and the smaller grain size. The distribution for the normally incident 1.5 keV Ne + ions obeyed the cosine law. An estimation of surface topography changes and the grain size effect on angular distribution was made.

Dose and energy dependence of ion-induced release of deuterium from molybdenum by low energy hydrogen ions

Abstract The ion-induced release of deuterium trapped in molybdenum at room temperature was investigated by subsequent bombardment with protons using incident energies ranging from 0.5 to 6 keV for deuterons and protons. The release curve of deuterium pre-implanted at 1 keV due to proton bombardment at energies from 1 to 6 keV consisted of two exponential decay curves. The two decay constants, i.e., release cross sections, decreased with increasing dose of the implanted deuterium. Two distinct peaks were observed in the release spectra of deuterium pre-implanted at energies from 3 to 6 keV due to proton bombardments at the corresponding energies. The release spectra and the amount of release of D 2 were dependent on the implanted deuteron dose, deuteron energy, proton energy and condition of pre-implanted target. The results are discussed in view of radiation damage effects.

Dose and microstructural effects on surface topography change and sputtering yield in polycrystalline molybdenum bombardment with 2 keV Ne+ions

Abstract The topographic change and sputtering yield in polycrystalline molybdenum due to the bombardment with normally incident 2 keV Ne + ions have been investigated to study dose and microstructural effects at room temperature in three differently fabricated molybdenum samples. The surface topography changes with increasing dose and depends on the microstructure of the samples. The cones were formed at a dose above 4 × 10 18 ions/cm 2 , and the number density and height of the cone increased with increasing dose. The distribution of cones was localized, and the aggregation of impurities including implanted neon atoms on cones was not observed. The sputtering yields in the three molybdenum samples were found to be the same value ( 0.90 ± 0.10 ) atoms/ion for 2 keV Ne + ions, in spite of different surface topography changes caused by sputtering.