M. Balden

Tungsten: an option for divertor and main chamber plasma facing components in future fusion devices

The tungsten programme in ASDEX Upgrade is pursued towards a full high-Z device. The spectroscopic diagnostic of W has been extended and refined and the cooling factor of W has been re-evaluated. The W coated surfaces now represent a fraction of 65% of all plasma facing components (24.8 m(2)). The only two major components that are not yet coated are the strikepoint region of the lower divertor as well as the limiters at the low field side. While extending the W surfaces, the W concentration and the discharge behaviour have changed gradually pointing to critical issues when operating with a W wall: anomalous transport in the plasma centre should not be too low, otherwise neoclassical accumulation can occur. One very successful remedy is the addition of central RF heating at the 20-30% level. Regimes with low ELM activity show increased impurity concentration over the whole plasma radius. These discharges can be cured by increasing the ELM frequency through pellet ELM pacemaking or by higher heating power. Moderate gas puffing also mitigates the impurity influx and penetration, however, at the expense of lower confinement. The erosion yield at the low field side guard limiter can be as high as 10(-3) and fast particle losses from NBI were identified to contribute a significant part to the W sputtering. Discharges run in the upper W coated divertor do not show higher W concentrations than comparable discharges in the lower C based divertor. According to impurity transport calculations no strong high-Z accumulation is expected for the ITER standard scenario as long as the anomalous transport is at least as high as the neoclassical one.

New Weight-Loss Measurements of the Chemical Erosion Yields of Carbon Materials under Hydrogen Ion Bombardment

Total erosion yields of graphite and carbon materials under hydrogen and deuterium bombardment measured with the weight-loss method are presented for ion energies between 15 eV and 8 keV in the temperature range 300 to above 1000 K. The temperature of the maximum of the chemical erosion increases from below 600 to above 850 K with ion energies from 15 to 300 eV. Chemical erosion yields obtained by weight-loss measurements exceed yields measured mass-spectrometrically always by a factor of about two. Collector experiments show that a fraction of the eroded particle sticks to walls and, therefore, reduces the yield measured by mass spectrometry. A synergistic effect of neutrals in the ion beam on the chemical erosion yield can be excluded.

Synthesis of diamond fine particles on levitated seed particles in a rf CH4/H2 plasma chamber equipped with a hot filament

The first successful growth of diamond layers on levitated seed particles in CH4/H2 plasma is presented. The particles were grown in a rf CH4/H2 plasma chamber equipped with a tungsten hot filament. The seed diamond particles injected in a plasma are negatively charged and levitated under the balance of several forces, and diamond chemical vapor deposition takes place on them. The SEM images show that the crystalline structures are formed after the coagulation of islands. The micro-Raman spectroscopy of the particle grown after several hours shows the clear peak assigned to diamond.

Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Subsurface morphology changes due to deuterium bombardment of tungsten

Recrystallized polycrystalline tungsten was exposed to a deuterium plasma beam with high flux (1022 D m−2 s−1) and low energy (38 eV D−1) to fluences up to 1027 D m−2. The sample temperature was varied between 320 and 800 K. The three-dimensional morphology of blister-like structures and the grain orientation were investigated by scanning electron microscopy combined with focused ion beam cross-sectioning and electron backscattering diffraction. Cracks with distorted areas ( 480 K) were observed beneath the surface. The surface blister-like structures and the defects underneath are correlated along crystallographic orientation of the W grains in accordance to the low-indexed slip system {110}111. The defects are mobile and accumulate under deuterium loading. Samples exposed near room temperature do not form such large cavities by subsequent heating up to 1300 K. Deuterium bombardment above 700 K does not lead to blister-like structures.

Surface morphology and deuterium retention in tungsten exposed to low-energy, high flux pure and helium-seeded deuterium plasmas

Blistering and deuterium retention in re-crystallized tungsten exposed to low-energy, high flux pure and helium-seeded D plasmas to a fluence of 1027?D?m?2 have been examined with scanning electron microscopy, thermal desorption spectroscopy, and the D(3He,p)4He nuclear reaction at 3He energies varied from 0.69 to 4.0?MeV. In the case of exposure to pure D plasma (38?eV?D?1), blisters with various shapes and sizes depending on the exposure temperature are found on the W surface. No blisters appear at temperatures above 700?K. The deuterium retention increases with the exposure temperature, reaching a maximum value of about 1022?D?m?2 at 480?K, and then decreases as the temperature rises further. Seeding of 76?eV He ions into the D plasma significantly reduces the D retention at elevated temperatures and prevents formation of the blisters.

Improvement of the thermo-mechanical properties of fine grain graphite by doping with different carbides

The possibilities for optimization of doped fine grain graphites with high thermal conductivity and high thermal shock resistance are demonstrated at laboratory scale. A mixture of MCMB powder and different carbides (B4C, TiC, VC, ZrC and WC) was used as starting material. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. A direct proportionality between the mean crystallite height, Lc, and the thermal conductivity at room temperature was found for all materials except for the B4C- and the ZrC-doped graphites. With increasing graphitization temperature the open porosity of all doped materials becomes gradually closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. The addition of carbides does not strongly influence the mechanical properties of pure graphite. A high ratio flexural strength to Young’s modulus was achieved.

Properties of tungsten coatings deposited onto fine grain graphite by different methods

Tungsten coatings on fine grain graphite with film thicknesses in the micrometer range were prepared by three different methods: electron beam evaporation, magnetron sputtering, and arc deposition. They were compared with respect to properties relevant for the application of such systems in the nuclear fusion experiment ASDEX Upgrade. The content of light impurities was determined by sputter XPS. A deposition parameter study for magnetron sputtered coatings is presented; the formation of compressive stress is identified to be the major drawback of this method. Furthermore, experiments with thermal loading by an electron beam facility are presented.

Crystal structure characterisation of filtered arc deposited alumina coatings: temperature and bias voltage

Using a filtered vacuum arc deposition device, stoichiometric aluminum oxide (Al2O3) films, with thickness ranging from 20 nm to several microns, were produced under various substrate bias voltages and temperatures. Analysis of the resulting alumina crystal structures was performed with transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. Depending on the negative substrate bias voltage, the deposition temperature required to form α-Al2O3 could be reduced. A crystal phase diagram showing the effect of bias and temperature is presented. Also, preliminary hydrogen permeation measurements of these coatings deposited on thin palladium foil show a good barrier performance as compared with uncoated samples.

Deuterium inventory in the full-tungsten divertor of ASDEX Upgrade

The deuterium inventory in tungsten-coated divertor tiles used during the first full-tungsten plasma-facing wall phase of ASDEX Upgrade was measured by various methods of analysis. The D inventory in the inner divertor was still dominated by codeposition with residual carbon, whereas it was dominated by trapping in the thicker vacuum plasma sprayed tungsten layers at the outer divertor. The total inventory in the divertor area decreased by a factor of 5–10 compared with the period of carbon-dominated plasma-facing wall.