H. Plank

Re-emission and thermal desorption of deuterium from plasma sprayed tungsten coatings for application in ASDEX-upgrade

Abstract The trapping and release of deuterium implanted with an energy of 100 eV in wrought and in plasma sprayed tungsten of different manufacture and structure has been investigated by means of re-emission as well as thermal and isothermal desorption spectroscopy. The experimental data for wrought tungsten are compared with model calculations with the PIDAT code in order to estimate the parameters governing diffusion, surface recombination and trapping in tungsten. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for the implantation parameters used in this work. The mobile hydrogen concentration in tungsten during the implantation is of the same order of magnitude than the trapped one, being released after the termination of the implantation. The fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The temperature needed for the release of the trapped deuterium (∼ 600 K) are considerably lower than for graphite, due to the smaller trapping energy (≤ 1.5 eV).

Codeposition of hydrogen with beryllium, carbon and tungsten

Abstract The trapping of energetic deuterium codeposited with beryllium, carbon and tungsten has been measured on a silicon collector probe at room temperature. The subsequent release of the trapped deuterium at elevated temperatures was determined by thermal desorption spectroscopy. At room temperature, deuterium codeposits both with carbon and BeO with a ration of 0.41 D-atorns/C-atom and 0.38 D-atoms/BeO, respectively. No codeposition of deuterium with tungsten is observed. The thermal release of codeposited hydrogen from BeO begins at 400 K. All hydrogen is released at temperatures above 800 K.

Preferential sputtering of carbides under deuterium irradiation — a comparison between experiment and computer simulation

Abstract The erosion behaviour of SiC, TiC and WC, i.e. sputtering yields and changes of surface composition due to preferential sputtering, is investigated under deuterium bombardment at room temperature in the energy range between 20 eV and 1.5 keV. Surface concentrations are measured by means of Auger Electron Spectroscopy and are compared both with concentrations calculated by using experimental sputtering yields of pure elements and with results obtained from TRIDYN computer simulations. Thus, the contribution of different factors to compositional changes, such as physical sputtering (TRIDYN), chemical erosion (experimental yields) and radiation induced solid state reactions, can be studied. It is found that the dominant erosion mechanism for carbides is physical sputtering at ion energies above about 5 times the threshold energy. A surface layer depleted of C is formed at lower ion energies due to preferential erosion of C. In SiC, this is due to chemical erosion of C. In TiC, evidence is given that, at lower ion energies, the surface composition is governed by bombardment induced solid state reactions between Ti and C. In WC, the large mass ratio between W and C results in preferential erosion of C due to threshold effects.

Hydrogen trapping in and release from tungsten: Modeling and comparison with graphite with regard to its use as fusion reactor material

Abstract Trapping and release of deuterium implanted in tungsten is investigated by modeling the results of reemission, thermal and isothermal desorption experiments. Rate coefficients and activation energies for diffusion, trapping and detrapping are derived. Hydrogen atoms are able to diffuse deep into tungsten, establishing a solute amount of the same order of magnitude as the trapped one. This ‘diffusion zone’ exceeds the implantation zone by more than two orders of magnitude, even at room temperature. The solute amount of hydrogen in tungsten depends only slightly on the incident ion energy, but scales with implantation fluence. This high amount of solute hydrogen is the main difference of tungsten compared to graphite where nearly all hydrogen is trapped in the implantation zone, the solute amount being orders of magnitude lower. The resulting unlimited accumulation of hydrogen in tungsten deep in the material down to the backward surface disadvantages tungsten as fusion reactor material with regard to hydrogen recycling properties.

Erosion of Si- and Ti doped graphites due to deuterium irradiation

During irradiation of carbon materials with hydrogen ions hydrocarbon molecules are formed resulting in an enhancement of the erosion yield. At temperatures around 800 K hydrocarbon molecules are released in a thermal activated process, while at low temperatures and low ion energies physical sputtering of lightly bound hydrocarbon radicals enhances the erosion yield. Doping of carbon materials with B, Si and Ti results in a reduction of its chemical reactivity with hydrogen ions. While B reduces drastically the thermal activated process it does not alter the sputtering of hydrocarbons at low energies. For isotropic graphites doped with 10at% Si (LS10) and 10at% Ti(LT10) it is shown that preferential erosion of carbon leads to enrichment of the dopant at the surface. The thermal activated hydrocarbon emission is reduced already at low ion fluences for LS10 and LT10, while the low energy process is only reduced after high fluence irradiation and carbon surface depletion in the case of Ti doping. Depending on the microstructure of the material a very pronounced surface topography delevopes. Carbidic grains protect the underlying carbon material from erosion until a columnar structure evolves. Due to the high threshold for physical sputtering of Ti the total erosion yield for LT10 shows the predicted threshold behaviour for physical sputtering.

Deposition and hydrogen content of carbon films grown by CH+3 ion‐beam bombardment

Carbon deposition and hydrogen codeposition is investigated as a function of ion energy, fluence, and target temperature at normal incidence by bombardment of silicon and pyrolitic graphite substrates with mass selected CH+3 molecules. An amorphous hydrogenated carbon layer (a‐C:H) is formed in a thickness range of 40–130 nm at a fluence of 3×1018/cm2. The deposition process, the re‐erosion phenomenon, the hydrogen content, and the H/C ratios of the carbon films are studied between 300 and 1000 K in the ion energy range from 150 eV to 3 keV. The experimental results are compared with those of TRIDYN computer simulations and previous experimental results of carbon sputtering by atomic H+ and C+ beams in order to obtain a better understanding of the interaction between hydrocarbon ions and the carbon‐based wall materials in fusion devices.

Surface composition modifications of carbides and doped graphites due to D+ ion bombardment

Abstract Surface composition modifications due to D + ion bombardment with energies between 10 eV and 1 keV at temperatures ranging from 300 K to 1000 K and the consequences for the sputtering yields and chemical erosion have been investigated for SiC, TiC and graphites doped with 10 at.% Si or 10 at.% Ti. At D + ion energies below about 100 eV, a steady-state surface enrichment of the metal has been observed both in the carbides and in the doped graphites. The maximum enrichment was nearly 100 at.% in SiC, about 75 at.% in TiC and about 70 at.% and 60 at.% in Si- and Ti-doped graphite, respectively. In SiC the temperature and the ion energy dependence of the Si surface enrichment reflects the chemical erosion, i.e. the surface depletion of carbon. The surface enrichment of the metal in TiC is caused by the threshold energy of 33 eV for Ti sputtering by D + irradiation. In both doped graphites after D + bombardment, the development of a cone structure has been observed. The tops of these cones consist of Si- or Ti-rich grains shielding the underlying graphite material from erosion. Thus, the steady-state metal surface enrichment under D + bombardment can be explained by considering the surface topography and the metal surface enrichment of the corresponding carbide.

Surface modifications and erosion yields of silicon and titanium doped graphites due to low energy D+ bombardment

Abstract Influences of low energy D+ ion bombardment and target temperature on surface topography, surface concentration and erosion yield of carbon based binary compounds were investigated. The samples contained 10 at.% Si and 10 at.% Ti, respectively. The surface concentration was determined in situ by Auger electron spectroscopy and the topography ex situ by scanning electron microscopy. During low energy D+ bombardment a pronounced conelike surface developed with silicon respective titanium rich ‘caps’ protecting the underlaying carbon rich shafts from erosion. The average dopant surface concentration was up to 7 × the bulk concentration. The erosion mechanism was determined by surface concentration and chemical state of the surface: At high temperatures carbidic bindings dominated, while at room temperature a mixture of graphite and carbide covered the surface.

Deposition of amorphous hydrogenated carbon films due to hydrocarbon molecule ion-beam bombardment

Abstract Carbon deposition and hydrogen codeposition are investigated as functions of ion energy, fluence and target temperature at normal incidence on silicon and graphite by bombardment with mass selected CH2+, CH3+, CH4+ and CD4+ molecule beams. An amorphous hydrogenated carbon layer (a-C:H) is formed in a thickness range of 40 to 130 nm by CH3+ and CH2+ bombardment up to a fluence of 3 × 1018/cm2. The deposition process, the re-erosion phenomenon and the H C ratio of the a-C:H films are studied between 300 and 1100 K in the ion energy range from 0.15 to 3 keV by means of ion beam analysis and Auger Electron Spectroscopy (AES). The experimental results are compared with TRIDYN computer simulation and previous experimental results of carbon sputtering by atomic H+ and C+ beams in order to get a better understanding of the interaction between hydrocarbon ions and the carbon-based wall materials in fusion devices.

Erosion behaviour and surface composition modifications of SiC under D+ ion bombardment

Abstract The erosion behaviour of fibre reinforced SiC under deuterium ion bombardment is investigated in the ion energy range between 20 eV and 1 keV. The sputtering yields are compared with earlier results obtained from SiC samples with different structure and fabrication. At D+ energies below 100 eV chemical erosion of C is found which results in a surface enrichment of Si. The surface composition modifications are investigated by Auger Electron Spectroscopy. The surface concentrations have been measured as a function of target temperature up to 1000 K. It is found that the Si surface enrichment reflects the ion energy and temperature dependence of the chemical erosion of C in SiC. At low D+ energies a Si rich thin altered layer covering the SiC target is formed.