R. Schwörer

Temperature-dependent sputtering of doped graphites and boron carbide

Abstract The temperature dependence of the sputtering behaviour of various low-Z materials is presented. The hydrocarbon production under deuterium bombardment is investigated as a function of the target temperature. Carbon doped with beryllium or boron, as well as boron carbide show a strong reduction in the chemical sputtering yield. The reduction of the radiation-enhanced sublimation yield (RES) above 1200 K is small and not uniquely correlated with the dopant concentration. A variety of doped graphites is compared for 1 keV D+ sputtering at 300, 800 and 1470 K. The resulting erosion yields are discussed in terms of dopant concentration and the microstructure of the materials.

Thermal desorption of D2 and CD4 from bulk-boronized graphites

Abstract Thermal desorption spectra of D2 and CD4 from bulk-boronized graphites containing 3 to 38 wt% B as well as from pure graphite and stoichiometric B4C were measured. The samples were implanted with 3 keV D+3 ions at temperatures between 130 and 700°C to fluences between 2.3 × 1016 and 2.3 × 1019 D/cm2 prior to the desorption. An enhancement of D2 release is observed with increasing boron content, while the CD4 release decreases correspondingly. The desorption peak maximum of D2 is shifted towards lower temperatures with increasing boron concentrations. Apparently, it is the boron in solid solution which mainly influences the trapping and release behaviour of deuterium. The formation of CD4 is suggested to proceed via a precursor state. The formation of the complete CD4 molecule takes place only close to the desorption temperature. The reduction of binding energy for D has only a minor influence on ion-beam-induced detrapping.

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.

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.

Migration of tungsten eroded from divertor tiles in ASDEX Upgrade

Abstract Tungsten migration in ASDEX Upgrade was investigated by analysis of deposition on long term sample probes and by erosion and deposition studies using two probe manipulator systems in the midplane and the outer divertor plate respectively. Divertor retention was determined by measuring the total tungsten production in the divertor and comparison with the spectroscopically measured tungsten content in the confined plasma. The fraction of tungsten deposited on limiters in the main chamber compared to deposition in the divertor was determined by injecting tungsten into the main plasma using laser ablation and determining the deposition of these atoms on collector probes. The ratio of inner divertor to outer divertor tungsten deposition was obtained from surface analysis of long term sample probes.

DEPOSITION OF HYDROGENATED CARBON LAYERS FROM CH3+ ION-BEAMS

Carbon deposition and hydrogen codeposition is investigated as function of ion energy, fluence and target temperature at normal incidence by bombardment of hydrocarbon molecuIes onto Si and graphite. An amorphous hydrogen/carbon (a-C: H) layer is formed in a thickness range of 40 to 130 nm by CH 3 + bombardment up to a fluence of 3 × 10 18 /cm 2 . The deposition process and the re-erosion phenomenon of the carbon film is also studied. 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 have a good understanding of the interaction between hydrocarbon molecular ions and the carbon-based wall in fusion devices

Dynamics of B sublimation from boron doped graphite USB15

Abstract The diffusion and release of boron in USB15 at temperatures above 1700 K has been investigated. The boron depth distribution has been measured using the 11B(p, α0)8Be nuclear reaction after heating the sample for different times and at different temperatures. After prolonged heating a boron depleted carbon layer develops due to outdiffusion of boron in solid solution and the dissolution of the B4C precipitates. Additionally, the high temperature erosion of USB15 under 1 keV D+ bombardment at a flux of 2 × 1015 D+/cm2 s has been studied. The radiation enhanced sublimation (RES) is limited by the boron concentration at the surface. A model which relates the reduction of RES to the surface concentration of boron predicts a stronger suppression of RES for larger ion fluxes.