Total Erosion Yield Research Articles

Temperature dependence of the erosion behavior of deuterium beam exposed tungsten-doped carbon films (a-C:W)

Tungsten-doped amorphous carbon films (a-C:W) are used as model system to study the deuterium (D) retention behavior and the erosion behavior of metal containing co-deposited layers. In our recent studies, the implantation temperature was increased towards an ITER relevant range. Pure and 7.5at% tungsten-doped carbon films have been exposed to a deuterium beam of 200eV/D at different implantation temperatures up to 1300K and at a fluence of 1024D/m2. Rutherford backscattering spectrometry with a 1.5MeV +H beam was performed to obtain the total erosion yield. For each implantation condition and for each structure of doped films, the total erosion yield is reduced significant, compared to pure films. The temperature dependence of the total erosion yield is less pronounced for doped films.

Ti-doped isotropic graphite: A promising armour material for plasma-facing components

Finely dispersed Ti-doped isotropic graphites with 4 at.% Ti have been manufactured using synthetic mesophase pitch ‘AR’ as raw material. These new materials show a thermal conductivity at room temperature of ∼200 W/mK and flexural strength close to 100 MPa. Measurement of the total erosion yield by deuterium bombardment at ion energies and sample temperatures for which pure carbon shows maximum values, resulted in a reduction of at least a factor of 4, mainly due to dopant enrichment at the surface caused by preferential erosion of carbon. In addition, ITER relevant thermal shock loads were applied with an energetic electron beam at the JUDITH facility. The results demonstrated a significantly improved performance of Ti-doped graphite compared to pure graphite. Finally, Ti-doped graphite was successfully brazed to a CuCrZr block using a Mo interlayer. These results let assume that Ti-doped graphite can be a promising armour material for divertor plasma-facing components.

Coupling glacial erosion and tectonics at active orogens: A numerical modeling study

Climate change indirectly alters the distribution of tectonic uplift at active orogens by modifying the action of surface processes, which in turn alters mountain topography. The impact of alpine glaciation on tectonic activity is explored here. The predictions of previous analytical, critical wedge models are compared with the output of a numerical model that explicitly couples rock uplift produced by convergence with surface erosion. Glacial and fluvial and erosive processes are calculated over a two‐dimensional grid, which uses an ice evolution model to calculate the rate of glacial erosion due to ice sliding. Tectonic uplift is determined by a two‐dimensional finite element convergence model. The simulations performed support the predictions of previous one‐dimensional analytical modeling of fluvially dominated orogens: Increasing precipitation decreases orogen width by the one quarter power. The simulations show that glacially dominated landscapes have similar dependencies. The total glacial erosion yield is determined to be proportional to the glacial coverage, sublinearly dependent on the precipitation rate, and linearly dependent on the glacial erosion constant that relates erosion with sliding speed. Climate also determines the distribution of tectonic uplift in the model. If the equilibrium line altitude (ELA) is high and glacial processes are limited to peak regions, the center of the mountain range experiences the highest rate of rock uplift. Extensive glaciation increases rock uplift rates on the flanks of the range. The style of glacial erosion is also important: A simple glacial buzzsaw does not create the same tectonic response as models that physically simulate glacial erosion. The simulations support the idea that a climate cooling of the magnitude recorded in the late Cenozoic has the potential to more than double the rate of rock uplift in appropriate orogens. The implications of the model are discussed for observations from the Olympic Mountains of Washington State, the Southern Alps of New Zealand, the Alaskan coastal range, and the southern Andes.

Characterization and erosion of metal-containing carbon films

The annealing of magnetron-sputtered carbon films doped with up to 8.5 at.% W, Ti, V, and Zr prior to the erosion experiments led to the formation of carbide cyrstallites of several nm. The total erosion yield by 30 and 200 eV D−1, impacting as D3+ projectiles, at 300 K and ∼750 K was determined from film thickness changes measured by ion beam analysis. Doping with W, Ti, V, and Zr always reduces the total erosion yield by a factor of up to 20 compared to a pure C film. The temperature dependency of the metal erosion was observed. The methane production yield was studied by mass spectrometry. The CD4 production yield decreases less than the total yield or even increases. This implies that the distribution of the chemically eroded species is changed by the dopants. It can be speculated that the re-deposition of carbon is reduced by doping.

Chemical erosion by deuterium impact on carbon films doped with nanometer-sized carbide crystallites

The erosion by 30 and 200eV/D at temperatures between 300 and 1100K was investigated for magnetron-sputtered films, consisting of carbon and metal (2–7at.% W, Ti, Zr) present as nanometer-sized carbide crystallites. The total erosion yield was determined from weight-loss measurements and film thickness changes measured by RBS. The chemical erosion yield was obtained from the CD4 signal of mass spectrometry. The total erosion yield of doped films is reduced by a factor of 3–20 compared to pure C films. The CD4 production yield decreases less implying that the distribution of the chemically eroded species was changed by the dopants. Therefore, measuring only CD4 production or its spectroscopic signature could yield to misleading values and interpretations.

Chemical weathering in high‐sediment‐yielding watersheds, New Zealand

We have determined the chemical erosion yields for fifteen watersheds in New Zealand, ranging in size from 12.2 to 2928 km2. These rates, coupled with previously measured physical erosion yields, allow us to compare these two modes of landscape denudation. The physical erosion yields are some of the highest measured in the world. Although in most instances the chemical erosion yields are only a small fraction of the total erosion yields, the absolute values are very high. Our data strongly support the notion that chemical erosion rates are greatly influenced by the yield of physical erosion and that the rapid production of fresh surfaces as a result of high physical erosion rates and subsequent denudation is critical to the high chemical erosion yields observed.

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.

Thermal stability and chemical erosion of the silicon doped CFC material NS31

The thermal stability of the 8–10 at.% Si doped 3D CFC material NS31 was investigated using scanning electron microscopy (SEM) and ion beam analysis. Total erosion yields and CD 4 production yields were determined by bombarding with monoenergetic deuterium ions. During heating to 1800 K SiC crystallites of a few μm size are created on the surface. A Si depletion at the surface and an enrichment layer (20–50 at.% Si) beneath the surface of about 5–10 μm thickness were observed. They are explained by the distribution of the crystallites of μm size and the evaporation of Si from the surface. After heating a NS31 mock-up tile to temperatures above 2300 K in a power load test, the Si concentration in a surface layer of more than 5 μm thickness drops below 3 at.% Si. The chemical erosion is reduced for the initial and heated samples by a factor of 2–3 compared to pure graphite. NS31 shows sputtering yields between those of graphite and silicon carbide depending on the Si concentration of the individual surface.

Model for the chemical erosion of graphite due to low-energy H+ and D+ impact

A methane erosion yield model has been developed using the principal atomistic reactions outlined by Küppers and co-workers [eg., A. Horn, A. Schenk, J. Biener, B. Winter, C. Lutterloh, M. Wittmann, and J. Küppers, Chem. Phys. Lett. 231, 193 (1994)] with additional terms to account for the energy of the incident particles, namely, kinetic ejection and damage deposition. Furthermore, modifications were made to the previous models by using distributed activation energies for methyl and hydrogen release as well as an activated Eley-Rideal abstraction process. Fitting of this model to experimentally measured methane yield data shows excellent agreement, except for low energy (⩽25 eV) impact at temperatures above ∼800 K. We have provided a sound physical basis for the behavior of the free fitting parameters and conclude that most of the processes associated with low-energy impact on pyrolytic graphite leading to methane production have been incorporated. Possible extensions of the model to include heavy hydrocarbons and total chemical erosion yields are also discussed. Due to the lack of comprehensive experimental data on the flux dependence of hydrocarbon erosion, the flux dependence of the fitting parameters could not be explored and so the effect of flux density requires further modeling considerations.

Chemical sputtering measurements in Tore Supra by aftershot mass spectrometry outgassing studies

One of the main issues of the high-density, high-radiation plasma-edge scenario for long-pulse operation in future fusion devices is to reduce physical sputtering by lowering the edge electron temperature. If carbon-based materials are used as plasma-facing components in next generation machines such as ignition device to test engineering concepts, the role of chemical with respect to physical sputtering increases as the net erosion mechanism limiting the component’s lifetime. This is due to the fact that the physical sputtering yield decreases stronger when decreasing the ion energy in the 30–150 eV energy range, although very recent laboratory experiments have concluded an important chemical sputtering yield decrease at low deuterium impact energies (down to 10 eV). Because laboratory experiments cannot reproduce the real plasma-wall interaction conditions, principally the high bombarding flux conditions, it is important to determine experimentally and in situ the chemical sputtering yields. The global chemical sputtering yield for a tokamak with carbon walls has been estimated in Tore Supra by analyzing the aftershot molecular outgassing with a differentially pumped mass spectrometer. Quantitative measurements of molecular outfluxes during plasma operation are difficult due to the strong cracking probability of the neutral species. The aftershot outgassing method takes advantage of the fact that just after the termination of the plasma, the molecular desorption responsible for chemical sputtering continues, although in a decreasing way. This avoids the plasma perturbation effect due to molecule cracking. Even though the analysis of the spectrum is difficult due to the complex decomposition pattern of organic compounds containing hydrogen and deuterium, the characteristic time evolution of the different outgassing species can help to deconvolute the mass spectrum. The main product is CD4 and the measured chemical sputtering yield (YCD4=methane outflux from the wall/deuterium flux to the wall) is 3×10−3<YCD4<9×10−3 for D2 discharges. The contribution of second order hydrocarbons and of CO to the total chemical erosion yield is also studied. The dependence of the yields on plasma flux and surface conditions is discussed, as well as the pros and cons of the method used.

Erosion of Pyrolytic Graphite and Ti-doped Graphite due to High Flux Irradiation

The erosion of pyrolytic graphite and titanium doped graphite RG-Ti above 1,780K was investigated by 5keV Ar beam irradiation with the flux from 4x1019 to 1x1021 m−2·s−1. The total erosion yields were significantly reduced with the flux. This reduction would be attributed to the reduction of RES (radiation enhanced sublimation) yield, which was observed in the case of isotropic graphite with the flux dependence of RES yield of φ−0.26 (φ: flux) obtained in our previous work. The yield of pyrolytic graphite was roughly 30% higher than that of isotropic graphite below the flux of 1020 m−2·s−1 whereas each yield approached to very close value at the highest flux of 1x1021 m−2·s−1. This result indicated that the effect of graphite structure on the RES yield, which was apparent in the low flux region, would disappear in the high flux region probably due to the disordering of crystal structure.In the case of irradiation to RG-Ti at 1,780K, the surface undulations evolved with a mean height of about 3μm at 1.2×1020 m−2·s−1, while at higher flux of 8.0×1020 m−2·s−1 they were unrecognizable. These phenomena can be explained by the reduction of RES of graphite parts excluding Tic grains.

Chemical erosion of pyrolytic graphite by low-energy H + and D + impact

Recent developments have lead to prospects of lower plasma edge temperatures in the divertor region of tokamaks and, consequently, a growing interest in the interaction of graphite with low-energy hydrogen ions (10s of eV). Here we present experimental measurements of methane and heavier hydrocarbon yields of pyrolytic graphite (300 K–1000 K temperature) for H + impact energies of 10–200 eV. Heavier hydrocarbons contribute significantly to the total chemical erosion yield at low energies. Comparison of CH 4 yields due to H + irradiation of graphite with similar results for CD 4 yields due to D + shows a smaller isotopic effect than anticipated based on previous H + and D + measurements. Comparisons are also made between the experimental results and a recently proposed model.

Erosion of Pyrolytic Graphite and Ti-doped Graphite due to High Flux Irradiation.

The erosion of pyrolytic graphite and titanium doped graphite RG-Ti above 1,780K was investigated by 5keV Ar beam irradiation with the flux from 4x1019 to 1x1021 m−2·s−1. The total erosion yields were significantly reduced with the flux. This reduction would be attributed to the reduction of RES (radiation enhanced sublimation) yield, which was observed in the case of isotropic graphite with the flux dependence of RES yield of φ−0.26 (φ: flux) obtained in our previous work. The yield of pyrolytic graphite was roughly 30% higher than that of isotropic graphite below the flux of 1020 m−2·s−1 whereas each yield approached to very close value at the highest flux of 1x1021 m−2·s−1. This result indicated that the effect of graphite structure on the RES yield, which was apparent in the low flux region, would disappear in the high flux region probably due to the disordering of crystal structure.In the case of irradiation to RG-Ti at 1,780K, the surface undulations evolved with a mean height of about 3μm at 1.2×1020 m−2·s−1, while at higher flux of 8.0×1020 m−2·s−1 they were unrecognizable. These phenomena can be explained by the reduction of RES of graphite parts excluding Tic grains.

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.

Reduction of radiation-enhanced sublimation of graphite under high flux beam irradiation

Radiation-enhanced sublimation (RES) of isotropic graphite was studied under high flux beam irradiation. Irradiation was performed with a 5 keV Ar beam with a maximum flux of 1.5 × 10 21 m −2 s −1, about two orders of magnitude higher than that of previous beam experiments. It is found that the total erosion yield of isotropic graphite at 1980 K under high flux conditions of 1 × 10 21 m −2 s −1 is reduced to a value of about 3 in comparison with a yield of 5.9 under low flux conditions of 3.5 × 10 19 m −2 s −1. The dependence of the RES yield (total yield subtracted by physical sputtering yield) on the flux φ ( φ > 1 × 10 20 m −2 s −1) is as φ −0.26, which is much stronger than that obtained in the low flux region ( φ −0.07, φ < 1 × 10 19 m −2 s −1). The RES model based on diffusion and annihilation of C self-interstitials including stable defects for the sinks of interstitials is discussed in terms of the flux dependence of RES yield.

Comparison of the chemical erosion of carbon/carbon composites and pyrolytic graphite

Abstract The production of hydrocarbons due to 300 and 1000 eV H+ bombardment of an AEROLOR carbon/carbon composite sample is compared with that of pyrolytic graphite. Within the experimental uncertainty, no difference was observed in either the total erosion yield or the distribution of hydrocarbons, as a function of temperature, between two orientations of the carbon/carbon composite and the pyrolytic graphite sample. For pyrolytic graphite, the ion energy dependence of the hydrocarbon formation rate was also studied.

Hydrocarbon Formation on Carbon Surfaces Facing a Hydrogen Plasma

The hydrocarbon formation of carbon materials exposed to hydrogen atoms and ions is reviewed. It turns out that surface conditions and modification play a major role in hydrocarbon formation. Graphite exposed to hydrogen plasma exhibits surface properties similar to those of amorphous hydrogenated carbon films and redeposited carbon films. The total chemical erosion yield of these carbon materials is similar for thermal atomic hydrogen and for energetic hydrogen ions, reaching maximum values of ∼0.1 eroded carbon per incoming hydrogen. However, the spectrum of formed hydrocarbons is determined by the energy of the impinging hydrogen. By a thermal H0 exposure, CH3 is formed together with equal amounts of C2Hx and C3Hx. With an energetic H+ bombardment, the main reaction product is CH4 with minor contributions of C2Hx and C3Hx. The amount of C2Hx and C3Hx formation decreases with increasing H+ energy. Hydrocarbon formation at low energies and high flux densities, as in the scrape-off layer (SOL) of fusion devices, is characterized by a broadening of the temperature dependence together with a slight decrease of the absolute erosion yield. Similar results have been obtained by an in situ study of the hydrocarbon formation in the SOL of the Tokamak Experiment for Technology Oriented Research (TEXTOR) plasma using a sniffer probe system.

Erosion and Redeposition of Graphite by Hydrogen Plasmas

The results of erosion and redeposition studies of graphite by hydrogen plasma bombardment in the PISCES facility are reviewed. The total erosion yields of several types of graphites have been measured during plasma exposure with ion fluxes of up to 2 x 10/sup 18/ cm/sup -2/ . S/sup -1/, ion energies of 50 to 200 eV, and sample temperatures of 50 to 950/sup 0/C. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, and pyrolytic graphites, and a four-directional carbon-carbon (C-C) composite weave. The erosion rates of all the graphites tested are about equal, suggesting that surface damage by the ion bombardment results in similar erosion yields. The C-C composite weave material showed an increased weight loss during initial exposure, and then equal or lower erosion yields compared to the other graphites.

Erosion of graphite by high flux hydrogen plasma bombardment

The total erosion yields of several types of graphites have been measured during high ion flux exposure in the PISCES plasma facility. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, pyrolytic, and C-SiC-coated graphites, and a ‘4-D’ C-C composite weave. Erosion experiments with ion fluxes of up to 2 × 1018 cm−2 · s−1, at ion energies of 50 to 200 eV and sample temperatures of 50 to 950°C, are reported. In the absence of redeposition, the erosion rates of all the graphites tested is essentially the same. The C-C composite weave material showed equal or lower erosion yields compared to the other graphites over the range of temperatures tested. The maximum total carbon erosion yield at 600°C increased by a factor of two for 50 eV ion bombardment and by a factor of five for 200 eV ion bombardment, compared to the reference values found for room temperature samples. At temperatures above 800°C, the chemical erosion is suppressed and the erosion yield reaches values expected for physical sputtering. A direct comparison with two ion beam experiments with an ion flux three orders of magnitude lower than used in PISCES showed total erosion rates within a factor of two, indicating that chemical sputtering is not significantly suppressed by high ion fluxes. The results show that the chemical sputtering of graphite limiters or divertor plates in tokamaks will vary by a factor of only two to three over a limiter temperature range of 0 to 800°C if the incident ion energy is less than 100 eV as is observed in tokamak experiments. The net erosion is also strongly reduced by reionization in the plasma and redeposition of both hydrocarbons and physically sputtered carbon.

The reaction of energetic O 2+, thermal O 2, and thermal O 2/ar + on graphite and the use of graphite for oxygen collector probes

The bombardment of graphite with energetic oxygen leads initially to a retention of oxygen within the graphite and subsequently to a reemission of CO and CO 2, increasing continuously with further implantation. After saturation of the graphite surface layer with oxygen, the reemission of CO and CO 2 reaches a steady state in which the implanted oxygen ion reacts to CO or CO 2 resulting in a chemical erosion yield close to 1. Since physical sputtering occurs simultaneously, the total erosion yield for graphite is nearly 1 for low energy oxygen and greater than 1 for energies above 500 eV. By a subsequent heating of the targets the retained oxygen is released completely in the form of CO and CO 2. Pyrolytic graphite collects nearly all the implanted oxygen until a critical irradiation dose is reached beyond which the retention behaviour decreases. This critical dose depends on the oxygen energy and on the irradiation temperature. These results were applied in developing a new method for measuring the oxygen impurity flux in the scrape-off region of fusion machines as demonstrated by exposed probes in TEXTOR under various discharge conditions.