Radiation-enhanced Sublimation Research Articles

Upgrading of Plasma Wall Interaction Model for Tokamak Transient Modeling Code AINA 2.0, Used in Safety Studies of ITER Plasma Instability Events

In this contribution, an upgraded model for plasma-wall interaction in the AINA code is presented. The AINA code is a comprehensive hybrid code comprising a global balance plasma dynamics model and a radial and poloidal thermal analysis of in-vessel components. AINA is an evolution of the SAFALY code, which was initially adopted to assess ITER EDA plasma safety events and quantitatively investigate plasma instability events in nuclear fusion reactors such as ITER.The new erosion code module includes algorithms for the most relevant plasma wall interaction phenomena that will take place in the ITER vessel during the steady state of the normal operation. Physical sputtering, radiation enhanced sublimation (RES), and chemical erosion algorithms have been added to the previous thermal sublimation algorithm. The erosion results from these models have been benchmarked with results for ITER normal operation from the B2-Eirene code.The new erosion model had to be tested with external data for particle fluxes over the wall, because the AINA code does not presently have the ability to model those particle fluxes. However, with the new results, the impurity transport model parameters have been re-calibrated and some useful conclusions have been extracted.

Study of carbon erosion under ion bombardment at high temperature: Application to the thermal protection system of Solar Probe+

Solar Probe+ (SP+) will employ a combination of in situ measurements and imaging to achieve the mission’s primary scientific goal: to understand how the Sun’s corona is heated and how the solar wind is accelerated. The Solar Probe+ thermal protection system (TPS) can be a carbon–carbon (C–C) composite shield, reaching temperatures of 1600–2100 K and submitted to radiation effects including physical sputtering, chemical erosion, sublimation and radiation-enhanced sublimation (RES). In this study, different numerical codes were used to describe the behaviour of a carbon-based thermal protection system. The physical sputtering yield of carbon by hydrogen and helium ions is less than 2%. The maximum implantation yield of ions is more than 85% and the implantation depth is around 100 nm for hydrogen and 150 nm for helium. This could have a great influence on the material properties and its chemical erosion, characterized by the production of hydrocarbons (acetylene and methane) and consequently the degradation of the composite and the pollution of the measurement of the onboard instruments. Finally, an experimental study is also done on polycrystalline graphite to observe the various erosion mechanisms of the material under extreme conditions.

Molecular sputtering of fullerite by low-energy bismuth ions

The interaction of low-energy bismuth ions with a fullerite surface at ion energies in the range from 50 to 200 eV and at target temperatures from 100 to 270°C has been studied experimentally. Investigation of the structure of the condensates formed by a flow of the eroding target material has revealed that the emitted flow consists of C60 molecules and bismuth atoms. The erosion of fullerite is explained by a superposition of three main processes, namely, thermal evaporation, radiation-enhanced sublimation, and physical molecular sputtering, which dominate in different temperature ranges.

Overview of Erosion Mechanisms, Impurity Transport, and Deposition in TEXTOR and Related Modeling

This paper gives an overview of the research activities at TEXTOR on impurity production, impurity transport through the plasma, and then deposition. First, laboratory experiments on chemical erosion by hydrogen and oxygen and radiation-enhanced sublimation are described, followed by the main part, which concentrates on the TEXTOR data of impurity release, impurity transport, and redeposition. The differences between the behavior of high-Z and low-Z materials are discussed. Many of the TEXTOR experiments are carried out using special limiter locks, but the overall carbon balance of net erosion sources and net deposition zones are also shown. Finally, modeling of erosion and dedicated transport experiments are addressed.

Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation*1

A suite of FORTRAN subroutines/functions to generate data using empirical formulas for physical sputtering of mono-atomic targets for any elemental incident ion (atom), chemical erosion of graphite, Radiation Enhanced Sublimation (RES) of graphite, the number and energy backscattering coefficients for any elemental incident ion (atom) on a compound target and thermal evaporation of graphite is presented. Since chemical erosion, RES and thermal evaporation depend on the surface temperature of graphite, a subroutine implementing the 1-D heat diffusion equation to determine the temperature of any plasma-facing graphite surface is implemented. As an example to illustrate the use of these subroutines/functions, a simple model for the erosion of a plasma-facing surface, consisting of a simple collisionless sheath model, a 1-dimensional steady state heat diffusion model and 0-dimensional steady state particle balance at the target is developed and a sample listing of the program is presented. Title of program: Plasma Surface Interaction Codes (PSIC) Catalog identifier: ADTR Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADTR Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computers for which the program has been designed for and others on which it has been tested: Program is designed for any computer with a FORTRAN-77 compiler. It has been tested on a Linux PC with g77, Absoft f77, f90, f95, Fujitsu f95, Lahey f95 Operating systems under which the program has been tested: Linux, UNIX Programming language used: FORTRAN-77 Memory required to execute with typical data: Negligible (executable is 178 861 bytes; add to it the memory used by the Fortran library linked to) No. of bits in a word: 16 No. of processors used: 1 Has the code been vectorized or parallelized?: No No. of bytes in distributed program, including test data, etc.: 11 619 No. of lines in distributed program, including test data, etc.: 1015 Distribution format: tar gzip file Nature of physical problems: 0-D plasma surface interaction model used to demonstrate calls to subroutines for physical sputtering yield, chemical erosion yield of graphite, RES of graphite, backscattering coefficients, thermal evaporation of graphite and 1-D heat diffusion in graphite. Method of solution: Semi-empirical formulas are used for the PSIs and analytical formulas are used for thermal evaporation and 1-D heat diffusion in graphite. Restrictions on the complexity of the problem: All the PSIs (except for the backscattering coefficients) are valid only for a monoatomic target. The heat diffusion solution is not valid for non-uniform plasmas and for targets for which the heat diffusion is not isotropic. Typical running time: Example program takes much less than 0.01 second Universal features of the program: None

Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation

A suite of FORTRAN subroutines/functions to generate data using empirical formulas for physical sputtering of mono-atomic targets for any elemental incident ion (atom), chemical erosion of graphite, Radiation Enhanced Sublimation (RES) of graphite, the number and energy backscattering coefficients for any elemental incident ion (atom) on a compound target and thermal evaporation of graphite is presented. Since chemical erosion, RES and thermal evaporation depend on the surface temperature of graphite, a subroutine implementing the 1-D heat diffusion equation to determine the temperature of any plasma-facing graphite surface is implemented. As an example to illustrate the use of these subroutines/functions, a simple model for the erosion of a plasma-facing surface, consisting of a simple collisionless sheath model, a 1-dimensional steady state heat diffusion model and 0-dimensional steady state particle balance at the target is developed and a sample listing of the program is presented. Program summary Title of program: Plasma Surface Interaction Codes (PSIC) Catalog identifier: ADTR Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADTR Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computers for which the program has been designed for and others on which it has been tested: Program is designed for any computer with a FORTRAN-77 compiler. It has been tested on a Linux PC with g77, Absoft f77, f90, f95, Fujitsu f95, Lahey f95 Operating systems under which the program has been tested: Linux, UNIX Programming language used: FORTRAN-77 Memory required to execute with typical data: Negligible (executable is 178 861 bytes; add to it the memory used by the Fortran library linked to) No. of bits in a word: 16 No. of processors used: 1 Has the code been vectorized or parallelized?: No No. of bytes in distributed program, including test data, etc.: 11 619 No. of lines in distributed program, including test data, etc.: 1015 Distribution format: tar gzip file Nature of physical problems: 0-D plasma surface interaction model used to demonstrate calls to subroutines for physical sputtering yield, chemical erosion yield of graphite, RES of graphite, backscattering coefficients, thermal evaporation of graphite and 1-D heat diffusion in graphite. Method of solution: Semi-empirical formulas are used for the PSIs and analytical formulas are used for thermal evaporation and 1-D heat diffusion in graphite. Restrictions on the complexity of the problem: All the PSIs (except for the backscattering coefficients) are valid only for a monoatomic target. The heat diffusion solution is not valid for non-uniform plasmas and for targets for which the heat diffusion is not isotropic. Typical running time: Example program takes much less than 0.01 second Universal features of the program: None

Erosion of high- Z metals with typical impurity ions

This paper reviews the erosion properties of high- Z plasma facing materials due to the bombardment with typical impurity ions. The different impurity species present in today’s fusion experiments are compared in terms of their importance for erosion of high- Z metals, showing that C is the main source of erosion. As example results are presented here from controlled ion beam experiments on the temperature dependent erosion and deposition processes occurring during bombardment of W by keV C-ions. It is shown that radiation enhanced sublimation (RES) and diffusion are responsible for the observed temperature dependence. A new code was developed to simulate the influence of thermally activated processes on the complex temperature dependence of protective C layer growth on W during simultaneous bombardment by keV C and eV hydrogen isotopes. To model RES, diffusion and chemical erosion of C the Monte Carlo program TRIDYN was coupled with a newly developed diffusion code DIFFUSEDC and a chemical erosion module YCEHM. This combination was validated by comparing calculated W erosion yields in D plasma with C as main impurity to spectroscopically measured values from ASDEX Upgrade as well as ion beam experiments using CH 3 +.

A New Type of Multielements-Doped, Carbon-based Materials Characterized by High-thermoconductivity, Low Chemical Sputtering, Low RES Yield and Exposure to Plasma

Low-Z materials, such as carbon-based materials and Be, are major plasma-facing material (PFM) for current, even in future fusion devices. In this paper, a new type of multielement-doped carbon-based materials developed are presented along with experimental results of their properties. The results indicate a decrease in chemical sputtering yield by one order of magnitude, a decrease in both thermal shock resistance and radiation-enhanced sublimation, an evidently lower temperature desorption spectrum, and combined properties of exposing to plasma.

Comparison of surface modification and erosion of high-Z metals due to carbon ion implantation

In this work we investigated the erosion and carbon deposition behavior of different high-Z materials, namely W, Ta, Pt and Au during irradiation by 2.4 keV C 12 ions in the temperature range from 300 to 1073 K. During the bombardment the sample's weight was monitored and information about erosion and deposition was deduced from the dependence of the weight change on the ion fluence. We found a strong temperature dependence of the weight change. Two processes were found to contribute to the temperature dependence, namely diffusion and radiation enhanced sublimation (RES) of the deposited carbon. For modeling of the weight-change measurements these two processes were investigated in separate experiments. Diffusion profiles of C in W were measured and evaluated using a concentration dependent diffusion coefficient D(C). The carbon erosion due to RES at elevated temperatures was investigated in C self-sputtering experiments. The information gained during these two experiments was then used in computer simulations of the temperature dependent weight change of W and Ta samples during C bombardment. In order to model the simultaneous diffusion and erosion, the Monte Carlo code TRIDYN was coupled to the newly developed diffusion code DIFFUSEDC, which solves the general form of Fick's second law with a concentration dependent diffusion coefficient using a finite difference approach. By using this model we were able to reproduce the experimental results from the weight change measurements. The model allows extrapolation to temperatures and ion fluxes not accessible in ion beam experiments.

Erosion of carbon materials under high fluxes

Experimental results of graphite erosion under high fluxes by using a high flux irradiation device in Osaka University are reviewed. Results of chemical sputtering experiments by 5 keV D3+ beams with fluxes from 0.5 × 1021 to 3.9 × 1021 D/m2 s showed an increase in peak temperatures from 900 K to 980 K with flux for isotropic graphite and CFC. These peak temperatures are higher than those of low flux results (750 K-850 K, ≤ 1020 m-2 s-1). The peak yields did not show clear flux dependence. Results of radiation-enhanced sublimation (RES) experiments by 5 keV Ar+ beams with fluxes from 3.5 × 1019 to 1.5 × 1021 Ar/m2 s showed a significant decrease in the RES yields with flux for isotropic graphite and pyrolytic graphite. The flux dependence of the RES yields (total yields minus physical sputtering yields) was about ϕ-0.26 (ϕ: flux), while the flux dependence in fluxes from ∼1017 to ∼1019 Ar/m2 s was about ϕ-0.07. Comparison between models and experiments for chemical sputtering and RES are also shown.

Overview on the Effects of Dopants on Chemical Erosion and RES of Carbon-Based Materials

For carbon-based materials physical sputtering, chemical erosion, and radiation enhanced sublimation (RES) are the primary erosion processes due to ion impact. For H impact the influence of dopants (e.g. B, Si, Ti) on the radiation enhanced chemical erosion (Ytherm) at temperatures around 800 K is discussed. For B doping the reduction of the erosion yield could be described by a reduction of the activation energy for H release. At low surface temperatures and low H ion energies a reduction of the chemical enhanced physical sputtering yield (Ysurf) due to dopants is observed and discussed. It is unclear if the reduction is just caused by modification of the surface composition due to preferential erosion of carbon. The surface composition and the erosion yield depend on ion fluence, dopant distribution, production procedure and operational history of the target. On atomic scale the dopant distribution causes changes in the chemical bonds and reactions and, therefore, in the chemistry necessary for both chemical erosion regimes (Ysurf, Ytherm). Also, the basic processes of RES – interstitial production, migration, recombination, and thermal desorption – could be influenced. At high temperatures, at which RES dominates, the stability of the material and sublimation of the dopant has to be taken into account.

Development of multi-element doped graphite and its modification of chemical erosion

A series of B-, Ti- and Si-doped graphite has been recently developed in China as low- Z plasma facing materials (PFMs) for reducing the chemical sputtering (CS) and radiation enhanced sublimation (RES). The erosion experiment indicates that the CS yield of doped graphite at 1 keV and 50 eV D + bombardment was decreased by factor of 5 and 20–30%, respectively, in comparison with that of pure graphite. Raman spectrum analysis and scanning electron microscope (SEM) observation showed that the doped graphite was more stable under H + bombardment.

Erosion and surface morphology of graphite materials under high flux beam irradiation

Surface morphology and its relation to the erosion mechanism of graphite materials (pyrolytic graphite (PG) and isotropic graphite) irradiated by high flux beams were studied. The irradiation was made by a 5 keV Ar beam with a maximum flux of 1.3 × 10 21 Ar/m 2 s with irradiation angles and fluences, and sample temperatures as experimental parameters. In the case of PG, the surface morphology was different between irradiation at RT (physical sputtering) and at elevated temperatures (1980 K, radiation enhanced sublimation (RES)). In addition, irradiation at oblique incidence (60°) tended to flatten the surface at RT, while at 1980 K the surface roughness developed to some extent. No clear flux dependence of the surface morphology was observed at 1980 K as long as the mean eroded depth was similar, though the RES yield was flux-dependent (decrease with flux). In the case of isotropic graphite, the distinctive surface morphology observed for PG was not found.

Particle flux and temperature dependence of carbon impurity production from an inertially-cooled limiter in Tore Supra

A visible endoscope system and an infrared camera system have been used to study the flux of carbon from an inertially-cooled graphite limiter in Tore Supra. From the variation in the carbon flux with plasma parameters, new data have been obtained, describing the dependence of radiation enhanced sublimation (RES) and chemical sputtering on incident ion flux. Other characteristics of RES under plasma operation conditions have also been studied. The dependence of RES on incident deuterium particle flux density is found to be in reasonable agreement with the expected particle flux scaling over a range of particle fluxes varying by a factor of , extending the present scaling to higher flux density values. Chemical sputtering has been observed, but only in regions of the limiter with low incident deuterium fluxes. Values inferred for the chemical sputtering yield are similar to those measured with a temperature-controlled test limiter in Textor.

Erosion Issues for Long-Pulse Operation of a Volumetric Neutron Source

The purpose of a volumetric neutron source is the development and testing of the nuclear components of a fusion reactor. The main issue in this case is very long pulse operation, such as 2 weeks at a time, to elicit the nuclear effects to be studied. Operation at this pulse length will cause extreme erosion if the edge plasma cannot be tailored appropriately. Typical erosion rates that can be expected at some of the plasma-facing components such as the divertor target or the divertor baffles, without specifying a particular type of device, are analyzed. Accurate predictions of erosion and redeposition require not only knowledge of the erosion mechanism but also detailed knowledge of the plasma parameters, plasma flows, and their spatial distributions, as well as temperature distributions of plasma-facing components and other parameters. It is, therefore, a very difficult task to predict erosion/redeposition rates and patterns for future machines. Nevertheless, some estimate is needed of expected erosion rates, crude as they may be, so future machines for long-pulse operation can be designed. For that purpose, physical sputtering is examined only as a basis for erosion estimates and does not take into account the important processes of chemical sputtering and radiation-enhanced sublimation or the complicated redeposition processes. Even with this simplified approach, one can grasp the order of magnitude of erosion rates that will be encountered when a plasma device is operated for long pulses and at high-duty cycles.

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.

Impurity release and deposition processes close to limiter surfaces in TEXTOR-94

Measurements on the formation of hydrocarbons on plasma exposed surfaces performed by mass- and optical emission spectroscopy in TEXTOR is reported. The temperature dependence of hydrocarbon formation and the contribution of the hydrocarbon source to the CII ion densities near the limiter has been observed using a graphite limiter which is externally heatable up to 1400 K. It has been found that hydrocarbon formation occurs in a broad temperature region decreasing only for target temperatures above 1300 K and that hydrocarbons contribute to about 40% to the CII light. Strong methane release has been observed on copper and stainless steel limiters positioned at the LCFS while it is negligible on molybdenum and tungsten limiters under similar plasma edge conditions. Local transport and redeposition of molecules have been studied by gas injection of methane and silane through holes in the limiter surfaces and by local Monte Carlo calculations. Local deposition efficiencies between 4 and 7% have been measured for injected methane and silane. Monte Carlo calculations show, in general, a larger redeposition probability depending only little on local plasma parameters but significantly on the assumptions of the sticking and release properties of redeposited ions and radicals on the surface. For higher surface temperatures possible carbon release by radiation enhanced sublimation (RES) has been investigated. No increase of carbon release could be observed demonstrating that carbon release from RES is negligible under these conditions. Possible reasons for this behavior are discussed.

Impurity release from low- Z materials under light particle bombardment

Recent progress in the understanding of the erosion of low- Z materials under bombardment conditions characteristic of magnetic fusion experiments is reviewed. The role of physical sputtering, chemical sputtering and radiation-enhanced sublimation in tokamaks is considered, and observations are related to laboratory measurements. The role of physical sputtering is largely understood, and tokamak measurements, under conditions where physical sputtering is expected to dominate, can be well predicted, except in the energy range near the sputtering threshold. Chemical erosion and radiation-enhanced sublimation are less-well understood, and predictions of erosion yields under tokamak conditions require assumptions (primarily related to energy and flux density dependence) which do not have a solid experimental basis. Also, only a few quantitative results from tokamaks are available to confirm predictions, and those which are available are not always consistent.

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.

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.