Erosion

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Based on the articles on “Particle Induced Erosion of Be, C and W in Fusion Plasmas” [1] [2]

Contents

Motivation and Scope

The term 'erosion' in itself, encompasses processes such as physical sputtering, chemical reactions leading to the formation of volatile particles, radiation enhanced sublimation (which occurs for carbon-based materials), and thermal sublimation. The major deleterious effects of erosion include: a reduced lifetime for the plasma-facing material, contamination of the fusion plasma, and tritium uptake due to codeposition of eroded material with the hydrogen fuel.

Basic Features of Particle-Induced Erosion Processes

Physical sputtering occurs via collisional interactions between impacting projectile atoms and target atoms, leading to the ejection of some of the target atoms. This process occurs for all materials for incident particle energies above a certain threshold, which is characteristic of the target-projectile combination; the physical sputtering yield is not a function of temperature.

The occurrence of chemical erosion depends on the projectile-target combination and its mutual reactivity. Chemical erosion can occur at all incident particle energies. For example, in the case of carbon, H impact leads to the formation of hydrocarbons, with yields peaking in the 500-1000 K temperature range: reactions occur even at sub-eV impact energies, with no evidence of an energy threshold. At present, the mechanisms associated with chemical erosion of carbon due to hydrogen impact are not fully understood. Recent modelling advances, however, provided new insights in to the complex physical/chemical interactions. Oxygen impact on carbon produces CO2 and CO. Combined H and O also leads to the formation of some water. For Be and W, chemical erosion is also possible e.g., O impact on W produces a variety of tungsten oxides, WxOy. The chemical erosion of carbon and carbon-based materials are presented in the APID Vol. 7A [1] and stored in the IAEA numerical database ALADDIN.

Radiation-enhanced Sublimation (RES) has only been observed in carbon based materials and is induced by energetic particle impact at temperatures above ~ 1200 K. The present understanding of RES is based on the formation of interstitial-vacancy pairs in the implantation zone by energetic incident atoms (chemically inert or otherwise). At sufficiently high temperatures the interstitial C atoms diffuse to the surface, and subsequently leave the surface with ‘thermal’ energy. This model of RES agrees well with experimental observations, with the exception of flux dependence predictions. The model predicts a decrease of RES yield with increasing incident particle flux to the power of (-0.25). Experimental results generally show a power of ~ (-0.1). Since RES results from atom displacements, this process (like physical sputtering) only occurs above an incident particle energy threshold.


Figure 1. Erosion yields due to hydrogen and deuterium impact of carbon are presented at various energies, illustrating the characteristics of physical sputtering, radiation-enhanced sublimation and chemical erosion


The contribution of these erosion processes to the total erosion yield depends on both target and projectile characteristics. Fig. 1. shows the relative role of physical sputtering, chemical erosion and RES for hydrogen and deuterium impact on carbon for different H and D energies, as a function of carbon temperature. For both H and D, we note that physical sputtering yields are only applicable for energies above the sputtering threshold (~ 40 eV for H and ~ 33 eV for D). Physical sputtering is a function of energy, and the yield for 1000 eV is higher than those at 100 eV. Chemical erosion becomes significant for temperatures below ~ 1000 K, with the chemical erosion yield being dependent on both target temperature and projectile energy. The chemical erosion yield versus temperature curves are characterized by a maximum. The monotonically increasing yield for temperatures above ~ 1000 K for the 100 and 1000 eV cases is due to radiation-enhanced sublimation. For the sub-eV and 10 eV cases only chemical erosion occurs, as these energies are below the thresholds for both physical sputtering and RES.

Erosion Data Derived from Tokamaks

The task associated with the derivation of erosion data for fusion applications is extremely difficult due to the complexity of the plasma-materials interaction processes and the structure of the tokamak devices themselves. Determination of erosion yields requires spatially resolved measurements of incident particle fluxes (and particle energies) and the fluxes of the released particles, formed via the various erosion processes.

To obtain and compare erosion data, knowledge of the conditions under which the data have been generated is required. In laboratory experiments, using ion or atom beams, control over the impacting particles, in terms of both fluxes and energies is sufficiently good. Determination of the eroded particle fluxes is also well in hand since recycling and redeposition do not occur. Thus consistent results have been obtained in different laboratories. In ion-beam experiments in low energies, especially in the sub-100 eV range, some control is lost over the exact energy due to the possible formation of energetic neutrals via charge exchange. To overcome the flux limitations of ion-beam experiments, erosion studies are also being performed in laboratory plasma devices, with fusion relevant fluxes and energies. The complexity of fusion devices makes it even more difficult to measure erosion yields in tokamaks. Indirect methods are used to determine the flux densities and energies of the impacting and eroded particles. The most advanced method is emission spectroscopy in the plasma edge. However this method is plagued with interpretation difficulties.

Another in situ tokamak technique, especially used for the determination of chemical erosion, is the sniffer probe in TEXTOR. This system acts as a small pump limiter in the scrape-off layer where a plasma column impinges on a heatable graphite strip. Detection is via partial pressure rise of the residual gas in the probe cavity. Uncertainties in the determination of yields arise from: changing surface conditions due to redeposition; additional reaction products due to reflected hydrogen atoms at nearby walls; and the interaction of the plasma column with the residual gas. Taking these uncertainties into account, the measured yields should be an upper limit of real value.

Available data

Chemical Erosion data of Carbon-Based Materials

Available with fitting coefficients and the analytic functions in APID Volume 7 [1]

Comprehensive Set of Chemical Erosion Data from Various Laboratories

Available at the IAEA ALADDIN database

Physical Sputtering of Elemental Targets and Compounds

Available at the IAEA ALADDIN database

Energy dependence of physical sputtering at normal incidence

  • Be
  • C
  • W
  • Compounds

Angular dependence of physical sputtering

  • Be
  • C
  • W
  • Compounds

Radiation-Enhanced Sublimation

Available at the IAEA ALADDIN database

  • Temperature dependence
  • Energy dependence
  • Angle dependence
  • Flux dependence

References

  1. 1.0 1.1 1.2 A. A. Haasz, J. A. Stephens, E. Vietzke, W. Eckstein, J. W. Davis and Y. Hirooka Chemical Erosion of Carbon-Based Materials, Atomic and Plasma-Material Interaction Data for Fusion, Vol. 7 Part A (2000)
  2. W. Eckstein, J. A. Stephens, R. E. H. Clark, J. W. Davis, A. A. Haasz, E. Vietzke and Y. Hirooka Physical Sputtering and Radiation-Enhanced Sublimation , Atomic and Plasma-Material Interaction Data for Fusion, Vol. 7 Part B (2000)
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