Hydrogen isotopic effects on the chemical erosion of graphite induced by ion irradiation

Abstract

This theoretical study investigates the dynamic behavior of chemical erosion of graphite due to hydrogen-isotope-ion bombardment. Ion energies range from 10 to 1000 eV and target temperatures range from 300 to 1100 K. The computer code employed was TRIDYN. The chemical erosion processes under investigation included surface-related and thermally activated hydrocarbon emission processes.The proposed simulation model of this study was fitted to actual measurements by implementing surface-related and thermally activated coefficients. In addition, the proposed simulation model of this study was improved over our previous model by incorporating a depth-dependent probability for out-diffusion of hydrocarbons. The local reduction of carbon density due to either physical sputtering or chemical erosion was also taken into consideration. Furthermore, application of the proposed simulation model of this study was extended to include all hydrogen isotope ions (i.e. hydrogen, deuterium, and tritium). It was found that all the calculated, fitted, and measured results are in good agreement. In particular, the results calculated from the proposed simulation model of this study surpass previous ones in the low ion energy region in which chemical erosion is of vital importance.

Steady-state and transient hydrocarbon production in graphite by low energy impact of atomic and molecular deuterium projectiles

“…Redeposition of such hydrocarbon species has serious consequences on the tritium inventory accumulated in future burning-plasma machines. While models have been developed to explain general features of chemical sputtering by energetic hydrogen ions and their isotopes [3][4][5][6][7], many details of the processes underlying chemical sputtering are still incompletely understood. The conversion step from CH 3 radical to CH 4 molecule during room-temperature-target chemical sputtering of graphite by energetic hydrogen ions has not been conclusively identified [8], and, moreover, has not been successfully incorporated into current chemical sputtering MD simulations [9,10].…”

“…The conversion step from CH 3 radical to CH 4 molecule during room-temperature-target chemical sputtering of graphite by energetic hydrogen ions has not been conclusively identified [8], and, moreover, has not been successfully incorporated into current chemical sputtering MD simulations [9,10]. The hydrocarbon release mechanism during room-temperature chemical sputtering is still the subject of discussion, ranging from projectile-induced kinetic ejection [3,9], to purely diffusive release [6,7]. In addition, there are unresolved technical issues leading to differences between various measurement techniques [11] for determination of the total C chemical sputtering yield.…”

Isotope dependence of chemical erosion of carbon

“…Carbon-based divertor tiles are common in many existing fusion machines as well as in the current phase of ITER. Plasmasurface processes which involve interactions of hydrogen isotopes with carbon may play an important role in these machines [1][2][3][4].…”

“…For D (T) projectiles DE C is a factor of 1.72 (2.25) larger than that for H projectiles, which influences the probability for breaking CÀC bonds at a given E P [2]. Estimates for E P > 50 eV predict that the sputtering yield for D impact should be a factor of $3 larger than that for H impact [2,3,23].…”

“…For D (T) projectiles DE C is a factor of 1.72 (2.25) larger than that for H projectiles, which influences the probability for breaking CÀC bonds at a given E P [2]. Estimates for E P > 50 eV predict that the sputtering yield for D impact should be a factor of $3 larger than that for H impact [2,3,23]. At lower impact energies, where chemical sputtering dominates, some models have predicted similar moderate enhancements [2,9] while others foresee a much bigger enhancement [3].…”