Modeling tungsten and carbon sputtering by carbon at elevated temperatures

Abstract

Abstract. The planned use of carbon strike plates adjacent to tungsten tiles in the ITER divertor will lead to W erosion by sputtered C ions. The W sputtering yield by C decreases with increasing surface concentration of the implanted C. However with increasing temperature, the C self-sputtering yield increases, reducing the rate of C surface coverage. This study attempts to model the temperature dependent C self-sputtering process and its impact on W erosion by using modified surface binding energies (SBEs) in TRIDYN. The SBEs are benchmarked against C self-sputtering yields measured in this study and W irradiation by C is simulated and compared to previous experimental results. Good agreement between simulation and experiments is seen at 670 K, while small deviations are observed at 770 K due to surface roughening. At 870 K, significant deviation is observed which can be attributed to enhanced surface roughening as well as diffusion effects.

An overview of sputtering-related processes occurring at mixed surfaces formed by simultaneous C+ and D+ irradiation of W

“…The dotted line is a least square fit to the experimental data. (b) -average areal density of C + implanted during the steady-state bombardment of W with 6 keV C + (7% C fraction in the total flux) and 3 keV D + as a function of surface temperature; data from [20] and [31]. [35].…”

“…At T < 870 K, the temperature dependence of this process can be modeled in TRIDYN by modifying the surface binding energies (SBEs) used in [31]. It must be stressed that varying the SBE in TRIDYN is merely an approximation to include effects beyond the binary collision model, such as chemical sputtering, radiation enhanced sublimation or surface morphology into the prediction of the experimentally observed carbon erosion [32].…”

“…To benchmark these values, experimental C self-sputtering yields at 670 -870 K were measured by irradiation of C films using 6 keV C + [31]. Next, the experimental sputtering yields were compared to TRIDYN calculations.…”

“…The input SBEs that resulted in the best match between calculated and experimental self sputtering yields were then selected. These benchmarked SBEs were used to model the fluence-dependent W sputtering and C + implantation by 6 keV C + at 670 -870 K and compared to experimental data [31] (see Figure 6). At T  770 K, the TRIDYN model of reducing the carbon SBEs could describe, within 10-20%, the increased W sputtering and decreased C implantation rate with increasing temperature.…”

“…However, since contributions of chemical re-erosion of C also decrease at elevated temperature (as discussed in Section 4), increased C self sputtering was invoked to explain the experimental results in a manner similar to the explanation used for pure C + bombardment [31]. The SBEs obtained by the best match between the TRIDYN simulations and experimental data in [31] were used as input parameters in the simulations of combined C + + D + irradiation of tungsten.…”