Sputtering of Pure Elements

2 Physical Sputtering 9.2.1 Sputtering of Pure Elements Kinetic Effects 

For light ions incident on heavy materials, the threshold energy, Eth, is determined by the energy which can be transferred to target atoms and can be analytically approximated by [6] 

For Be and W, experimental data and the fit for the sputtering yield at normal incidence are shown in Fig. 9.f for D ions as a function of incident particle energy. Physical sputtering data are available for both materials from energies close to the threshold energy (9eV for D on Be and 200 eV for D on 

W) up to the high keV range and have been collected in data summaries [5, 10,11], The data shown in Fig. 9.1 are taken from [5] very similar data exist for H and He ion bombardment. All experimental data shown were obtained from the weight loss of polished samples after ion bombardment. In the case of Be, clean surfaces could only be obtained at temperatures above 900 K where Be diffuses through the surface oxide layer [12], while at lower temperatures, oxidation from the residual gas reduced the erosion yield [5]. 

Additionally, the incident impurity ions in a fusion device will be multiply charged, e.g., a charge state of 4 can be assumed for Be, C, O, and even higher values for W ions. This will result in increased acceleration of the ions in the sheath potential such that the most probable energies for multiply charged ions in a divertor plasma with Te = 10 eV will be around 200 eV, i.e., well above the threshold energy. 

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