Energy-controlled and mass-selected beam of fluorocarbon ion (CF3+,CF2+) at energies from 50 to 400 eV are directed at Si and SiO2 surfaces, in order to elucidate etch mechanisms and SiO2/Si etch selectivity. During the ion beam injection, the time evolution of neutral radicals desorbed from the etching surface is measured by appearance mass spectrometry. At the same time, in situ surface analysis is also carried out by x-ray photoelectron spectroscopy. The etching rate of a clean Si surface by CF3+ injection is initially high but decreases with time due to the formation of a thin surface layer containing F and C atoms. Finally, a few-nanometers-thick stationary SixCyFz overlayer is formed on the Si surface for ion fluence >3×1016 cm−2, along with saturation of CF2 and SiF2 radical desorption from the surface. Comparison is made with molecular dynamics simulations of CF3+ interactions with Si surfaces. CF2+ injection on the Si surface yields rapid formation of the SixCyFz layer and lower desorption of the CF2 radical compared with the CF3+ injection case. On the other hand, SiO2 etching by CF3+ takes place at an energy above 50 eV. During steady-state etching, a SixOyFz overlayer is formed with C atoms hardly observed. The measured desorption rates of etch products (CO and SiF2) show the same energy dependence of SiO2 etching yield per ion. CF2 desorption during the SiO2 etching is very low compared with Si etching.

Since a high performance operation is needed in the future fusion reactor, it is expected that the operation with Internal Transport Barrier (ITB) is utilized to improve core plasma confinement in the reversed magnetic shear. In this study, firstly, the simulation results are compared with the experimental result, and the validity of the model is checked. Secondly, the effects of the magnetic shear and the pellet injection on ITB formation are simulated in tokamak reactor using the Toroidal Transport Analysis Linkage code TOTAL. In this simulation, the ITB formation is confirmed in the condition that magnetic shear is nearly zero and the density gradient exists, and it found that the reversed magnetic shear configuration is effective in the operation with ITB.

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The operation with Internal Transport Barrier (ITB) is expected as a high performance operation. ITB is utilized to improve core plasma confinement in the reversed magnetic shear. It is considered that the changes of core plasma profile by the ITB cause changes of impurity transport. In a large fusion reactor, high-Z materials will be used as plasma facing components because high loads of heat and particles concentrate there. However, high-Z impurities from these components cause large radiation loss and dilute the fuel even if the amount of impurities is small. Therefore, in this study, firstly, the ITB formation which includes the effects of the magnetic shear and perturbed profiles by the pellet injection was simulated using the Toroidal Transport Analysis Linkage code TOTAL. Secondly, we analyzed transport of the tungsten impurities using an impurity model in TOTAL code, and compared the impurity profile in the case with ITB to the one without ITB in the tokamak reactor. The impurities decreased in the ITB formation region when ITB was formed, and the outward flux of total impurity density was observed there. It can be expected that outward flux of impurities is generated by the temperature and the density gradients.

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