Measurement of fractal dimension of plasma facing wall

The paper described the interaction between carbon materials and the marginal plasma. Especially, the structural change of sample surface by sputtering and re-adsorption of material components was tried to evaluate by use of a fractal dimension. When D is defined as the fractal dimension of sample surface, the relation μ∝σ-D/2 is set up, where σ is molar number of gas occupied monolayer on the sample surface and μ molecular cross section of gas. Ethanol and heptane were used as adsorbed gas. The volume of micro pore and the amount of adsorbed gas on the surface area were determined by various plasma irradiation times. The change of fractal dimension of carbon materials to be used in the tokamak fusion reactor was studied under the DC plasma at the hydrogen pressure 500 Pa. The following results were obtained: (1) The fractal dimension was about 2.5 to be determined by the adsorbed amount of gas at one point of BET method. (2) The fractal dimension had a tendency to increase with plasma irradiation time and attained to about 3 after 50 hour. (3) The volume of micro pore was not changed with the plasma irradiation. (S.Y.)