Relationship between fuelling profile and structure and structure formation in boundary region of fusion plasmas

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The objective of this research is to understand relationship between fuelling profile and spatial structure of edge pedestal (i.e. its width and gradient) which is self-formed in the boundary region of fusion plasmas. A new fuelling device gas-jet has been installed on JT-60U in collaboration with CEA-Cadarache, France. Gas jet can inject supersonic gas into plasmas and it has intermediate fuelling profile between those of gas-puffing and pellet injector which are so far used in JT-60U. It was critical issue for the installation to avoid damage of the vacuum seal placed in the injector head owing to high baking temperature for the JT-60U vacuum vessel (〜300 degree C). By applying high heat resistant material, high feeding gas pressure during the baking to reduce the stresses on the seal and reduction of baking temperature to 〜80 degree C, gas jet was successfully injected in the large tokamak plasma for the first time. Density jumps were observed with the background gas pressure of 4-6 bar and the frequency of 4-10 Hz. Although light from the gas jet was almost emitted outside the separatrix, quick decreases in the ion temperature were observed even at a normalized minor radius of 〜0.8. The gas jet could directory affect the plasma parameters at this point. Inside this point, cold pulse propagated into the central region. The pedestal electron pressure determined by the edge pedestal structure was kept constant, because the temperature was decreased with increasing the density. Enhancement of the edge pressure was not observed with gas-jet, while it was observed with high-field-side pellet injections. The pedestal ion temperature was also decreased with the gas-jet. Confinement degraded when the density was increased with the gas-jet. In future, it is important to investigate parameter dependence of the pedestal structure in the wide experimental ranges for background gas pressure and frequency.