科研之友
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摘要
Vertical profiles of edge impurity emissions have been measured in upper half region of elliptical plasmas at horizontally elongated plasma cross section in large helical device (LHD). The vertical profiles near upper O-point located just below helical coil are analyzed to study the plasma edge boundary of the ergodic layer consisting of stochastic magnetic field lines with connection lengths of 30 <= L(c) <= 2000 m. As a result, C(3+) ion emitting CIV spectrum is identified as the ion existing in the farthest edge of the ergodic layer. The peak position of CIV (312.4 angstrom: 1s(2)3p (2)P(1/2,3/2)-1s(2)2s (2)S(1/2)) vertical profile does not change at all in a wide temperature range of 150 <= T(e)(rho - 1) <= 400 eV, whereas it moves inside the ergodic layer when T(e)(rho - 1) is reduced below a threshold temperature, e.g., 130 eV at R(ax) - 3.75 m configuration. It is found that the C(3+) ion exists at the boundary between ergodic layer and open magnetic filed layer at which the L(c) distributes in lengths of 5 to 30 m. The result indicates that the edge boundary near the O-point in LHD is determined by a starting point of the open filed layer, where a tokamak-like steeper edge temperature gradient is formed, although the edge boundary is quite obscure at the X-point region. Any plasma does not exist between the edge boundary and the vacuum vessel. The CIV profile at the O-point is simulated using a three-dimensional edge transport code of EMC3-EIRENE in which the magnetic field structure in vacuum is used for the ergodic layer. A clear discrepancy of 8 mm is found in the peak positions of CIV between measurement and simulation for magnetic configurations with thick ergodic layer, i.e., R(ax) - 3.90 m, while only a small discrepancy of 3 mm is observed for those with relatively thin ergodic layer, i.e., R(ax) - 3.75 m. It suggests that the discrepancy is caused by a modification of the magnetic filed due to the presence of plasma pressure.
- 出版日期2011-8
