Fusion Science and Technology / Volume 58 / Number 1 / July 2010 / Pages 91-102
Chapter 3. Confinement and Transport / Special Issue on Large Helical Device (LHD)
Impurity transport has been studied in the Large Helical Device (LHD) with different diagnostic approaches based on an active method that combine carbon pellet injection with visible bremsstrahlung measurement and three passive methods for radial profile measurements of Ar and Fe K X-ray lines, Zeff, and extreme ultraviolet (EUV, 500 Å) impurity line emissions, in addition to usual passive spectroscopy. The existence of an inward convective velocity is confirmed in the edge region ( > 0.6) using the active method, whereas no convection is required in the core region ( < 0.6). The electron density dependence is weak for the diffusion coefficient (typically D = 0.15 to 0.25 m2 /s) for densities of 1 to 5 × 1013 cm-3 but is strong for the inward convective velocity, which varies in the range of V(a) = -0.2 to -1.5 m/s. The inward V in helium plasmas (-0.4 m/s at = 0.8 and the central density, ne [approximately] 4.0 × 1013 cm-3) is nearly half that in hydrogen plasmas (-0.7 m/s). This difference suggests a charge state dependence of fuel ions predicted by the neoclassical theory. Radial profiles of impurity transport coefficients of argon and iron have been studied using spatially resolved soft X-ray pulse-height analyzers. The impurity transport has also been studied in extremely high density discharges achieved by H2 pellet injection based on the passive spectroscopy and Zeff profile measurement. A flat Zeff profile is obtained at ne = 2.5 × 1014 cm-3 with values of 1.1 Zeff 1.2, suggesting no existence of impurity accumulation and radially constant impurity partial pressure. Finally, radial profiles of impurity lines in the EUV range are analyzed with the transport coefficients.