Measurements of the Impurity Flow Velocity and Temperature in Deuterium and Hydrogen Plasmas in the Divertor Legs of the Stochastic Layer in LHD

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This paper presents carbon impurity flow velocity and temperature measurements in the divertor region for a wide density range n_e = (1 - 14) ×10¹⁹ m⁻³, central electron temperature T_e = 1.5 - 3.5 keV, and with total neutral beam injection power of 9 - 12 MW. The isotope effect on the transport is studied in hydrogen (H) and deuterium (D) discharges. Flow velocities for D plasma are systematically slower, by the factor of 1.4 - 2, compared to H plasma. For all selected discharges, the carbon ions flow toward the divertor in both H and D plasmas. Different velocities are obtained depending on the charge states. For C⁺ and C²⁺ they are in the range of 10 - 30 km/s, and 5 - 20 km/s for C³⁺ ions. It is also found that there is no change of flow direction even in the lowest density, where the impurity transport model predicts flow toward upstream in the thermal force dominant regime. In H discharges velocities increase proportionally to the plasma density, while in D discharges this dependency is weaker. Possible mechanism to interpret these observations is discussed based on the parallel momentum balance of impurity transport.