The coupling of impurity ions with scrape-off layer plasma ions

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Abstract
One-dimensional analysis of momentum transport equations of electrons, plasma ions, and one impurity charge state shows that both the nonambipolar current and the thermal forces can influence the coupling of impurity and plasma ions. The solution indicates that, in general, a steady-state drift can exist between the impurity and plasma ions. In cases where thermal forces are negligible, the impurity ions are strongly coupled to the plasma ions only if the transport is ambipolar. In this case the plasma ions and the impurities move as a single fluid. However, if nonambipolar currents are present, a steady-state drift is possible between the plasma ions and the impurity ions even if thermal forces are neglected. In this case, the imbalance due to the electron frictional force (∼Z2) acting on the impurity charge state and the plasma ions is balanced by the impurity and plasma ion friction.