The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a test fluid which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Results for typical impurities (O, Fe, He, etc.) show that collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong target recycling), causing impurity flow reversal and subsequent accumulation outside the recycling region. The criterion for flow reversal is roughly M <λi/λT where λi is the hydrogen ion mean free path and λT is the temperature gradient length. Self-sputtering at the target plates is calculated, showing the importance of fractional impurity acceleration in addition to the charge-statedependent electrostatic energy gain.
Observation of impurity accumulation and its compatibility with high plasma performance in W7-X