Monte Carlo modelling of impurity ion transport for a limiter source/sink

In relating the impurity influx Φ10 [atoms per second] into a plasma from the edge to the central impurity ion density n1(0) [ions-m~3], it is necessary to know the value of τ1SOL, the average dwell time of impurity ions in the scrape-off layer. It is usually assumed that τ1SOL = Lc/cs, the hydrogenic dwell time, where Lc is the limiter connection length and cs is the hydrogenic ion acoustic speed. Monte Carlo ion transport results are reported here which show that, for a wall (uniform) influx, τ1SOL is longer than Lc/cs, while for a limiter influx it is shorter. Thus for a limiter influx n1(0) is predicted to be smaller than the reference value. Impurities released from the limiter form ever large 'clouds' of successively higher ionization stages. These are reproduced by the Monte Carlo code as are the cloud shapes for a localized impurity injection far from the limiter.