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

Abstract

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 τ₁^SOL, the average dwell time of impurity ions in the scrape-off layer. It is usually assumed that τ₁^SOL = L_c/c_s, the hydrogenic dwell time, where L_c is the limiter connection length and c_s is the hydrogenic ion acoustic speed. Monte Carlo ion transport results are reported here which show that, for a wall (uniform) influx, τ₁^SOL is longer than L_c/c_s, 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.