Implementation into DIVIMP of a drift-kinetic model derived from the Fokker-Planck equation to examine the parallel-to-B velocity component of impurity ions in divertor-tokamak plasmas

Dirk Reiser's drift-kinetic (DK) model is compared to the existing fluid approximation (FA) model of the DIVIMP (DIVertor IMPurity) algorithm. The DK model provides a more general equation of impurity ion transport by accounting for the decrease in Coulomb collision cross-sections with increasing relative speed of approaching ions. The FA model is shown to be an approximation of the DK model at low relative speeds. Model comparisons are conducted for carbon ions in strong and weak collisional plasmas using (1) linear temperature and velocity gradients and (2) the Scrape-Off-Layer (SOL) options 12 and 13 of DIVIMP. The steep gradients of weak collisionality were discovered to invalidate the DK model. For strong collisionality with impurities initially at rest both models show no divertor leakage, however, when initial impurity velocities are high the FA model overestimates the Coulomb effects of friction and velocity diffusion and again prevents any upstream leakage of impurities.