A kinetic model for divertors and pumped limiters

Abstract

In divertors and pumped limiters, plasma flows along magnetic field lines from the scrape-off layer to a material wall where it is neutralized. The neutrals refluxing from the wall will undergo ionization and charge exchange, producing cold ions. This problem has been examined by using a largely analytical, collisionless kinetic model which includes the effects of charge exchange. Cases with uniform magnetic field and with magnetic fields possessing a constriction (as in a bundle divertor) are treated. The cold ions increase the electric potential over that which otherwise would exist. Solutions are found with a peak in the potential near the plate, with cold ions flowing down the potential gradient either toward the scrape-off zone or towards the wall, depending on where they are produced. This is in agreement with the predictions of fluid theories and numerical kinetic calculations. The net effect is to reduce the heat flux in hot ions to the divertor plate, desirable for reducing sputtering loss. This analytical approach can be used to produce computationally efficient routines to determine particle and energy losses from the scrape-off layer which are suitable for inclusion in transport codes.