Plasma dynamics and collisional magnetic presheath structure in unmagnetized divertor plasma in fusion devices

Abstract

A model of strongly ionized plasma dynamics in a slightly inclined magnetic field with ions, unmagnetized with respect to ion–ion collisions, typical for detached regimes, is presented. It is demonstrated that far from the material surface, the parallel dynamics remains almost the same as in the magnetized plasma. In the wall vicinity, parallel plasma motion is transformed into perpendicular classical diffusion originating from electron–ion collisions, forming a diffusive layer. At a distance of few ion gyroradii from the material surface, a collisional magnetic presheath is formed, where ions move toward the surface with the velocity of the order of the sound speed, in spite of the fact that an ion exhibits many collisions before reaching the surface. For typical inclination angles of magnetic field of the order of few degrees, the electrostatic potential in the diffusive layer and in the presheath deviates considerably from the Boltzmann potential for electrons, which is typical for collisionless magnetic presheath. Moreover, the normal electric field in some regions is directed away from the surface, which is important for impurities to interact with the surface.