Particle-in-cell simulations of heat flux to tokamak divertors in an oblique magnetic field

Abstract

The management of excessive heat flux to tokamak divertors is one of the most important issues, especially for long-pulse operations of EAST. Different simulation codes based mainly on fluid models have been developed to study the physical processes in edge plasma, including the heat flux to divertor target plates. In these simulations, only the neutral region is included and the heat flux is estimated by using energy transmission coefficients based on a simplified sheath model. In this paper, particle-in-cell simulations are conducted to study the heat flux flowing to the EAST divertor target plates by taking the sheath into account. It is found that the parallel electron velocity distribution function (VDF) in the sheath is a cut-off Maxwellian distribution so the electron sheath energy transmission can be obtained analytically, while ion VDFs in the sheath strongly depend on the incident angle between the magnetic field and divertor target plates. For a small incident angle, in the sheath, the ions move along open orbits and are accelerated mainly in the direction perpendicular to the plane formed by the magnetic field and target plates due to the Lorentz force. An analytical expression for the ion heat flux to the target plates is presented and the obtained results are in good agreement with the simulation results.