Hydrocarbon Redeposition on Plasma Facing Walls Intersecting Magnetic Field at Shallow Angles

Abstract

In present nuclear fusion devices, hydrocarbons resulting from the chemical sputtering of carbon-based walls redeposit on other areas of the wall after transport in plasmas, forming hydrogen-rich carbon layers. A particle-in-cell calculation of a sheath region between the plasma and the wall is incorporated into the transport simulation of methane (CH4) and the fragments in the plasma. The effect of the magnetic field intersecting the wall surface at shallow angles on the redeposition characteristics is studied, taking the reflection and sticking on the wall into account. The redeposition rate is rather slowly increased with increasing angle between the magnetic field line and the surface normal, whereas it strongly depends on the plasma temperature and the sticking probability, S, of hydrocarbons (CHx) returning to the surface. By assuming S = 1, the redeposition of large molecular ions (CH3+ and CH4+) is suppressed at shallow angles (θ> 85°), whereas the redeposition of atomic C ions is enhanced. For zero sticking (S = 0), the redeposition is dominated by C ions at high temperature, whereas at low temperature, it is dominated by neutral C atoms.