Modelling of deposition of hydrocarbon films underneath the divertor and in the pumping ducts of ITER

This paper presents the results of a modelling study conducted to estimate the deposition of hydrocarbon radicals in the regions of the ITER divertor hidden from the plasma (e.g., underneath the private flux region dome and in the pumping ducts). A Monte-Carlo code based on the approach described in Ref. [M. Mayer et al., J. Nucl. Mater. 313–316 (2003) 429] has been developed and applied for the actual ITER geometry. It allows for a full 3-dimensional treatment of particle trajectories and follows individual hydrocarbon particles until they either stick on the surfaces or leave the system. The cases analysed clarify the influence of several parameters such as the sticking coefficient of the hydrocarbon species, the pressure of the background gas, and the geometry. Consistent with experimental findings from tokamaks, this assessment shows that particles with relatively high sticking probability (>0.01) deposit primarily underneath the divertor structure, and only species with very small sticking probability (⩽10−3) may reach and stick along the inner surfaces of the pumping ducts.