Multi-staged ERO2.0 simulation of material erosion and deposition in recessed ITER mirror assemblies

In reactor class devices, with long pulse, high fluence plasma operation and with diagnostic systems that cannot easily be maintained, a critical issue is the deposition/erosion of wall materials and its impact on key optical diagnostic components - mirrors. A workflow for recessed volume simulation was developed for the three-dimensional Monte-Carlo code ERO2.0 (Romazanov, 2020), consisting of multiple simulation stages starting from first wall erosion to more focussed volumes around the key components. This complex workflow has been found to be mandatory to have meaningful statistics of interaction processes on the mirror because the physical fluxes of particles into recessed volumes are extremely low compared to first wall fluxes, leading to unsatisfying statistics in the case of standard Monte-Carlo simulations. The workflow is introduced with the example of a diagnostic mirror assembly in equatorial port 12 behind panels 14 and 15 in the diagnostic first wall in ITER, assuming a beryllium first wall of the main vessel, a stainless steel mirror unit and two molybdenum mirrors embedded in a port plug for neutron shielding. The simulation case corresponds to an H-mode burning plasma case from SOLPS modelling, for which the incoming flux distributions of deuterium neutrals into the recessed volume were simulated by the neutrals transport code EIRENE.