Simulation of the gas density distribution in the large vacuum system of a fusion-relevant particle accelerator at different scales

•Gas density in SPIDER (ITER full-size Beam Source) affects negative ion acceleration.•Single beamlet TPMC shows dependence of pressure profiles on velocity distribution and wall scattering.•View Factors method used to calculate Beam Source flow partition among side and front conductance.•Gas density distribution in SPIDER is studied considering previous results, vessel and vacuum system.•Accommodation at wall & scattering: molecular dynamics show dependence on gas type and temperature.

The accurate determination of the gas density distribution in negative ion accelerators plays a substantial role for the protection of the components and the efficiency of the system. The presence of background gas in between the electrodes has the highest impact on the beam properties and on the heat loads on the electrodes. The full-scale ITER beam source and extractor test facility SPIDER is studied considering the large vacuum vessel (4 m diameter), the pumping system, the plasma source (hydrogen gas filling pressure of ∼0.3 Pa) and the geometry of the in-vacuum components. On a smaller scale, the beam source and the multi-aperture electrodes (provided with apertures having an inner diameter of ∼12 mm) is accurately modeled. In these applications, the gas-surface interaction plays an important role and is therefore studied with dedicated models (on the nano-scale) to improve the predictive capability of molecular gas flow simulations.