Multi-scale simulation of gas density distribution in a fusion-relevant particle accelerator

The accurate determination of the gas density distribution in ion accelerators plays a substantial role for the protection of the com ponents 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 electrode s. SPIDER, a full-scale ITER beam source and extractor test facility, is studied considering the large vacuum vessel (4 m diameter), the pumping system, the plasma source (hydroge n gas filling pressure of ~0.3Pa) and the presence 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 ~12mm) is accurately modeled. In these applications, the gas-surface interaction is found to play an important role, and is therefore studied with de dicated models (on the nano-scale) to tune the parameters of the large scale simulation.