Does viscous and transient effects, dissociation, heating and surface scattering play a role in the gas density distribution along SPIDER accelerator?

SPIDER is the full-scale beam source prototype of the ITER heating neutral beam. To support the incoming operation, we study for the first time a number of aspects linked to the presence of background gas in the multiaperture accelerator, which may occur in the early phase of SPIDER operation. In high filling pressure oprerations (1Pa), viscous effects will probably play a role. In short pulse operation, transients might introduce variability in the conditions seen by the beam. Dissociation, with the ensuing presence of atomic hydrogen along the extractor and accelerator, constitutes an additional gas target for the ion beam, exhibiting higher stripping probability. Gas heating is possible by indirect effects, due to beam-gas or beam-surface interaction. Finally, gas evacuation from the ion source may be favoured by non-diffuse scattering at surfaces, possible in non-isothermal gas flows. These aspects are sudied by a 3D Direct Simulation Monte Carlo method, recently implemented in the Avocado code. The implementation is validated first by comparison against cases available in bibliography. The described effects are studied by parametric analyses.