Design process for the new RFX-mod2 DNBI duct

The following technical note describes the process that led to the new and final duct design for the RFX-mod2 DNBI, taking into account the necessity to transition from open-cycle main cryopumps to closed-cycle ones with lower pumping speed. A first approximation of the gas pressure just downstream the 4th acceleration grid was obtained by leveraging the similarity between lumped electrical nets and vacuum systems. Subsequently, the resulting data have been used into Molflow+, a vacuum simulation Monte Carlo code, in order to get an estimate of the pressure and density profiles along the DNBI main axis. In the meanwhile, another code was used to evaluate the beam cross-section power profile at different position along the beam line. Altogether, results from the Molflow+ and beam cross-section profile simulations, have been used to run an optimization code, based on a lumped elements model, which produced parameters to correctly dimension a duct configuration for maximum output current. Finally, the validity of the results provided by the optimization code have been put to the test with another Monte Carlo code, capable of simulating trajectories and species switching of the single particles that compose the beam. Though the last simulation code was not able to reproduce the experimental results from past experiments, probably due to some of them being influenced by a substantial amount of outgassing coming from the duct, the accordance between its results and the ones from the lumped model optimization step has been determined to be high enough to proceed to the actual design phase that, lastly, produced a proper CAD model for the new duct.