Conductance modelling of ITER vacuum systems

This paper presents the full network model of the ITER torus vacuum system including the toroidal and radial conductances inside the divertor cassettes, the by-pass leaks between and inside the cassettes, the conductances between the divertor cassettes and the vacuum vessel and inside the pumping ducts up to the cryopumps. The model was built in ITERVAC which is a program for smooth simulation of the mass flow through ducts at isothermal conditions in a wide flow regime including the difficult to describe transitional flow regime that is most relevant to the application for ITER. Simulations were performed for the burn and dwell mode of ITER; deuterium was used as model gas within the simulations. The results show for the burn phase (with four torus cryopumps in pumping mode) a strong back streaming of gas into the plasma which can be 30% higher than the actual throughput to the torus cryopumps. For the dwell phase the effect of pumping with four and eight torus pumps was investigated, respectively. It is shown in this paper that the throughput can be increased between 30% (end of dwell) and 60% (start of dwell) by using eight pumps.