Pressure distribution simulation for SNS ring vacuum systems

Brookhaven is responsible for the design and construction of the US Spallation Neutron Source (SNS) accumulator ring and beam transport lines. Ultrahigh vacuum of a few nano-Torr is required for the accumulator ring to minimize the residual gas ionization. To size the pumps and to optimize the pump locations, the pressure distribution in the vacuum systems has to be calculated based on chamber conductance, the static and dynamic outgassing of the chamber walls and the beam components. A computer program, based on finite differential approximation, was used to model the pressure in the accumulator ring, using the measured outgassing rates of chambers with and without TiN coating. The simulation results indicate that the designed vacuum will be achieved after prolonged pumping due to the high outgassing associated with the low secondary electron yield coatings.