PBFA-II vacuum system design using a lumped parameter computer model

The PBFA-II vacuum vessel which houses the power flow lines and ion diode presented a difficult engineering problem in analyzing the pressure distribution throughout the chamber. The vessel utilizes a typical construction of stacked lucite and aluminum rings with their associated high outgassing loads. The transmission lines are a series of stacked cones and toroids that form an inter-connected network of annular pathways. Calculating the steady state pressure distribution required solving 30 simultaneous equations, and any transient solution was virtually impossible by hand. This paper describes a computer model developed using the direct analogy between fluid flow parameters and electrical parameters. This model can then be solved as a lumped parameter electrical circuit using the differential network analysis program, SCEPTRE. The overall design of the vacuum system, including the choice of helium cryopumps and water vapor cryopumps to handle the anticipated heavy water vapor load, is also discussed.