AVS 46th International Symposium
    Vacuum Technology Division Tuesday Sessions
       Session VT-TuP

Paper VT-TuP6
A Unique Cryogenic Pumping System for Space Simulation Chambers

Tuesday, October 26, 1999, 5:30 pm, Room 4C

Session: Poster Session
Presenter: A.D. Ketsdever, Air Force Research Laboratory
Authors: A.D. Ketsdever, Air Force Research Laboratory
F.M. Lutfy, University of Southern California
E.P. Muntz, University of Southern California
Correspondent: Click to Email
To investigate the behavior of spacecraft interactions with the space environment and their own ambient atmosphere (caused by material outgassing or propulsive maneuvering), extremely low chamber background pressures and low backscattering rates from chamber walls are required. For meaningful spacecraft-thruster interaction studies, large pumping rates are also required to maintain background pressures at acceptable levels with high propellant flow rates. For the pumping rates required by some applications, cryogenic pumping appears to be the only adequate choice. In order to have a manageable chamber size, a unique geometrical configuration for the cryogenic pumping system was developed. The pumping system consists of outer liquid nitrogen panels which reduces the heat transfer from the outer chamber walls (300 K) to the inner gaseous helium panels (20 K). The inner cryogenic arrays consist of many radial fins which serve several purposes. First, the fin arrangement increases the available pumping surface area by an order of magnitude over a simple cylindrical geometry. Second, there are gaps between the radial fins which allow heavy ions and radiant heat to impact the graphite covered liquid nitrogen panels. In this way, sputtering from energetic ions is reduced and the liquid nitrogen panels remove most of the heat generated by the thruster. Finally, the fins reduce the solid angle for backscattering from non-condensing chamber surfaces to a particular area of interest. With this configuration, a total chamber pumping scheme has been adopted which uses all of the interior chamber surfaces to condense gases thus reducing backscattered molecules as much as possible.