An Analysis of Gas Flow in the BaBar Drift Chamber

An analysis based on the Green’s function for the diffusion equation indicates that the BaBar drift chamber could be purged of air down to 1 ppm in 8 hours with a purge gas velocity in the chamber of 0.043 cm/s, corresponding to one volume change every 2 hours. Anecdotal evidence from CLEO and ALEPH supports this claim. • It is proposed that the gas be fed into the chamber through 64 holes of 1/8′′ ID on the rear endplate at the lattice points of the electronics card-cage superstructure, and the gas be taken out through 64 corresponding holes in the front endplate. The average flow velocity in these tubes during purging would be 180 cm/s. • An analysis based on Landau’s jet solution to the Navier-Stokes equation indicates that the gas flow near the endplate between feed holes will be sufficient to insure good purging there. • Pressure drops in the gas feed system are the order of 0.1′′ of water. • The leakage of chamber gas out of (and air into) a feedthrough hole during a 5-minute wire repair would be only 20 cm on the rear endplate, and 130 cm on the front endplate. • The endplates will be surrounded by plena flushed with dry nitrogen at a flow rate of about one volume change per hour.