Frank,  after writing thae last email I checked my code again.  During the gap in
the bunch train my code does not create
any primary electrons.   At a fixed location in the pipe the primary electron
creation rate is constant at 3.e16 electrons
per meter per second  for t< 0.5 microseconds and it is zero for t > 0.5
microseconds.  Looking at your curve it might
be that you continue to generate primaries during the bunch gap.    This would
give a non-zero asymptotic value.
You will need to check reflectivities etc. to find what is right for LHC.   Cheers
-- Mike