  The Fermi National Accelerator Laboratory (Fermilab) Linac accepts 750 keV H-
ions from the front end and accelerates them to 400 MeV for injection into the
Booster rapid cycling synchrotron. Day-to-day drifts in the beam longitudinal
trajectory during regular operation are of the order of several degrees. They
are believed to cause additional losses in both the Linac and the Booster and
are addressed by empirically adjusting cavity phases of front end and Linac RF
cavities. This work explores a scheme for expressing these drifts in terms of
phase shifts in the low-energy part of the Linac. Such a description allows for
a simplified visual representation of the drifts, suggests a clear algorithm
for their compensation, and provides a tool for estimating efficiency of such
compensation.
