  This paper introduces a new method for stacking beams in the longitudinal
phase space. It uses RF barriers to confine and compress beams in an
accelerator, provided that the machine momentum acceptance is a few times
larger than the momentum spread of the injected beam. This is the case for the
Fermilab Main Injector. A barrier RF system employing Finemet cores and
high-voltage solid-state switches is under construction. The goal is to double
the number of protons per cycle on the production target for Run2 and NuMI
experiments.
