  The FNAL accelerator complex is poised to reach MW neutrino beams on target
for the exploration of the dark sector physics and rare physics program spaces.
Future operations of the complex will include CW linac operations at beam
intensities that have not been seen before \cite{PIP2,RCS_LOI}. The ambitious
beam program relies on multi-turn H$^{-}$ injection into the FNAL Booster and
then extracted into delivery rings or the Booster Neutrino Beam (BNB) 8 GeV HEP
program. A new rapid-cycling synchrotron (RCS) will be required to reach the
LBNF goal of 2.4 MW because of intense space-charge limitations. There are many
accelerator engineering challenges that are already known and many that will be
discovered. This proposal calls for an intermediate step that will both
facilitate the operation of Booster in the PIP-II era and gain operational
experience associated with high power injection rings. This step includes the
design, construction and installation of a 0.8 GeV accumulator ring
(upgradeable to 1+ GeV) to be located in the PIP-II Booster Transfer Line
(BTL). The PIP-II accumulator ring (PAR) may be primarily designed around
permanent magnets or use standard iron core magnet technology with an aperture
selected to accommodate the desired high intensity protons at 0.8 GeV.
