  Measurements of Cosmic Microwave Background (CMB) anisotropy have served as
the best experimental probe of the early universe to date. The inflationary
paradigm, inspired in part by the extreme isotropy of the CMB, is now a
cornerstone in modern cosmology. Inflation has passed a series of rigorous
experimental tests, but we still do not understand the physical mechanism or
energy scale behind inflation. A general prediction of inflation and one that
can provide certain insights into inflationary physics is a background of
primordial gravitational waves. These perturbations leave a distinct signature
in the CMB B-modes of polarization. The EPIC (Experimental Probe of
Inflationary Cosmology) study team has investigated several CMB polarization
mission concepts to carry out a definitive measurement of the inflationary
B-mode polarization spectrum. In this report we study a mission with an
aperture intermediate between the two missions discussed in our previous
report. EPIC-IM's increased aperture allows access to a broader science case
than the small EPIC-Low Cost mission. In addition to the search for
inflationary gravitational waves, the increase aperture allows us to mine the
scale polarization and lensing shear polarization signals down to cosmological
limits, so that we extract virtually all the cosmological information available
from the CMB. In addition, a modest number of channels operating at higher
frequencies allows for an all-sky measurement of polarized Galactic dust, which
will provide a rich dataset for Galactic science related to magnetic fields.
Using a combination of a large sensitivity focal plane with a new optical
design, and an efficient 4K mechanical cooler, EPIC-IM realizes higher
sensitivity than EPIC-Comprehensive Science mission.
