  The search for the polarized imprint of primordial gravitational waves in the
cosmic microwave background (CMB) as direct evidence of cosmic inflation
requires exquisite sensitivity and control over systematics. The
next-generation CMB-S4 project intends to improve upon current-generation
experiments by deploying a significantly greater number of highly-sensitive
detectors, combined with refined instrument components based on designs from
field-proven instruments. The Precursor Small Aperture Telescope (PreSAT) is
envisioned as an early step to this next generation, which will test prototype
CMB-S4 components and technologies within an existing BICEP Array receiver,
with the aim of enabling full-stack laboratory testing and early risk
retirement, along with direct correlation of laboratory component-level
performance measurements with deployed system performance. The instrument will
utilize new 95/155GHz dichroic dual-linear-polarization prototype detectors
developed for CMB-S4, cooled to 100mK via the installation of an adiabatic
demagnetization refrigerator, along with a prototype readout chain and
prototype optics manufactured with wide-band anti-reflection coatings. The
experience gained by integrating, deploying, and calibrating PreSAT will also
help inform planning for CMB-S4 small aperture telescope commissioning,
calibration, and operations well in advance of the fabrication of CMB-S4
production hardware.
