Design and development of an ambient-temperature continuously-rotating
  achromatic half-wave plate for CMB polarization modulation on the POLARBEAR-2
  experiment
  We describe the development of an ambient-temperature continuously-rotating
half-wave plate (HWP) for study of the Cosmic Microwave Background (CMB)
polarization by the POLARBEAR-2 (PB2) experiment. Rapid polarization modulation
suppresses 1/f noise due to unpolarized atmospheric turbulence and improves
sensitivity to degree-angular-scale CMB fluctuations where the inflationary
gravitational wave signal is thought to exist. A HWP modulator rotates the
input polarization signal and therefore allows a single polarimeter to measure
both linear polarization states, eliminating systematic errors associated with
differencing of orthogonal detectors. PB2 projects a 365-mm-diameter focal
plane of 7,588 dichroic, 95/150 GHz transition-edge-sensor bolometers onto a
4-degree field of view that scans the sky at $\sim$ 1 degree per second. We
find that a 500-mm-diameter ambient-temperature sapphire achromatic HWP
rotating at 2 Hz is a suitable polarization modulator for PB2. We present the
design considerations for the PB2 HWP, the construction of the HWP optical
stack and rotation mechanism, and the performance of the fully-assembled HWP
instrument. We conclude with a discussion of HWP polarization modulation for
future Simons Array receivers.
