Retrofit of the POLARBEAR-2a Cosmic Microwave Background Instrument

POLARBEAR-2a (PB-2a) is the first receiver installed of Simons Array, a millimeter-wave observatory located in the Atacama Desert of northern Chile at an altitude of 5,200 m which studies the cosmic microwave background (CMB) polarization anisotropies. Simons Array is designed for sensitivity to a broad range of angular scales, allowing measurements of large-scale B-modes for primordial gravitational wave background searches and small-scale CMB-lensing for neutrino physics constraints. PB-2a has operated since 2018 and observes with arcminute-scale resolution at 90 and 150 GHz bands, using alumina lenses to re-image the sky onto a kilo-pixel focal plane of antenna-coupled transition-edge sensor bolometers. Multi-layer anti-reflection coatings on its optics allow simultaneous dual-band observations of the CMB. A continuously rotating half-wave plate modulates polarized signal to mitigate low-frequency noise and beam-related systematics. Signal from these bolometers are read out using frequency-multiplexed electronics, with bolometers in series with lithographed superconducting Nb LC resonators.

We present on the planned retrofit of PB-2a in 2023 and the lab characterization of the replacement hardware, which upgrades the sensitivity, stability, and operability of the instrument.