Design and performance of wide-band corrugated walls for the BICEP Array detector modules at 30/40 GHz

BICEPArray is a degree-scale Cosmic Microwave Background(CMB) experiment that will search for primordial B-mode polarization while constraining Galactic foregrounds. BICEPArray will be comprised of four receivers to cover a broad frequency range with channels at 30/40, 95, 150 and 220/270 GHz. The first low-frequency receiverwill map synchrotron emission at 30 and 40 GHz and will deploy to the South Pole at the end of 2019. In this paper, we give an overview of the BICEPArray science and instrument, with a focus on the detector module. We designed corrugations in the metal frame of the module to suppress unwanted interactions with the antenna-coupled detectors that would otherwise deform the beams of edge pixels. This design reduces the residual beam systematics and temperature-to-polarization leakage due to beam steeringand shape mismatch between polarized beam pairs. We report on the simulated performance of single- and wide-band corrugations designed to minimize these effects. Our optimized design alleviates beam differential ellipticity caused by the metal frame to about 7% over 57% bandwidth (25 to 45 GHz), which is close to the level due the bare antenna itself without a metal frame. Initial laboratory measurements are also presented.