Radiative modeling and cryo-engineering for LIGO Voyager prototype

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is considering a near-future cryogenic upgrade (Voyager) to its current detectors to increase strain sensitivity. The Voyager upgrade will involve radiative cooling of 200 kg silicon test masses to 123K and the use of 2-um lasers for GW detection. The strength of radiative coupling is largely dependent on surface emissivities, which must be high enough to offset heating from high-power laser fields. The Mariner upgrade at the Caltech 40m interferometer aims to prototype Voyager technologies. We discuss efforts to model the radiative cooldown of 40m core optics, in order to inform optimal coating and shielding design choices for Mariner. We present an emissivity measurement setup to estimate and verify material emissivities within specification tolerances. The experiment uses a cryostat to cool samples to 123K and obtain temperature data. Robust estimates of emissivities and system parameters are achieved using Markov-Chain Monte Carlo (MCMC) methods. This work will contribute to the design of efficient radiative cooling for cryogenic interferometry.