Developing Highly Multiplexed Detectors for MUSTANG2

Clusters of galaxies – the largest gravitationally-bound objects in the universe – provide important tests of cosmology and serve as rich astrophysical laboratories for events such as mergers and AGN outbursts, the two most energetic classes of events to happen since the Big Bang. As tracers of the evolution of structure, clusters of galaxies represent an important cosmological tool being exploited by a host of current and future projects such as Planck, SPT, ACT, DES, the Simons Observatory, CMBS4, LSST, Euclid, and eROSITA. However, the mass scaling relationship used to extract parameters relies on X-ray measurements which are biased towards the high density regions of clusters, whereas Sunyaev-Zel’dovich effect (SZE) surveys measure the integrated signal on larger scales beyond where X-rays can typically probe. Maps of the SZE in galaxy clusters at 90 GHz with the 100-meter Green Bank Telescope (GBT) are providing a combination of unprecedented mapping speed, resolution, and sky coverage which will unlock the full potential of these complex objects. Operating on NRAO’s Green Bank Telescope (GBT) with 9 resolution, the 4.2 field of view is well-matched to cluster observations. MUSTANG2 has a strong track record of demonstrated performance for cluster measurements and a host of other observations. The Green Bank Observatory recently received an NSF grant to enable real-time laser metrology of the GBT dish which will increase MUSTANG2 observing efficiency by up to a factor of two. MUSTANG2 has a 215-pixel bolometer array of micro-strip coupled TES detectors combined with a microwave SQUID readout system providing background-limited performance at 90 GHz. The microwave SQUID multiplexer has been demonstrated by NIST to multiplex 400 bolometers with no degradation in noise performance of the detectors. The SQUID readout system developed and employed by MUSTANG-2 represents a significant step forward in bring down the cost of ~105 element arrays down to tens of dollars per detector. The SQUID readout works by inductively-coupling a bolometer circuit to a radio frequency SQUID, which is then also inductively coupled to a microwave resonator. Absorbed power on the bolometer circuit changes the input flux to the SQUID, which in turn causes a change in the SQUID’s self-inductance, thus shifting the tone of the resonator. The resonators are interrogated with a frequency comb and the transmitted phase and amplitude of the probe tones traces the incident power on the TES. MUSTANG2 was the first astronomical instrument to utilize this technology successfully on the sky and it is now being developed for other ground-based system and is likely to be a leading technology candidate for the next cosmic microwave background satellite.