Thermal Kinetic Inductance Detectors for a CMB space observatory

The 2020 decadal survey identified a CMB space observatory as a possible probe for the 2030s. Such an experiment would measure B-mode polarization to unprecedented levels while separating primordial and galactic components, and would test a large section of parameter space of inflationary models. This observatory would follow DOE's CMB S4 experiment with enhanced foreground control and full sky maps. However, there is considerable work to be done on instrument development to ready NASA and the CMB community for this mission. We propose to extend our team's thermal kinetic inductance detector (TKID) performance to the loading levels, speeds, and multiplexing density expected in such a future probe mission. TKIDs use the kinetic inductance in high Q resonators as bolometric thermal sensors, and the array design uses passive RF multiplexing similar to traditional KIDs. These detectors more gracefully multiplex than TESes, so can greatly simplify the design and integration of a future satellite borne instrument, and reduce the risk of damage during handling and launch by minimizing the wirebond count. TKIDs also offers improved sensitivity over KIDs (whose GR noise equals the CMB photon shot noise for most commonly used materials) and improved cosmic ray immunity thanks to their released membranes. In our prior work, we optimized our detectors for 150GHz under atmospheric loading at the South Pole. This work anticipated a field testing campaign that is on hold while the NSF upgrades polar infrastructure. In the process of preparing for this field test, we have demonstrated that we can make high yield (>85%) antenna-coupled TKID arrays with background limited noise levels (40aW/rtHz), high optical efficiency (in excess of 30%, end to end through a camera), and properly synthesized beams and bands. These laboratory measurements show our instrument would perform well in a real observing campaign, and we will keep our hardware ready for future opportunities. However, detectors for space will need to be redesigned to have lower internal noise and faster response times. The reduced optical loading in space forces a reduction in internal noise, specifically the phonon (G) noise through longer legs, the generation recombination (GR) noise through lower Tc and larger volumes, and the amplifier and TLS noise though higher detector responsively. At the same time, longer detector legs require a lower heat capacity, which we can achieve by shrinking the bolometer island and the indicator while using a higher resistivity material to maintain similar inductance. Our team has been developing novel WSiMn alloys to assist with this goal of higher resistance and lower Tc. We have also demonstrated a speedup in detectors by operating them in a mode of strong electrothermal feedback Our specific tasks include: -Advancing our preliminary WSiMn recipes to repeatably make resonators with Tc~0.2-0.4K and high Q to limit GR noise. -Using AlMn films to down scatter phonons and limit GR noise by surpassing phonon recycling physics. -Developing resonators at lower frequencies and higher Q to increase multiplexing density and responsively -Studying the noise of low G detectors to demonstrate background limited performance at expected space loading levels. -Experimenting with reading out multiple detectors operated with strong and negative electrothermal feedback.

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