CMB-S4 Readout Components Demonstrate Thermal and Electrical Performance for Next-Generation Cosmic Microwave Background Studies

The quest to understand the very early universe drives the development of increasingly sensitive instruments, and the next generation Cosmic Microwave Background experiment, CMB-S4, represents a significant leap forward in this pursuit. Wilber Dominguez, Darcy R. Barron, and colleagues from the University of New Mexico, alongside Zeeshan Ahmed from SLAC National Accelerator Laboratory, Amy N. Bender from Argonne National Laboratory, and Sandra Diez and Malcolm Durkin from the National Institute of Standards and Technology, now present crucial characterisation of prototype readout components for this ambitious project. Their work addresses a key challenge in scaling up detector technology, namely balancing the need for increased bandwidth with the constraints of thermal load and power dissipation. By meticulously evaluating the thermal and electrical performance of wiring and superconducting detector arrays, the team demonstrates progress towards optimising the readout system for CMB-S4 and unlocking its potential to reveal new insights into cosmic inflation and the origins of the universe.

This system employs time-division multiplexing (TDM) to read signals from a vast array of detectors, requiring extremely sensitive components cooled to millikelvin temperatures. The team is developing a cryogenic system to accommodate a large number of detectors, utilizing transition-edge sensor bolometers, which demand precise temperature control and readout. This innovative approach allows for scaling the experiment while maintaining signal integrity.

The system incorporates superconducting quantum interference devices (SQUIDs) to amplify the extremely weak signals from the detectors, alongside integrated cryogenic electronics (ICE) for signal processing and networking within the cryostat. Pulse tube cryocoolers provide the initial cooling stages, and specialized cryogenic cabling minimizes heat leaks and maintains low temperatures. Understanding the thermal properties of materials like woven ribbon cable and various polymers is critical for optimizing the thermal design of the system. Researchers are addressing challenges related to thermal management, noise reduction, and bandwidth limitations to ensure the system’s performance.

Precise calibration and long-term stability of the detectors are also key areas of focus. The team is leveraging existing technologies, such as SQUID array amplifiers and the Simons Array, a precursor experiment, to refine their designs. They are also investigating the effects of cryocooler inclination on performance, ensuring reliable operation in a complex cryogenic environment. This research focuses on material characterization, cryogenic design, readout electronics, and signal processing, all integrated to create a highly sensitive detector system for CMB-S4.

Cryogenic Wiring and Thermal Conductivity Measurements

To accommodate an increased detector count, the CMB-S4 project engineered a new cryogenic wiring system and meticulously characterized its thermal and electrical performance. Researchers addressed the competing demands of thermal load and bandwidth by precisely measuring the thermal conductivity of key materials, including Teflon and FEP, between 4 and 300 Kelvin, utilizing data from existing databases and extrapolating existing measurements. To accurately determine the thermal conductivity of FEP, which lacked comprehensive documentation, the team performed measurements with varying material amounts, enabling them to gauge uncertainty in their technique. By comparing measurements of single and dual FEP strips, they established a reliable baseline for assessing the thermal performance of the readout wiring.

The study revealed that the shielding and insulation of the twisted pair cables significantly contributed to the overall thermal conductivity, more than doubling it compared to bare twisted pairs. Researchers further investigated heat-sinking techniques and their impact on power deposition, finding substantial variations based on thermal interface quality. Electrical testing focused on prototype cold readout configurations, including the ICE readout system, which features a short cryogenic wiring run. The team evaluated superconducting signal amplifiers (SSAs) in both series and parallel configurations to optimize gain and noise. By varying shunt resistances, scientists altered the SSA’s dynamic impedance, influencing the overall system bandwidth. The study demonstrated that shunting SSAs provides a means to tune the bandwidth without requiring new SSA designs, a crucial innovation for maximizing signal fidelity in the CMB-S4 project.

Efficient 80-Channel Readout for CMB-S4 Detectors

Scientists are developing a groundbreaking readout system for the next generation of cosmic microwave background (CMB) experiments, known as CMB-S4. This ambitious project aims to observe the faint polarization patterns in the CMB, relics of the early universe, and requires a significant leap in detector technology. The team intends to deploy 500,000 transition-edge sensor (TES) bolometers, necessitating a highly efficient and scalable readout system based on time-division multiplexing (TDM). This technique allows multiple detectors to be read out by a single electronic chain by rapidly switching between them.

To achieve a multiplexing factor of 80, a substantial increase over previous designs, researchers focused on optimizing the bandwidth and thermal performance of the readout components. Detailed thermal tests were conducted on prototype cabling, consisting of numerous wires packaged as twisted pairs, each utilizing manganin conductors with fluorinated ethylene propylene (FEP) insulation and stainless steel shielding. Measurements confirmed the thermal conductivity of the manganin and stainless steel at cryogenic temperatures, aligning with established data. Comparisons were made to similar fluoropolymers like polytetrafluoroethylene to estimate the thermal conductivity of FEP at cryogenic temperatures.

The team employed a precise measurement technique, suspending a heater resistor on a copper plate by the wire samples under test, and carefully monitoring the temperature difference between the heated plate and the cold stage. This allowed for accurate characterization of the thermal conductivity of the assembled cable. The design prioritizes minimizing thermal load on the cryostat stages, ensuring compatibility with the cryocooler’s performance and other system components. These measurements are crucial for validating the thermal model of the TDM readout system and ensuring the feasibility of achieving the desired multiplexing factor for CMB-S4.

Cryogenic Wiring Thermal Limits Characterized

This research details the characterization of prototype readout components developed for CMB-S4, a project designed to investigate the early universe and cosmic inflation. The team focused on understanding the thermal and electrical limitations of increasing detector multiplexing rates and bandwidth within the system, crucial for achieving the project’s scientific goals. Measurements of key components, including wiring and superconducting amplifier arrays, provide valuable data for optimizing the overall design. The study successfully measured the thermal conductivity of materials used in the cold readout wiring, including manganin, FEP, PTFE, and shielded twisted pair wires, at cryogenic temperatures.

These measurements are essential for accurately estimating thermal loads on the cryostat stages and ensuring the system operates within its thermal budget. Electrical characterization of superconducting amplifier arrays revealed that a newer design exhibits a greater voltage amplitude than an older design under comparable conditions. While decreasing dynamic impedance generally reduced voltage, the impact on bandwidth required further investigation. The authors acknowledge that additional work is needed to fully characterize the newer devices at typical operating temperatures and to identify other limiting factors affecting bandwidth and performance. These findings will inform future integrated tests and system design studies, ultimately contributing to the development of an optimized readout system for CMB-S4 and enabling its ambitious scientific objectives.