by Robin Cantor, STAR Cryoelectronics LLC, rcantor@starcryo.com; Stephen Boyd, University of New Mexico, stpboyd@unm.edu; Aritoki Suzuki, Lawrence Berkeley National Laboratory, asuzuki@lbl.gov
There are many fundamental questions about the universe that have intrigued human beings for generations: How did the universe begin and then evolve? And what are the constituents of the universe?
Physicists around the world are using experiments to find answers to such fundamental questions, including theoretical models and experimental evidence. The Simons Array is an example of one such experiment, used to observe Cosmic Microwave Background (CMB) radiation, an afterglow from the universe created when it was just 380,000 years old.
The experiment utilizes superconducting, ultrasensitive detectors and readout electronics to achieve unsurpassed sensitivity. STAR Cryoelectronics, a CSA CSM, and experimental physics groups at the University of New Mexico and Lawrence Berkeley National Laboratory have co-developed a new class of Superconducting Quantum Interference Device (SQUID) amplifiers that can be used as first-stage amplifiers for detector readout electronics.
These devices feature both power dissipation and input inductance that is 10 times lower than amplifiers used for previous generation experiments, while at the same time maintaining the required amplifier gain and noise performance.
POLARBEAR-2b, the second cryogenic receiver to be deployed for the Simons Array, is shown in Figure 1 and will be equipped with 200 of these SQUID amplifiers to unlock the mysteries of the universe. The group at LBNL is also exploring whether these amplifiers will be suitable to read out the detectors used to study the nature of neutrinos and dark matter in other upcoming experiments.
The focal plane array of POLARBEAR-2b, shown in Figures 2 and 3, is an array of 7,600 superconducting bolometer detectors. Future planned experiments will have a larger number of detectors in order to extend the sensitivity of these CMB observatories.Reading out such large numbers of detectors is a significant technological challenge. The readout technology developed and deployed by the POLARBEAR and South Pole Telescope teams is based on a frequency-division multiplexing (DfMUX) scheme. With this approach, signals from 40-68 detectors are read out using a series SQUID array amplifier (SSAA). Unique frequency channels are assigned to each detector in the frequency range 1.5 MHz to 5.5 MHz using superconducting resonators, and each channel is probed using a unique frequency tone.
There is a current effort to develop new SSAAs specifically for DfMUX. One of the goals is to reduce power dissipation to enable placement of the SSAAs directly on a focal plane operating at 100~250 mK rather than at 4 K, a much higher temperature that is farther away from the focal plane. Such a setup will reduce the parasitic impedance associated with interconnections between the focal plane array and remotely located SSAAs, the dominant source of crosstalk between channels. In addition, integrating the SSAAs directly on the focal plane improves stability, increases scalability, lowers noise and offers higher multiplexing factors.
Figure 2. Photograph of the POLARBEAR focal plane array (400 mm diameter), tiled with seven hexagonal detector array wafers, each with 271 pixels. Image: POLARBEAR Collaboration
Figure 3. Photograph of a microfabricated detector. The sinuous antenna (diameter of 1.5 mm), a broadband dual-polarization antenna, is shown in center. The four rectangular structures that surround the antenna are transition edge sensor (TES) bolometers. Image: Aritoki Suzuki
A second goal of the SSAA development effort was to reduce the input inductance of the SSAAs, therein improving overall performance. The new SSAA devices were designed at the University of New Mexico using extensive simulations, circuit analytical tools and model calculations to determine the design parameters and layouts. The designs were fabricated at STAR Cryoelectronics using the company’s standard commercial SQUID foundry process.
The SSAA design for POLARBEAR consists of 112 SQUIDs connected, as seen in Figure 4, in order to achieve a large output voltage swing. The low critical current of the Josephson junctions (14 µA) results in a low power dissipation 500 V/A over a wide range of working points seen in Figure 6.
In summary, the low power dissipation and low input inductance of the new SSAAs enable the integration of these devices directly on the focal plane. Such a significant advance will result in improved performance and is essential for the integration of very large detector arrays with hundreds of thousands of detector elements. ■
Figure 4. Photograph of the SSAA chip for POLARBEAR with 112 SQUIDs connected in series. Image: STAR Cryoelectronics
Figure 5. Output voltage versus applied feedback current (flux) for a 112-SQUID SSAA. The individual plots are for SSAA bias currents ranging from 24 to 36 μA in 2 μA steps. Image: Stephen Boyd
Figure 6. Maximum transimpedance as a function of SSAA bias current. Image: Stephen Boyd
CSA Archive 