Components for Multiplexed DC-SQUID Readouts of Transition Edge Sensor Arrays

A SQUID tandem is a two-stage amplifier consisting of the front end SQUID at T << 1 K and the booster SQUID at T ∼ 4 K. We give an overview of the dc SQUID design for the tandem used in the recent 37-pixel frequency domain multiplexed (FDM) readout demonstration of X-ray calorimeters. The SQUID devices were fabricated with pillar-type Josephson junctions at the nominal J_C = 500 A/cm² critical current density. We further discuss possible improvements to the FDM tandem configuration, including (i) the class-B operation for better power efficiency, (ii) the input power combiner as an improvement over the simple T-junction summing, with the promise for larger multiplexing factors, and (iii) the new booster SQUID device. The new improved booster is a 8-parallel 66-series SQUID array fabricated with SWAPS Josephson junctions at the nominal J_C = 1.5 kA/cm2 critical current density, and it shows dV/dΦ > 30 mV/Φ₀ gain in the symmetric-slope operation, as well as Φ_N ≈ 0.05 μΦ₀ / Hz^1/2 flux noise.