The Microstrip SQUID Amplifier in ADMX

The Axion Dark Matter eXperiment (ADMX) relies equally on a low physical temperature T and low amplifier noise temperature TN to achieve a high Signal-to-Noise Ratio (SNR). The low noise amplifier is a Microstrip SQUID Amplifier (MSA) with TN of 200 mK, which compares favorably with the best available cryogenic transistor-based amplifiers with a TN of 1.5 K, allowing a search rate up to 56 times faster, depending on T. Here we present the operating principles of the MSA and some practical considerations of MSA design in a way we hope is accessible to those not otherwise familiar with superconducting electronics or amplifier design.