Niobium dc SQUID with microstrip input coupling as an amplifier for the axion detector

The output from the 1.4 K cavity of the axion detector at Lawrence Livermor e National Laboratory is currently amplified by a cooled semiconductor ampl ifier with a noise temperature of about 3 K. To achieve a lower system nois e temperature, one must cool the cavity to a lower temperature and use an a mplifier with a lower noise temperature. An amplifier based on a dc SQUID ( Superconducting QUantum Interference Device) has been developed in which th e input signal is coupled to one end of the microstrip formed by the input coil and the body of the SQUID. Gains of up to 20 dB have been achieved in the frequency range 100 - 900 MHz. At 4.2 K and with a room-temperature pos t-amplifier the system noise temperature ranged from 0.5 K +/- 0.3 K at 80 MHz to 3.0 K +/- 0.7 K at 500 MHz; particularly at the higher frequencies, the measured noise temperature was determined by the noise temperature of t he post-amplifier (about 80 K). With the SQUID cooled to 1.8 K and a cooled high electron mobility transistor, the system noise temperature was reduce d to 0.3 K +/- 0.1 K at 250 MHz and 0.25 K +/- 0.1 K at 365 MHz.