Construction and performance of a low noise inductive transducer for the Louisiana State University gravitational wave detector

The design, fabrication, and performance of an inductive transducer for a two-mode resonant mass gravitational radiation detector is described. The design of the transducer is based on a detailed noise model of the detector. The transducer combines a large dynamic mass, a large loaded quality factor, and transformer impedance matching to an integrated dc superconducting quantum interference device (SQUID). The transducer has been in continuous use on the Louisiana State University detector for two years. Its performance on the antenna is reported. The Q’s of the two resonant modes measure 6.7×106 and 2.3×106 at a transducer coupling coefficient of 0.74%. The electrical Q ranges from 2×105 to 9×105 and is dependent upon trapped magnetic flux in the transducer. The detector’s dominant noise source is the commercial SQUID amplifier, and not losses in the transducer. The predicted sensitivity of the detector equipped with our transducer agrees well with its measured burst strain sensitivity, h≊6×10−19. The detector’s gravitational burst strain sensitivity could be improved to 1.7×10−19 by coupling the transducer to a 200ℏ SQUID, while operating at 4 K.