Superconducting tunable-diaphragm transducer for sensitive acceleration measurements

A very sensitive resonant superconducting accelerometer has been developed as a component of a cryogenic gravitational-radiation detector. The device consists of a superconducting test mass and superconducting coils carrying a persistent current. The displacement of the test mass modulates the inductances of the coils and generates an ac magnetic field which is detected by a Josephson-junction magnetometer. The restoring force provided by the magnetic field is used to tune the resonant frequency of the transducer. The expected sensitivity of the system is better than 10−12gE/Hz1/2 (gE=9.8 m/s2) when used to detect accelerations at frequencies lower than 50 Hz. The system has been thoroughly tested and is being used to detect small accelerations of a gravitational-wave antenna caused by the Brownian motion and other external disturbances. When used as a resonant displacement sensor in a gravitational-wave detector cooled to 3 mK, this transducer is capable of converting a displacement of 4×10−20 m at 1 kHz into an electrical signal detectable with unity signal-to-noise ratio for 1-Hz bandwidth. The gravitational-radiation-flux sensitivity implied by this is 0.1 erg/cm2 Hz. This will make not only the observation of expected galactic events possible, but will allow one to extend the scope of observation beyond the Milky Way. The system can be modified to make a sensitive gravity gradiometer. When two accelerometers are coupled to the same Josephson-junction magnetometer with their transformer coils wound in the opposite sense, direct subtraction of acceleration signals can be accomplished. The system will be easy to build and mechanically rugged. The device in various applications is discussed and the theory of transducer energy coupling, frequency tuning, and parameter optimization is presented. Some experimental results confirming the theory are reported. Included are data showing the temperature dependence of the Q of a niobium diaphragm and the measurement of the low-frequency background acceleration of a magnetically levitated gravitational-wave antenna.