Michael J. Buckingham

A high stability microwave oscillator based on a sapphire loaded superconducting cavity

By combining the high mechanical rigidity and low loss tangent of cryogenic sapphire with the excellent shielding and low loss properties of a superconducting cavity, the authors have developed a microwave resonator with both high electrical quality factor and very high intrinsic stability even at relatively high power. They have implemented the sapphire-loaded superconducting cavity resonator in a novel phase-stabilized loop oscillator circuit and achieved Allan variances of around 10/sup -14/ for 1 to 1000 s integrating time. This level of stability is competitive with that of the best hydrogen masers, and in fact is superior for integrating times under a few tens of seconds. An overview of the oscillator system is presented and applications and prospects for further improvement in performance are discussed.<<ETX>>

A very high stability sapphire loaded superconducting cavity oscillator

Abstract We have implemented a sapphire loaded superconducting cavity resonator in a novel phase stabilised loop oscillator at 10 Ghz and achieved a fractional frequency stability of 10–14 for 3 to 300 seconds integration time and a resonator Q of 3 x 108. We present here an overview of the oscillator.

Ultra-stable cryogenic sapphire dielectric microwave resonators: mode frequency-temperature compensation by residual paramagnetic impurities

Residual paramagnetic impurities in a sapphire monocrystal multi-mode resonator can provide a means of temperature compensation so that the effect of temperature fluctuations on the frequency of an ultra-stable cryogenic microwave oscillator can be reduced to second order. Modes investigated at X-band in 3 and 5 cm diameter resonators exhibited a wide range of effective paramagnetic filling factors, and frequency-temperature turning points in the range 5 to 13 K. The observations agree well with theory.

Laboratory tests of a mobile superconducting gravity gradiometer

Abstract We describe a gravity gradiometer designed to measure off-diagonal components of the earths gravity gradient tensor, and intended for airborne geophysical exploration. The instrument consists of an orthogonal pair of mass quadrupoles pivoted about a common vertical axis. Each quadrupole is a rectangular bar of niobium having square cross section and with integrally machined flexural micro-pivots. Superconducting niobium pancake coils form the basis of the SQUID based transducers and provide supplementary magnetic springs to enable matching of mechanical parameters.

A sapphire oscillator for VLBI radio astronomy

The authors present a sapphire oscillator with a long hold-time cryostat and helium recovery system to overcome the inherent difficulty of using this liquid-helium-cooled frequency standard at a remote site. The oscillator has been used as a frequency reference for very long baseline interferometry (VLBI) radio astronomy observations. Data are presented which show that the performance of the oscillator is superior to a rubidium standard in this application. While laboratory data have demonstrated a fractional frequency stability of about 9*10-15 for integration times between 1 and 300 s the VLBI frequency stability measurements in this case are limited by the signal-to-noise ratio in the correlated data.

Performance of a superconducting gravity gradiometer

Abstract We report the laboratory operation of an rf SQUID-based superconducting gravity gradiometer designed to measure from an aircraft, off-diagonal components of the earths gravity gradient tensor. A detailed numerical model of the multi-mode electromechanical system shows excellent agreement with the experimental observations. We have demonstrated a common mode rejection ratio to linear accelerations of 180dB, as well as 60dB of active thermal compensation below 0.01Hz. Short term measurements show the instrument to be SQUID noise limited at 0.5Eo¨/√Hz in a frequency band from 50mHz to 1Hz. After correcting for the remaining effects of thermal fluctuations and flux creep the noise below 50mHz has a 1/f characteristic which is also dominated by the SQUID.

A prototype superconducting gravity gradiometer

We report the successful laboratory test of a single-axis gradiometer designed to measure a diagonal component of the earths gravitational gradient tensor. It Consists of a pair of accelerometers mounted with their sensitive axes vertical and in line. The difference in displacement of the accelerometers is proportional to the component of the tensor gradient and is sensed via the modulated inductance of a superconducting coil coupled by a superconducting transformer into an RF biased SHE SQUID with energy sensitivity 4 × 10-29J/Hz. Rejection of in-line common mode accelerations is achieved by trimming the natural resonant frequency of each accelerometer: the restoring force acting on an accelerometer test mass is partly magnetic and can be trimmed by adjusting the persistent currents in a pair of force coils. A common mode rejection ratio exceeding 95 dB has been achieved in the presence of linear accelerations \sim 10^{-3} ms-2, and a laboratory generated gradient of 30 Eo rms has been detected with a signal to noise ratio of about 100. The dependence of this signal on the distance between source and detector has the expected Newtonian form. Under quiet conditions the background noise level of the instrument is at present 3 Eo/ \sqrt{Hz} . ( 1 Eo = 10-9s-2.) This is close to the practical limit achievable for such a single axis configuration: a three axis instrument for geophysical application is under development.

A cryostat for measuring low field flux creep

We describe a SQUID-based magnetometer for measuring low field flux creep (LFFC) over the temperature range of 2–7.5 K. Temperatures below 4.2 K are achieved by pumping on a reservoir of liquid helium; the upper temperature limit is imposed by the SQUID magnetometer. Valid LFFC measurements to 105 seconds in niobium at 4.2 K require temperature regulation of the sample to within 1 μK. The white noise floor of the thermal system has been measured at 3–4 μK rms/√Hz, and the increased low frequency noise limits the temperature regulation to somewhat better than 1 μK at 10−5 Hz. Preliminary measurements of LFFC made using this magnetometer are presented.