M.J. van Duuren

High sensitivity magnetic flux sensors with direct voltage readout: double relaxation oscillation SQUIDs

The experimental sensitivity of double relaxation oscillation SQUIDs (DROSs) has been compared with theory and with the results obtained by numerical simulations. The experimental sensitivity ranges from 60 to 13h, where h is Plancks constant, for relaxation frequencies from 0.4 up to 10 GHz. For low frequencies the DROS characteristics can be explained by thermal noise on the critical currents. For high frequencies, the voltage-flux characteristics and the sensitivity are limited by the plasma frequency. The cross-over frequency is at 2 GHz, which is about 2% of the plasma frequency of the DROSs.<<ETX>>

Two-stage amplifier based on a double relaxation oscillation superconducting quantum interference device

A low-noise single-chip two-stage superconducting quantum interference device (SQUID) system with a double relaxation oscillation SQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was 1.3 μΦ0/√Hz, corresponding to an energy sensitivity of e≈27h. Owing to the large flux-to-voltage transfer of up to 3.6 mV/Φ0, the room-temperature preamplifier noise did not dominate the overall flux noise.

Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

Double relaxation oscillation superconducting quantum interference devices (DROSs) with a gradiometric signal SQUID and either a reference SQUID or a reference junction will be presented in this article. The devices are user friendly, particularly those with a reference junction. Because of the large flux-to-voltage transfer of ∂V/∂Φ=0.7–1 mV/Φ0, the devices can be operated in a flux locked loop based on direct voltage readout without loss of sensitivity. The typical white flux noise of the DROSs amounts to √SΦ=5–6μΦ0/√Hz, which corresponds to an energy resolution e=SΦ/2Lsq≃200 h. Coupled to an external planar first-order gradiometer, a white magnetic field sensitivity of √SB<2 fT/√Hz was measured inside a magnetically shielded room.

Multichannel SQUID magnetometry using double relaxation oscillation SQUID's

A highly sensitive first-order gradiometer based on double relaxation oscillation SQUIDs (DROSs) for multichannel use is presented. The white flux noise level of the bare DROSs is 4.5 /spl mu//spl Phi//sub 0///spl radic/Hz (/spl epsi/=275 h). With wire-wound first-order gradiometers, having a baseline of 40 mm, the white magnetic field noise equals 4 fT//spl radic/Hz. As a result of the high flux-to-voltage transfer of the DROSs of about 1 mV//spl Phi//sub 0/, this low noise level could be obtained with simple room-temperature flux locked loop electronics based on direct voltage readout. The relaxation frequency of the present DROSs is approximately 1 GHz. No degradation of the DROS characteristics caused by interference between relaxation oscillations in two adjacent channels has been observed, although the gradiometers are spaced by less than 1 mm. This shows that DROSs can be used in multichannel SQUID magnetometers.

Smart SQUIDs based on relaxation oscillation SQUIDs

Smart SQUIDs based on double Relaxation Oscillation SQUIDs (DROS) and a superconducting up-down counter have been developed. DROS and counter form a flux locked loop on one single chip. The DROS output consists of a series of pulses that controls the two up and down write gates of the counter. The pulsed output structure of the DROS constitutes the internal clock for this single-chip device. Several prototypes were built with a clock frequency of 100 MHz, a linear operation flux range of about 2.5 /spl Phi//sub 0/, and a white noise level of 6.5 /spl mu//spl Phi//sub 0///spl radic/Hz. The smart SQUID is in principle a promising device for application in multichannel SQUID systems.

Low input coil inductance SQUIDs for cryogenic current comparator applications

Dc SQUIDs with an optimal input coil inductance have been developed for a Cryogenic Current Comparator (CCC) that is used for the calibration of electrical standards. We studied a series of SQUIDs with input inductances in the range from 20-160 nH. The electrical properties like input current noise and flux to voltage transfer have been investigated. The CCC is an overlapping tube configuration and the tube itself is used as the pick-up coil of the flux transformer circuit of the SQUID. The coupling between CCC and flux transformer is in this case ideal and should have an optimal value when the effective overlapping tube inductance, typically in the range from 10-100 nH, equals that of the SQUID input coil (flux transformer theory). To compare with theory, sensitivity measurements on the SQUID-CCC have been performed in a special set-up where the effective overlapping tube inductance can be modified placing the CCC in a superconducting shield at various distances.

Frequency readout of relaxation oscillation superconducting quantum interference devices in the GHz regime

The output of relaxation oscillation superconducting quantum interference devices (ROSs) consists of a sequence of voltage pulses with a frequency that depends on the flux that is applied to the ROS. In this paper, a theoretical model for the flux‐to‐frequency conversion of a ROS is presented, and this model is validated in practice for oscillation frequencies up to 7 GHz. The experiments have been performed on more than ten different ROSs and the model was able to fit all measured data, which illustrates the versatility of the model. Furthermore, a simple flux locked loop based on frequency readout of a ROS in the GHz regime is presented. The measured flux noise, √SΦ=2.5μΦ0/√Hz, corresponding to an energy resolution e≊600h, is probably not intrinsic to the ROSs, but due to the readout electronics.

A 1-MHz low noise preamplifier based on double relaxation oscillation SQUIDs

A low noise and wideband preamplifier based on Double Relaxation Oscillation Superconducting Quantum Interference Devices (DROSs) has been realized. A major advantage of a DROS is that it can be operated in a simple flux modulation. So far, biomagnetic measurements performed in our group required only a limited bandwidth smaller than 100 kHz. Other applications, like for instance readout of radiation and particle detectors, demand a larger bandwidth. In this paper, we will discuss our efforts aimed at increasing the operational bandwidth of a DROS in flux locked loop. Presently, a flux locked loop scheme with a -3 dB bandwidth of 1.45 MHz has been built. With this system a white flux noise of 8 /spl mu//spl Phi//sub 0///spl radic/Hz was measured with a 1/f-corner frequency of 10 Hz. The slew rate was 2.5/spl middot/10/sup 5/ /spl Phi//sub 0//s. With the mutual input inductance of 6.7 nH, an input current noise of the preamplifier of 2.5 pA//spl radic/Hz was found and a current slew rate of 80 mA/s. We will discuss the suitability of our DROS-based preamplifier for readout of cryogenic particle detectors based on superconducting tunnel junctions.

3-channel double relaxation oscillation SQUID magnetometer system with simple readout electronics

Recently several approaches have been made to simplify the readout scheme of the standard dc SQUID. A double relaxation oscillation SQUID(DROS) consisting of a hysteretic dc SQUID and a reference junction in series shunted by an inductor and a resistor can provide a very large flux-to-voltage transfer coefficient. Thus, a DROS with direct readout with room temperature dc amplifier can be a good candidate for the next-generation SQUID magnetometer. We report on the development of a 3-channel magnetometer system based on DROS. The DROS is based on Nb/AlO/sub X//Nb Josephson junctions and the main feature of the system is its simple readout electronics.<<ETX>>

Nearly Quantum-Limited Squids for a Gravitational Wave Antenna

Nearly quantum-limited Superconducting QUantum Interference Devices (SQUIDs) have to be used in resonant mass gravitational wave antennas to reach a displacement sensitivity of the order of 10–21 m/√Hz in a bandwidth of 100 Hz around the resonance frequency of about 900 Hz. The design of these SQUIDs will be described. We show that a good coupling between the inductive readout circuit and the SQUID can be obtained by choosing a relatively large hole size of the washer-type SQUID configuration.