Dc Superconducting Quantum Interference Device Magnetometers Research Articles

PSIM simulations of a dc SQUID magnetometer

Purpose – This paper aims to report the modelling and simulation work that predicts the behaviours of both a Josephson junction (JJ) and a dc superconducting quantum interference device (SQUID). It is pertinent to predict the SQUID magnetometers’ behaviours via simulations, before subjecting them to real experiments because they are quite expensive to acquire, and can be easily damaged during test analysis. Design/methodology/approach – To achieve this, power simulation (PSIM) was used to model and simulate a JJ, using the basic equation that describes the effective current through it. A dc SQUID magnetometer, which is composed of two JJs, was then modelled and simulated using the modelled JJ. Thermal noise simulation is also included, to observe its effects on the magnetometer’s output. A directly coupled flux-locked loop circuit was later included in the simulation to amplify and linearise the SQUID’s output, which is usually sinusoidal. Findings – When steady bias currents were applied to the JJ, the resulting voltage across it was seen to oscillate. The JJ’s and SQUID’s voltage–current characteristics, and voltage–flux characteristics were also observed in the simulations, and the results respectively agree with the behaviours of a typical JJ and dc SQUID magnetometer. Originality/value – A way of simulating SQUIDs, without a superconducting simulation tool, is presented. The work provides a much simpler way of studying the behaviour of dc SQUID magnetometers, due to the easy accessibility and fast simulation capability of the software used, with an added advantage of being able to simulate the thermal noise effects, without having to import this facility from secondary software.

Experimental Realization of Josephson Junctions for an Atom SQUID

We report the creation of a pair of Josephson junctions on a toroidal dilute gas Bose-Einstein condensate (BEC), a configuration that is the cold atom analog of the well-known dc superconducting quantum interference device (SQUID). We observe Josephson effects, measure the critical current of the junctions, and find dynamic behavior that is in good agreement with the simple Josephson equations for a tunnel junction with the ideal sinusoidal current-phase relation expected for the parameters of the experiment. The junctions and toroidal trap are created with the painted potential, a time-averaged optical dipole potential technique which will allow scaling to more complex BEC circuit geometries than the single atom-SQUID case reported here. Since rotation plays the same role in the atom SQUID as magnetic field does in the dc SQUID magnetometer, the device has potential as a compact rotation sensor.

High-Tc dc SQUID Cooled by Pulse-Tube Cooler and Corrosion Measurements

We developed an YBCO high-Tc dc superconducting quantum interference device (SQUID) system cooled by a pulse-tube cooler. To stabilize and control the operating temperature of SQUID, a temperature controlling method, using dc SQUID itself as the temperature sensor, was developed. With the temperature controller, the temperature fluctuation could be reduced to about 50 mK and the dc SQUID magnetometer could keep locking for over 8 h. Using this dc SQUID magnetometer, we did some corrosion measurements in shielding. The field produced by the corrosion of aluminum plate with one side immersed in corrosion solution could be successfully detected.

Detection of early stage damage in carbon fiber reinforced polymers for aeronautical applications using an HTS SQUID magnetometer

We present an experimental characterization of multidirectional fibre composites based on eddy current testing using HTS dc SQUID (Superconducting Quantum Interference Device) magnetometers. The correlation between the mechanical tolerance of CFRPs with different thickness and the phase gradient of the magnetic field generated by damage is shown. The eddy current based SQUID NDE is used to detect damage not visible to the naked eye and to study the effect of an impact far from the point of impact. The aim of this work is to demonstrate the capability of our technique to provide information on early stage damage due to an impact process in an unshielded environment.

Noise performance of HTS solid and meshed dc SQUID magnetometers in external magnetic fields

The noise performance of thin film YBa 2Cu 3O 7− x (YBCO) directly coupled superconducting quantum interference device (SQUID) magnetometers fabricated on MgO substrates with step-edge junctions was investigated at 77 K in static magnetic fields. Dc SQUID magnetometers with solid pick-up loops and with meshed structures consisting of narrow (⩽4 μm) lines were compared. Our measurement results showed that solid structure dc SQUID magnetometers have comparable noise performance to those employing a meshed structure when operated in external magnetic fields. Neither our solid nor meshed dc SQUID magnetometers showed a significant rise in low-frequency noise in the presence of a field as large as 120 μT. The impact of YBCO film morphology was also found to influence the 1/ f noise performance.

An HTS dc SQUID system in competition with induction coils for TEM applications

A superconducting quantum interference device (SQUID) system using high- T c dc SQUID magnetometers setup for geomagnetic prospection is presented. In geomagnetic prospection using the transient electromagnetics method the SQUID system had to compete with conventional induction coils. In measurements above real geological targets the significantly better performance of the SQUID based sensor compared to those coils could be demonstrated. The lower noise of the SQUID system at low frequencies allows measurement of the transients 5–10 times longer, which results in an increase of the investigation depth of a factor more than 3. For the same data quality the measurement time can be reduced by a factor of 25–100 compared to coils.

Effects of flux dam on low-frequency noise in high-T/sub c/ SQUID magnetometers

We demonstrated that the low-frequency noise in a high-T/sub c/ superconducting quantum interference device (SQUID) magnetometer when an external magnetic field is changed could be reduced by forming slots in a flux dam. We designed and fabricated directly coupled dc SQUID magnetometers having a mesh structure and flux dams. In order to suppress the vortex motion in the flux dams, we formed 5-/spl mu/m-wide strip lines and slots across the grain boundary of the flux dams. The output of the magnetometer in a flux-locked loop (FLL) operation became stable and low-frequency noise was suppressed up to an applied field of 83 /spl mu/T in field cooling and 40 /spl mu/T for field change after zero field cooling. The importance of the structure of the flux dam in controlling the vortex motion is discussed.

Low-noise, single-layer YBa/sub 2/Cu/sub 3/O/sub 7-x/ DC SQUID magnetometers at 77 K

We describe a low-noise dc Superconducting QUantum Interference Device (SQUID) magnetometer that is fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on a 2 cm/spl times/2 cm bicrystal substrate. The magnetometer design consists of a single-turn pickup loop that is directly coupled to a low inductance SQUID. Using conventional flux-locked loop electronics with bias current reversal, the white flux noise of several of these magnetometers operated at 77 K is observed to be as low as 2.2 /spl mu//spl Phi//sub 0///spl radic/Hz above 10 kHz, increasing to about 5.7 /spl mu//spl Phi//sub 0///spl radic/Hz at 1 Hz. The field-to-flux conversion efficiency is measured to be 4.6 nT//spl Phi//sub 0/, resulting in a white magnetic field noise of to fT//spl radic/Hz above 10 kHz, increasing to 26 fT//spl radic/Hz at 1 Hz. >

Resistance Ratio Bridge Using Cryogenic Current Comparator with DC-Superconducting Quantum Interference Device Magnetometer

A resistance ratio bridge using a cryogenic current comparator (CCC) with a dc superconducting quantum interference device (dc-SQUID) was developed. Its structure, driving circuit and principle of operation were based on those of a resistance ratio bridge using a CCC with a commercially available rf-SQUID. Because of the small intrinsic noise of the dc-SQUID, its resolution was determined not by the dc-SQUID noise but by the thermal noise of the resistor being tested. The ratio of a resistor having a nominal resistance of 100 Ω and a resistor with that of 1 Ω was measured with an uncertainty of 1.1×10-8.

A high performance integrated DC SQUID magnetometer

The authors have carried out extensive mappings of the DC characteristics and flux noise of an integrated, DC superconducting quantum interference device (SQUID) magnetometer as functions of bias current and applied flux. The open-loop white flux noise at 1 kHz measured without flux modulation is Phi /sub n/ >

Impact of noise of linearity of SQUID feedback loops at high slew rate

A simplified digital DC SQUID (superconducting quantum interference device) system has been simulated to determine the degree of linearity in a digital flux-locked-loop (FLL), with 12-b D/A converter. The influence of comparator noise and quantization noise on the feedback loops corresponding to single- and two-pole integrators is investigated as a function of the normalized slew rate s/sub N/=s/s/sub max/. A simple approximation describing the attainable linearity up to a specific slew rate range is suggested. Measurements with and without a DC SQUID magnetometer in a digital FLL system yielded a satisfying agreement with simulations in the range 0.3<s/sub n/<or=1.<<ETX>>

DC SQUID preamplifier for DC-SQUID magnetometer

A DC SQUID (superconducting quantum interference device) magnetometer with DC SQUID preamplifier has been designed and experimentally tested. The total magnetometer noise was reduced to the intrinsic noise level of the input DC SQUID, and magnetometer cryogenic containment was minimized by using the DC SQUID preamplifier. Two identical thin-film DC SQUIDs with 15- mu m/sup 2/-area resistively shunted Nb-AlO/sub x/-Nb junctions were utilized. The resulting magnetometer had a coupled energy sensitivity equivalent to 3*10/sup -31/ J/Hz in the range from 1 Hz to 30 kHz and about 100-dB dynamic range. >

Integrated dc SQUID (superconducting quantum interference device) magnetometer with a high slew rate : Wellstood, Frederick, C. Heiden and John Clarke, 1984. Rev. scient. Instrums, 55(6):952–957. Dept. of Phys., Univ. of Calif., Berkeley, Calif. 94720, USA

Integrated dc SQUID (superconducting quantum interference device) magnetometer with a high slew rate : Wellstood, Frederick, C. Heiden and John Clarke, 1984. Rev. scient. Instrums, 55(6):952–957. Dept. of Phys., Univ. of Calif., Berkeley, Calif. 94720, USA