System integration and trade-offs of SQUID system for biomagnetic applications

We have been developing SQUID systems for various applications, with special emphasis on systems for biomagnetic applications. In 1987, Koyanagi and Kado fabricated and integrated a SQUID system made entirely of thin-film integrated SQUID. Research and development activities on multichannel SQUID system integration has been undertaken by various groups. The Superconducting Sensor Laboratory (SSL) project was one of the most significant of these activities. Such activities in North America and Europe have mostly been pursued by commercial groups, who have been fabricating commercial systems for end users. However, large-scale system distribution to end users is still impractical because of high costs and the lack of user-friendliness. As former members of the SSL project, some of us at the Kanazawa Institute of Technology began the development of SQUID systems aimed at overcoming previous problems such as high cost and the low level of user-friendliness. In this paper, we describe our concept of system integration and the level of the system’s sophistication.     

A dual SQUID linearization concept using a phase modulation scheme

A critical control system evaluation is presented of basic flux-locked loop systems. The development of a new superconducting quantum interference device (SQUID) linearization method is then described, where no magnetic flux feedback is necessary to cancel the applied flux. It is shown that a dual SQUID configuration will be able to produce a true phase modulation system that is easily demodulated with a phase-locked loop. The theoretical performance of the proposed configuration is verified by simulations, and the performance and limitations are discussed in detail. It is shown that small dc correction voltages at the output of the SQUID's significantly decrease output noise, as is the case with an increase in SQUID dc bias currents. An optional feedback system is also described for optimal performance of the dual SQUID configuration     

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.     

Modulation SQUID electronics working with high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> SQUIDs in open space

SQUID electronics optimized for operation in unshielded space with dc high-T _c superconducting quantum interference devices (HTS SQUIDs) are developed, manufactured, and studied. The dynamic characteristics of the SQUID electronics are studied with two magnetic-field sensors based on the HTS SQUIDs: a conventional SQUID sensor with a resolution of 100 fT/Hz^1/2 and a supersensitive SQUID sensor with a resolution of 15 fT/Hz^1/2 at frequencies exceeding 10 Hz and a resolution of 30 fT/Hz^1/2 at a frequency of 1 Hz. Stable operation of the magnetometric channel is demonstrated with both SQUID sensors under urban conditions. On the basis of a complex programmable logic device (CPLD), an ac bias can be realized in the SQUID and the modulation signal can be compensated in the feedback, bias-current, and desired-signal circuits. Such a compensation system is the most appropriate and versatile means of providing stable operation of the magnetometric channel in the presence of the SQUID ac bias, regardless of the type of high-temperature sensor and the configuration of the input contacts in the measurement probe.     

A wide dynamic range single-chip SQUID magnetometer

A single-chip SQUID magnetometer is described and demonstrated that integrates a SQUID sensor with feedback circuitry on the same chip. This chip has a very large dynamic range, determined by the sensitivity of the input SQUID and by the current-carrying capacity of the input superconducting lines. This chip can eliminate the need for the sophisticated room temperature circuitry currently used with conventional analog SQUID magnetometers and replace these electronics with a simple bi-directional counter. Furthermore, on-chip multiplexing can be easily implemented for use in multi-channel systems where arrays of more than 100 sensors are required for magnetic imaging. In addition, due to its extremely wide dynamic range and high slew rate, a system based on this chip can be operated in a relatively high magnetic field environment without extensive magnetic shielding     

Gradiometers based on superconducting quantum interference device for nondestructive testing

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.     

DC SQUID-based position detector for gravitational experiments

Experiments like the test of the weak equivalence principle at the Bremen drop-tower, Germany, and STEP (Satellite Test of the Equivalence Principle) require a position detector with an extremely high resolution to measure tiny displacements of free falling test bodies.In order to develop a SQUID position detector numerous configurations of test bodies and pick-up coils with different geometries were tested experimentally. As a function of the position of the test body the inductance of the pick-up coil was measured with a commercial LCR meter as well as with a LTS DC SQUID system. This SQUID system, which is developed and manufactured at the Jena University, provides high sensitivity and extremely low intrinsic noise, especially at low frequencies.This contribution will also discuss some recent results in measuring the motion of one body during its free fall over 109 m at the Bremen drop-tower.     

Implementation of new superconducting neural circuits using coupledSQUIDs

A novel superconducting neuron circuit and two types of variable synapses, which are based on superconducting quantum interferometer devices (SQUIDs), are presented. A neuron circuit with good input-output isolation and steep threshold characteristics is accomplished using a combination of a single-junction SQUID coupled to a double-junction SQUID. The quantum state of the single-junction SQUID represents the neuron state, and the output voltage of the double-junction SQUID, which is operated in a nonlatching mode with shunt resistors, is a sigmoid-shaped function of the input. Both variable synapse circuits are composed of multiple shunted double-junction SQUIDs. The first type changes its conductance value by using both superconducting and voltage states. The second variable synapse circuit changes its output current digitally by switching its bias currents. Besides numerical simulations of the circuit characteristics, we have fabricated superconducting neural chips in a Nb/AlO_x/Nb Josephson junction technology. The fundamental operation of each element and a 2-bit neural-based A/D converter have been successfully tested. A learning system with a variable synapse is also discussed     

Recent low temperature SQUID developments

Magnetometers based on superconducting quantum interference devices (SQUID's) fabricated from low (liquid helium) temperature superconductor materials are currently the most sensitive devices for measuring weak magnetic fields. A major application of low temperature SQUID's is in biomagnetism where multichannel systems with around 100 channels are required. The recent world-wide effort to develop biomagnetic multichannel systems has stimulated both the sophistication of existing SQUID concepts and the development of novel SQUID concepts optimized for biomagnetic applications. The latest SQUID designs are reviewed and their merits are evaluated. Both the noise and the dynamic behavior are discussed and a fair quantitative comparison between the different SQUID concepts is given     

Using a 77 K SQUID to measure magnetic fields for NDE

A bare HTS SQUID of commercial design was used in 77 K experiments concerning NDE. The SQUID was operated with flux-locked instrumentation to provide a noise floor of 80 pT/√Hz. The effective sensor area was measured to be approximately 70 μm² equivalent to an ideal point detector for NDE. The SQUID was used unshielded in a normal laboratory environment in a special purpose LN₂ cryostat positioned above a motorized computer-controlled scanning system. We measured magnetic fields associated with current flowing in wires and compared them with calculations. We also detected a simulated flaw in an aluminum plate using an eddy current technique and made a preliminary depth assessment by frequency sweeping. Although developments in electronic gradiometers and gradiometric SQUID's should make the use of single bare magnetometer SQUID's unnecessary, we show that these already have sufficient sensitivity for NDE research, even without flux-focusing washers or pick-up coils     

A 75-ch SQUID Biomagnetometer System for Human Cervical Spinal Cord Evoked Field

A 75-ch SQUID biomagnetometer system for the measurement of the cervical spinal cord evoked magnetic field (SCEF) was developed for the purpose of the noninvasive functional diagnosis of the spinal cord. The sensor array has 25 SQUID vector sensors arranged along the cylindrical surface to fit to the shape of the subject's neck. The magnetic fields, not only in the direction radial to the subject's body surface but also in the tangential direction, are observed in the area of 80 mm times 90 mm at one time. The dewar has a unique shape with a cylindrical main body and a protrusion from its side surface. The sensor array is installed in the protruded part. This design is optimized to detect magnetic signals at the back of the neck of the subject sitting in a reclining position. We applied the developed SQUID system to the cervical SCEF measurement of normal subjects who were given electric pulse stimulation to their median nerves at the wrists. The evoked magnetic signals were successfully detected at the cervixes of all subjects. A characteristic pattern of transition of the SCEF distribution was observed as a reproducible result and the signal components propagating along the spinal cord were found in the time varying SCEF distribution. We expect that the investigation of the propagating signal components would help to establish a noninvasive functional diagnosis of the spinal cord.     

High Tc SQUIDs with two or three junctions for application in disturbed environment

Different dc SQUIDs for galvanometer type high T_c SQUID sensors are investigated. The aim is to achieve a high flux-to-voltage transfer function and good sensitivity to the current originated in a directly coupled pickup loop. Furthermore, the stability against external fields is investigated. For these purposes 2 and 3 junction SQUIDs with various layouts are compared. The 3 junction SQUID showed no advantage for the application in a galvanometer type SQUID sensor. For the layout slim and long SQUID loops give best results in all respects.     

Signal amplification and signal to noise ratio improvements in thermally switching SQUIDs

The response of the circulating screening current to applied magnetic flux in a variety of DC SQUIDs has been studied in a regime in which thermal noise induces rapid switching between the internal flux states of the SQUID. We observe an unexpected jump of 10 dB to 25 dB in the amplitude and signal to noise ratio (SNR) at the output of the SQUID in response to input signals of frequency below the knee of the switching spectrum. The magnitude of the gain in SNR has been measured as a function of both barrier height and energy difference between local minima of the SQUID energy potential revealing new features of SQUID behavior. A new analysis is put forth for the DC SQUID which is able to reproduce the key features of these observations.     

Fundamental characteristics of the QFP measured by the DC SQUID

The fundamental characteristics are described of the quantum flux parametron (QPF), measured by a method in which the output signals of the QFP are detected with a DC SQUID. The DC SQUID linearly and continuously converts the output current of the QFP to voltage, allowing the output signal of the QFP to be measured as the voltage of the DC SQUID. The fundamental characteristics of the QFP have been experimentally confirmed in detail     

Impact of noise of linearity of SQUID feedback loops at high slewrate

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_N =s/s_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_n⩽1     

基于HTc rf SQUID的通信传感器技术

超导量子干涉仪(Superconducting Quantum Interference Devices,简记为SQUID)是迄今为止灵敏度最高的磁场传感器,高温超导射频量子干涉仪(HTc rf SQUID)更以其良好的实用性而备受业界关注。将HTc rf SQUID技术引入到低频段通信中,可以有效解决目前低频段通信中收信天线设备笨重、灵敏度不高的问题。文中分析了超导技术发展现状和HTc rf SQUID技术原理,总结了该技术的特点和应用于低频段通信的优势,结合CSAMT试验方法验证了应用HTc rf SQUID进行低频段收信的可行性,提出了HTc rf SQUID实用化的技术途径。     

Study of the magnetic coupling between a SQUID and wires integratedon a chip

Magnetic coupling between a SQUID and wires in an integrated circuit has been studied. Using test integrated circuits, the coupling is measured by varying the distance from the SQUID to each wire. The coupling between the SQUID and the wires with and without a groundplane decreases with an increase in the distance. The coupling for the wires with the groundplane is smaller than that for the wires without the groundplane at distances less than 595 μm. However, the rate of the decrease in the coupling for the wires with the groundplane falls off for distances more than 595 μm and the coupling converges to around 2.9 pH. From two dimensional simulations for the magnetic flux coupling, the origin of the residual inductance is found to be the coupling between the SQUID, and shielding current in the groundplane. The value of the distance, 595 μm, does not depend on the size of the SQUID. To decrease the coupling, the use of wires with stripline structure or coplanar structure is desirable     

Double relaxation oscillation SQUID with reference junction for biomagnetic multichannel applications

We have fabricated a four-channel SQUID gradiometer system based on double relaxation oscillation SQUIDs (DROSs) with a reference junction. For a biomagnetic multichannel system, we simplified the DROS design by using a single reference junction instead of the reference SQUID. The SQUIDs were fabricated from hysteretic Nb/AlO_x/Nb junctions using a simple four-level process. The DROSs provided very large flux-to-voltage transfers of typically 3 mV/Φ₀, enabling direct readout by a room-temperature dc preamplifier, and consequently simple flux-locked loop electronics were used. To realize a compact and reliable gradiometer, a first-order planar pickup coil was integrated on the same wafer with the SQUID. The flux noise of the gradiometer is about 5 μΦ₀/√Hz at 100 Hz, corresponding to a field noise of 8 fT/√Hz, measured inside a magnetically shielded room. A compact four-channel planar gradiometer system was implemented and operated to measure auditory evoked fields.     

Advances in SQUID magnetometers

The operation and noise limitations of de and RF SQUID's are outlined, and recent advances in their sensitivity are discussed. A model of the dc SQUID predicts an energy noise level per hertz referred to the SQUID of approximately8 k_{B}TL/R approx 8 k_{B}T(piLC)^{1/2}, whereL, R,andCare the SQUID inductance and the shunt resistance and capacitance of each Josephson junction. Some examples of dc SQUID's are described to show that their performance is generally in reasonable agreement with the model. The noise energy has improved from about 2 × 10^-30J. Hz^-1for a device withL = 1nH and a tunnel junction area of 10⁴µm²to about 2 × 10^-33J . Hz^-1for a device withL = 0.1nH and a microbridge resistance of 40 Ω. Further improvements axe expected in the near future. The model of the RF SQUID predicts a noise energy per hertz referred to the SQUID of[(pialpha^{2}phimin{0}max{2}/2L) + 2 pi alpha k_{B}Tmin{a}max{(eff)}]/omega_{RF}, where α is the intrinsic width of the distribution of flux transitions,Tmin{a}max{(eff)}is an effective amplifier noise temperature, and ωRFis the pump frequency. With one exception, the performance of the seven types of RF SQUID listed is in reasonable agreement with the model. The noise energy ranges from about 1.5 × 10^-29J . Hz^-1for a 20-MHz toroidal SQUID to 3.5 × 10^-31J . Hz^-1for 9-GHz reentrant toroidal SQUID; a somewhat better sensitivity has been reported for a 430-MHz device, apparently in conflict with the theory. In both dc and RF SQUID's, 1/fnoise (fis frequency) is likely to extend to higher frequencies as the white-noise level is decreased.     

Excess low-frequency noise in YBCO rf SQUIDs in weak magnetic fields

Thin-film HTS SQUIDs operated at 77 K and exposed to weak magnetic fields exhibit significant excess low-frequency noise arising from thermally-activated hopping of flux trapped in the superconducting film. We report an investigation of the dependence of this phenomenon on SQUID design and fabrication, measurement conditions and magnetic field history. The level of excess noise was directly related to the amount of flux penetrating the SQUID, and consequently was worse in large SQUIDs than in small SQUIDs due to the greater flux focussing of the larger SQUID. In SQUID fabrication, good film quality (high J_c) was found to be essential to minimize low frequency noise and careful patterning was required to avoid degrading the film. The method of cooling the SQUID was found to strongly affect the level of excess noise, with cooling in the magnetic field in which the SQUID was to be operated being preferable to zero-field cooling. The excess noise was typically 10 pTHz^−1/2 at 1 Hz for 150 pH rf washer SQUIDs having a white noise floor of about 1 pTHz^−1/2 operated in an applied field of 50 μT.