Dc SQUID System Research Articles

Highly scalable readout electronics for large multi-channel dc-SQUID systems

SQUID instruments for biomagnetic diagnostics, radiometry and astrophysics are often equipped with a large number of devices. For systems with more than a few tens of sensors, read-out with an equivalent number of independent components is not an appropriate solution to meet both, technical and financial requirements. In such cases, specifically tailored readout electronics are typically used. In this presentation, the general concept of a new highly scalable and flexible electronics is presented which is intended for operation of up to several 1000 SQUID channels in the linearizing flux locked loop mode. A prototype version of the electronics for the readout of a low-noise biomagnetic system involving 72 SQUID sensors is demonstrated. Measurement results regarding gain, bandwidth, noise, and power consumption will be presented. Finally, we discuss other potential applications in magnetometry and radiation detection with transition edge sensors or metallic magnetic calorimeters.

Practical dc SQUID system: Devices and electronics

In order to set up a liquid helium-cooled practical dc SQUID system with acceptable noise figure, we employ a weakly damped dc SQUID with a large flux-to-voltage transfer coefficient ∂V/∂Ф in a direct readout scheme (DRS) without flux modulation. Two preamplifiers are utilized: (1) AD 797 for a so-called “single chip readout electronics” (SCRE); (2) 6 parallel-connected bipolar transistors (PCBT). The latter reduces the preamplifier voltage noise Vn but increases its current noise In, which plays a leading role in the system low-frequency noise. We introduce a current feedback circuit (CFC) consisting of an inductor Li coupled to the SQUID with mutual inductance Mi to improve the noise performance. In this work, the preamplifier In contribution and CFC are analyzed. To evaluate the In suppression with CFC, two criteria are presented. Furthermore, we establish a dimensionless parameter ξ to describe CFC quantitatively. The system noise is compared with and without CFC using the two preamplifiers. For a dc SQUID with a loop inductance of 350pH, an intrinsic noise of about 5μФ0/√Hz and a corner frequency at 2Hz are measured using PCBT with CFC.

Micro-coil detection of nuclear magnetic resonance for nanofluidic samples

We have developed a novel dc SQUID system with a micro-coil input circuit to act as a local probe of quantum matter and nanosystems. The planar niobium micro-coil pickup loop is located remotely from the SQUID, coupled through a superconducting twisted pair. A high degree of coupling between the coil and the region of interest of similar dimensions (up to ∼ 100 microns) can be achieved. We report nuclear magnetic resonance (NMR) measurements to characterise the sensitivity of these coils to 3He in the gas phase at 4.2 K in a 30 mT magnetic field.

Application of LTS-SQUIDs in Nuclear Measurement Techniques

Low temperature superconducting quantum interference devices (LTS SQUIDs) are used to make precision measurements of electromagnetic fields in applications ranging from biomedicine to high energy physics. We have previously described an LTS SQUID-based device for nuclear physics which employs the Cryogenic Current Comparator principle (CCC). The CCC consists of a high-performance LTS DC SQUID system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. Theoretical investigations show that as external noise decreases, improvements in performance depend on the properties of the ferromagnetic core material embedded in the pick-up coil. Here we present the temperature- and frequency-dependence of several candidate ferromagnetic and nanocrystalline materials. We discuss these results in light of the optimization of the CCC sensor performance.

Percutaneous method for multiple epicardial monophasic action potential recordings during magnetocardiographic mapping in intact rats

A surgical approach for subdiaphragmatic-programmed electrical stimulation (PES) in mice has been reported by Gutstein et al. [D.E. Gutstein, S.B. Danik, J.B. Sereysky, G.E. Morley, G.I. Fishman, Subdiaphragmatic murine electrophysiological studies: sequential determination of ventricular refractoriness and arrhythmia induction, Am J Physiol Heart Circ Physiol. 285 (3) (2003) H1091-96.]. We developed a percutaneous method to drive a patented magnetocardiography-compatible amagnetic catheter (AC), specifically designed for multiple monophasic action potentials (Multi-MAP) recording, at the epicardium of spontaneously breathing small animals. The aim of this study was to test the feasibility of simultaneous magnetocardiographic (MCG) estimate of ventricular repolarization (VR) and Multi-MAP recording in 5 rats. Under fluoroscopic control, the AC was introduced with a sub-xyphoid puncture, in all animals, and moved at several epicardial sites until 4 stable MAPs were recorded, with a fixed inter-electrode distance of 1 mm. An unshielded 36-channel DC-SQUID system (sensitivity 20 fT/Hz 1 / 2) was used to record the MCG. MAP signals, differentially amplified and filtered at DC-500 Hz, were digitized at 1 kHz. The same AC was used for programmed PES at pacing cycles between 200 and 250 ms. Ventricular effective refractory period (VERP) was evaluated with the accuracy of 2 ms. MAP duration (MAPd) was measured, at 90% levels of repolarization. Simultaneous MCG and MAP recordings were successful in all animals. One rat died for respiratory arrest. Four animals tolerated well the procedure and survived. In WRs, MAPd 90% and VERP were 67 ± 9.4 ms and 69.6 ± 5.6 ms, respectively. This minimally invasive method is well tolerated, avoids animal sacrifice, can be used to validate the accuracy of surface MCG estimate of dispersion of VR and for assessment of arrhythmogenic potential of new drugs.

Magnetic shielding effects of a superimposed cylinder composed of BPSCCO and soft-iron cylinders: Dependencies of the air gap of the superimposed cylinder and the thickness of the soft-iron cylinder

Abstract The behavior of the magnetic flux density B in within a superimposed cylinder, constructed by the superposition of a soft-iron cylinder over a BPSCCO cylinder, is unknown. The present research examines and clarifies the effect of B in on the changes of the wall thickness W of the soft-iron cylinder, and the air gap distance δ between the soft-iron and BPSCCO cylinders. In addition, the magnetic noise power spectra of B in are examined for various values of δ and W , by the use of an HTS dc-SQUID system and a spectrum analyzer. The results lead to an important criterion for the design of an effective magnetic shielding vessel.

Analysis Theory of Asymmetric dc SQUID Driven by Thermal Noises**The project supported by Fok Ying Tung Education Foundation of China under Grant No. 71007 and National Natural Science Foundation of China under Grant No. 10275026

An analysis expression for the stationary probability distribution of asymmetric superconducting quantum interference device with two Josephson junctions (dc SQUID) driven by thermal noise is derived from two-dimensional Fokker–Planck equation, where the potential condition is satisfied. Two of the three asymmetric parameters, inductance asymmetric parameter η and critical current asymmetric parameter α, can be changed in this condition, but resistance asymmetric parameter ρ cannot be changed. The “ripple” phenomenon of circulating current can disappear with the change of coefficient α. The effects of asymmetric parameters on current-voltage relationship and transfer function of dc SQUID system are also represented.

High- Tc DC SQUID system cooled by pulse-tube cooler

We developed a high- T c DC SQUID system cooled by pulse-tube cooler. To avoid the influence of the wire resistance between SQUID and preamplifier, and to reduce the influence of the temperature fluctuation of pulse-tube cooler, DC coupling between SQUID chip and preamplifier was used and the flux locked loop worked in modulation mode. We also developed a temperature controller, using the DC SQUID as temperature sensor, to control and stabilize the operating temperature of the pulse-tube cooler. With the temperature controller, the DC SQUID system could remain locked for over 8 h.

Cryogenic current comparator for absolute measurements of the dark current of superconducting cavities for TESLA

A new SQUID based measurement system for detecting dark currents, generated by the TESLA (Tera Electron Volt Energy Superconducting Linear Accelerator) cavities is proposed. It makes use of the Cryogenic Current Comparator principle and senses dark currents in the nA range. To reach the maximum possible energy in the TESLA project is a strong motivation to push the gradients of the superconducting cavities closer to the physical limit of 50 MV/m. The field emission of electrons (dark current) of the cavities at strong fields may limit the maximum gradient. The absolute measurement of the dark current in correlation with the gradient will give a proper value to compare and classify the cavities. This contribution describes a Cryogenic Current Comparator (CCC) as an excellent tool for this purpose. The most important component of the CCC is a high performance DC SQUID system which is able to measure extremely low magnetic fields, e.g., caused by the extracted dark current. For this reason the SQUID input coil is connected across a special designed pick-up coil for the electron beam. Both the SQUID input coil and the pick-up coil form a closed superconducting loop so that the CCC is able to detect dc currents down to 1 nA//spl radic/Hz. Design issues and the application for the CHECHIA (horizontal test cryostat) cavity test stand at DESY are discussed.

An analysis theory of symmetric dc SQUID driven by thermal noises

An analysis expression for the stationary probability distribution of the symmetric superconducting quantum interference device with two Josephson junctions (dc SQUID) driven by thermal noise is derived from two-dimensional Fokker–Planck equations where the potential condition is satisfied. The analytical formulae for SQUID characteristics, e.g. the circulating current, the current–voltage relationship and the transfer function, are obtained by using the probability distribution. The ‘ripple’ phenomenon of the circulating current versus the applied flux, the effects of thermal fluctuations on current–voltage relationship and the optimum operating condition of dc SQUID system are represented.

Magnetic shielding effects of the superposition of a ferromagnetic cylinder over an HTS cylinder: magnetic shielding dependence on the air gap between the BPSCCO and soft-iron cylinders

Ideal magnetic shielded vessels can be realized by making use of a high-critical temperature superconductor (HTS). Use of such shielding does not, however, generally satisfy the maximum shielded magnetic flux density B s required of an HTS vessel for practical applications. The present authors have improved the value of B s by the superposition of a ferromagnetic cylinder over an HTS cylinder, termed the superimposed cylinder. The evaluation of the magnetic shielding effects was limited to the superimposed cylinder, in order to simplify the theoretical analysis. Nevertheless, little is known of the resulting characteristics and evaluation procedures when changing the size of the air gap δ between the HTS and ferromagnetic cylinders for applications of an external magnetic field B ex less than the value of B s. The values of B s were systematically measured by for different values of δ, and in addition, were theoretically analyzed. It was found that the experimental values of B s agreed well with theoretical values. Furthermore, measurements were taken of the magnetic noise power spectrum for various values of δ, with the use of the HTS dc-SQUID system and spectrum analyzer. From these results, the characteristics of the magnetic behavior for different values of δ within the superimposed cylinder were clarified. In addition, an important criterion is discussed for the design of an effective magnetic shielding vessel.

Nonadiabatic Geometric Quantum Computation with Asymmetric Superconducting Quantum Interference Device**The project supported in part by National Natural Science Foundation of China under Grant No. 19975036, and the Foundation of the Science and Technology Committee of Hunan Province of China under Grant No. 21000205

We propose a method of controlling the dc-SQUID (superconducting quantum interference device) system by changing the gate voltages, which controls the amplitude of the fictitious magnetic fields , and the externally applied current that produces the piercing magnetic flux for the dc-SQUID system. We have also introduced a physical model for the dc-SQUID system. Using this physical model, one can obtain the non-adiabatic geometric phase gate for the single qubit and the non-adiabatic conditional geometric phase gate (controlled NOT gate) for the two qubits. It is shown that when the gate voltage and the externally applied current of the dc-SQUID system satisfies an appropriate constraint condition, the charge state evolution can be controlled exactly on a dynamic phase free path. The non-adiabatic evolution of the charge states is given as well.

HTS dc SQUID systems for geophysical prospection

We describe the use of high T/sub c/ flip-chip dc SQUID magnetometers in systems for several geophysical prospection methods. For use in the Transient Electro Magnetics (TEM) method, a one-channel SQUID magnetometer system (vertical field component) was adapted to rough conditions in the field. We report on measurements in South Africa over a geophysically interesting target in real production mode. Transients up to 200 ms could be recorded, which is about a factor of 100 more than with conventional coils. For investigation of the magnetic field gradient a two-channel system, built as a first-order electronic gradiometer with a baseline up to 1 m, was designed and manufactured. It can be used, e.g., for surface exploration or for archeometry. The system can be moved in the Earth's magnetic field. We achieve a common mode rejection of 100. The reasons for the imbalance and the possibility of further improvement of the system are discussed.

Low-noise biomagnetic measurements with a multichannel dc-SQUID system at 77 K

Directly coupled dc-SQUID magnetometers made from the high-temperature superconductor (HTS) YBa/sub 2/Cu/sub 3/O/sub 7/ on symmetric 24/spl deg/ and 30/spl deg/ SrTiO/sub 3/-bicrystals have been optimized for low-noise operation in static magnetic fields. These magnetometers are encapsulated, for reliable operation in liquid nitrogen. In our small glass cryostat, allowing even mobile applications, up to 24 magnetometers can be mounted in three levels for gradiometer arrangements. With first-order gradiometers an external noise suppression of more than a factor of 100 is achieved. In a magnetically shielded chamber we find a typical noise /spl radic/S/sub B/ (77 K, 10 Hz) of less than 50 fT//spl radic/Hz and low-noise magnetocardiograms are recorded. Due to the low noise of the dc-SQUID sensors also electrically stimulated magnetoencephalography measurements were possible.

An HTS dc SQUID system for geomagnetic prospection

We have developed a highly sensitive dc SQUID system for geomagnetic prospection operating at 77 K. Because of the large voltage swing of our flip chip magnetometers it is possible to use a directly coupled SQUID electronics. Thus, the requirements for TEM measurements can be fulfilled: slew rate >1 mT s-1 and dynamic range >120 dB. In measurements above real geological targets the significantly better performance of the SQUID compared to conventional coils has been demonstrated. The lower noise of the SQUID system at late times allows us to measure the transients for 5 to 10 times longer, which results in an increase of the investigation depth by a factor of 2-3. For the same data quality a reduction of the measurement time between 25 and 100 compared to coils is possible.

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.

High-resolution magnetoneurography

Abstract Low noise dc-SQUID systems allow noninvasive measurements of magnetic fields generated by electric currents in human peripheral nerves. High-resolution magnetoneurography techniques were used to detect the magnetic fields of stimulated leg nerves with amplitudes of only 10–20 fT over the lower spine. A signal-to-noise ratio of about 10 was achieved after a special designed signal processing routine which was sufficient for an analysis of the source current, i.e. in particular the estimation of its location, strength, and spatial extent.

Mapping of tibial nerve evoked magnetic fields over the lower spine

Using a low-noise 49-channel dc-SQUID system spinal somatosensory evoked fields (SEF) were recorded which were generated by compound action currents evoked upon posterior tibial nerve stimulation. The SEF mapping showed the action current propagation along the sciatic nerve, lumbosacral plexus and cauda equina in parallel to simultaneously recorded electrical potentials (SEP). For a reliable intraindividual side-to-side comparison of spinal SEFs the right and left tibial nerves were stimulated in alternating order; this procedure minimizes artifactual inter-nerve SEF map differences due to eventual patient-to-sensor displacements which might occur in serial measurements. These large-area lumbar SEF mappings open up several clinical perspectives for magnetoneurography, in particular with respect to the 3D-localization of proximal conduction blocks.

Scanning DC-SQUID system with high spatial resolution for NDE

We have developed a scanning DC-SQUID system with a high spatial resolution for NDE. The SQUID system is composed of a concentric multiloop DC-SQUID gradiometer using Nb/AlOx/Nb Josephson junction, a non-magnetic scanning stage, a cryostat and a computer for gathering data and controlling the system. We have developed the concentric multi-loop gradiometer (d/sup 2/Bz/dr/sup 2/, r/sup 2/=x/sup 2/+y/sup 2/) to have a higher spatial resolution and reduce background noise. The gradiometer was integrated on a single 3/spl times/3 mm/sup 2/ Si chip. To reduce the standoff distance between the specimen and the pick-up coil, a cryostat with a thin bottom space was developed. A scanning stage was made of nonmagnetic materials and was driven by air pressure using a rod-less cylinder to avoid making magnetic noise.

High-T/sub c/ DC-SQUID system for nondestructive evaluation

We developed different types of thin film high temperature superconductor (high-T/sub c/) DC-SQUID magnetometers and gradiometers for application in a nondestructive testing system. We used these sensors in a liquid nitrogen dewar mounted above a computer controlled x-y table within a compact /spl mu/-metal shielded chamber. The planar galvanically coupled gradiometers based on step-edge or bicrystal Josephson junctions were investigated in shielded and unshielded environment. We discuss the influence of junction type and SQUID parameters depending on the device geometry on the sensitivity of our complete testing system. Our interest is focused in particular on the noise properties and magnetic field resolution at 77 K. Measurements of ac and dc current distributions will be shown as well as measurements of magnetic moments of different samples.