High-temperature Superconducting SQUID Research Articles

Estimation of Critical Current of HTS RF-SQUID

The operation of the RF SQUID is restricted by the condition that the inductance parameter β L must be in the range of 1−3. However, since both ends of the Josephson junction (JJ) of RF-SQUID are shorted, it is difficult to non-destructively estimate the critical current (IC ). Thus, we proposed a technique for the non-destructive measurement of the IC of a high-temperature superconducting (HTS) RF-SQUID ring by evaluating the behaviour of the flux in superconducting thin films using a SQUID magnetometer. A superconducting ring sample with JJ was placed below the HTS SQUID magnetometer and cooled down to 77 K. The change in the SQUID output was monitored on application of the magnetic field. When increasing the field, the waveforms indicated that the screening current of the ring sample exceeded the I C of the JJ, and the JJ became a normal-conducting state. As a result, we estimated the IC of the JJ of this sample as 134 μA using the values of mutual inductance and the coupling coefficient α between the coil and the sample.

Quantum interference effects in 1D parallel high-Tc SQUID arrays with finite inductance

The quantum interference effects of one-dimensional (1D) parallel arrays of high-temperature superconducting (HTS) SQUIDs were investigated experimentally and theoretically via the voltage–magnetic field responses for 4–81 Josephson junctions. The sensitivity of the arrays generally decreased as the number of junctions (and SQUIDs) in parallel increased, contrary to the predictions of models in the low (zero) inductance limit. A full theoretical description was developed to describe 1D parallel HTS SQUID arrays with finite inductances in an applied magnetic field, by extending the model for a single DC SQUID to multiple loops in parallel and including the flux generated by currents circulating through all loops in the array. Calculations were extended from SQUID arrays with equal loop areas to arrays with a distribution of loop areas, otherwise known as superconducting quantum interference filters. The model uses parameters relevant to HTS arrays, including typical variations (up to 30%) in HTS Josephson junction parameters, such as critical current and normal resistance. The effect of the location of the current biasing leads was also explored through the calculations. This model shows good agreement with experimentally measured 1D arrays of different lengths and highlights the importance of the geometry of the current biasing leads to the arrays when optimizing the array response.

Magnetomodulating Magnetometer Based on High-Temperature Superconductors

A magnetomodulation magnetic field sensor (MM-MFS) based on a high-temperature superconducting (HTSC) ceramic material in the Bi-2223 system was investigated. A cylindrical HTSC ceramic rod with a length of 10 mm and a diameter of 2 mm was used as a magnetically sensitive element. The MM-MFS operates in a differential mode, which automatically eliminates the effect of odd harmonics in the signal coil. The second harmonic response signal and magnetosensitivity of the sensor depend linearly on the frequency of the excitation field. The parameters of the sensor were experimentally determined, including a magnetosensitivity of about 500 V/T, a threshold sensitivity of about 20 pT, a magnetic field measurement range of approximately ±0.7 mT, and a dynamic measurement range of about 120 dB in the area ±15 µT. According to the estimates, the optimal choice of number of turns, selectivity, and identity of two parts of the excitation coil can significantly reduce the MM-MFS threshold sensitivity to below 1 pT. The MFS-MM can compete with HTSC SQUIDs in biomedical applications for noninvasive detection of magnetic particles and in various implantable devices and artificial organs.

SQUID NDE on Braided Carbon Fiber Reinforced Polymers With Middle-End Fibers Under Step-by-Step Tensile Loading

Three braided carbon fiber reinforced polymer (CFRP) samples with different middle-end fibers were fabricated. Damage mechanisms of the samples under step-by-step tensile loading were investigated using a nondestructive evaluation (NDE) method utilizing a high-temperature superconducting SQUID gradiometer. Commercial carbon fibers UTS50 were used for base braided fabrics, while UTS50, XN05 with higher electric resistivity, and XN60 with lower resistivity were used for the middle-end fibers. In the step-by-step tensile tests, observation of surfaces and measurements of stress-strain curves of the samples were carried out. At certain damage stages, the SQUID NDE method, which visualizes flowing currents in the samples, were applied to detect damage and assess the integrity of the carbon fiber yarns in the samples after removal of the loading. As a result, different damage mechanisms and stress-strain curves were observed and measured. From the results using the SQUID NDE method, it was shown that the current distributions in the samples were determined by the middle-end fibers at respective virgin stages, and the middle-end fibers XN60 were broken in some parts of the sample at its final fracture.

HTS planar gradiometer consisting of SQUID with multi-turn input coil and large pickup coil made of GdBCO coated conductor

We have investigated the fabrication of a high-temperature superconducting (HTS) gradiometer with long baseline for geophysical applications. The proof-of-concept gradiometer using a 1-turn pickup coil made of a GdBa2Cu3Oy coated conductor (GCC) and 5.5-turn input coil integrated on a SQUID was fabricated in our previous work. In this study, we have optimized the device structure to improve the frequency response, gradient field sensitivity and gradiometer balance. The fabricated flux transformer consists of a 6-turn planar gradiometric pickup coil and a 26-turn input coil made of an HTS thin film. A low-melting-point alloy was used to connect polished Ag surfaces of the CGG pickup coil and Au pads of the input coil. An HTS SQUID was formed on another substrate and stacked on the input coil. A mechanical balancing structure using three pieces of GCC as a superconducting shield was also implemented. The fabricated gradiometer showed a gradient field noise of 0.8fT/cmHz1/2 in the white noise regions, a gradiometer balance of 1/142, and a cutoff frequency of 9Hz corresponding to a 2mΩ contact resistance between the pickup coil and the input coil.

Observation of Polycrystalline Solar Cell Using a Laser-SQUID Microscope

Laser-SQUID microscopy is a technique for nondestructive inspection of the electrical properties of semiconductors. In laser-SQUID microscopy, a photocurrent is induced by a laser with an energy larger than the band gap of the semiconductor sample. The magnetic field induced by this photocurrent is detected by a SQUID. In our experiment the laser was focused on the surface of the sample, and a high-temperature superconducting SQUID was positioned behind the sample to detect the magnetic field from the photocurrent. The sample was raster scanned while the relative position of the SQUID and laser spot was fixed. The sample used was a commercially available polycrystalline silicon solar cell containing a p-n junction. It had several line electrodes and a transport electrode layer on the surface, and the reverse side was covered with a metal electrode. We studied a piece of this polycrystalline solar cell using a 1065 nm laser. Magnetic field induced by the laser was observed successfully. The produced image shows the inhomogeneity of the solar cell.

Development of new HTS-SQUID and HTS current sensor for HTS-SQUID beam current monitor

Two years ago, a prototype of a highly sensitive beam current monitor with a high-temperature superconducting (HTS) SQUID, an HTS current sensor and an HTS magnetic shield, that is, an HTS-SQUID monitor, was installed in the beam transport line of the RIKEN ring cyclotron (RRC). As a result, the beam intensity of a sub-μA beam was successfully measured by the prototype HTS-SQUID monitor. In fact, the intensity of a sub-μA 40Ar15+ (63 MeV/u) beam was successfully measured with a 500 nA resolution. However, the current resolution of the prototype HTS-SQUID monitor is not sufficient to measure the current of a uranium beam, which is accelerated in a new radioactive isotope (RI) beam facility called "RI Beam Factory" (RIBF). A minimum current resolution of 1 nA is required for the measurement of the uranium beam. Therefore, we are developing a new HTS-SQUID monitor so as to improve the current resolution. This new monitor consists of three parts, the HTS SQUID, an HTS current sensor and an HTS magnetic shield, and these parts have been separately developed this year. The high-permeability core that is installed in the two input coils of the HTS-SQUID is an extremely important part in this new HTS-SQUID monitor. A 50-fold improvement in gain was successfully realized using the high-permeability core compared with that obtained without the high-permeability core. Another key factor is the substrate of the HTS current sensor. A MgO ceramic tube was used for the substrate of the HTS current sensor in the prototype HTS-SQUID monitor. However, it was difficult to form the bridge circuit using the MgO ceramic substrate in the new HTS-SQUID monitor, because the bridge circuit that magnetically connects the HTS current sensor and the HTS-SQUID has to be three-dimensional. To solve this problem, silver (Ag) of 99.9% purity was adopted for the substrates of the HTS current sensor in the new HTS-SQUID monitor. Then the surfaces of the substrates were coated by a thin layer (70μm) of Bi2-Sr2-Ca1-Cu2-Ox (Bi 2212), which is an HTS material. We report the results of this development.

HTS SQUID NDE of Curved Surfaces Using Background Field Cancellation Techniques

Single-layer high-temperature superconducting SQUID gradiometers are well established for measuring small localized magnetic fields in the presence of much larger background interference. Such sensors have proved extremely useful for eddy-current nondestructive evaluation (NDE) of aircraft and other metallic structures. Most applications to date have involved either the sensor remaining stationary or being moved in a straight line in an unshielded environment. However it is desirable for many field mapping applications for the sensor to be rotated or tilted in the Earth's field whilst retaining full device sensitivity. We report on the development and performance of a portable NDE scanning system which uses background field cancellation techniques to allow the device orientation to change whilst mapping the surface of a curved wing structure.

Current and position beam monitor with HTS SQUIDs and HTS magnetic shield for the RIKEN Ring Cyclotron

This year, a prototype of a highly sensitive beam current monitor with a High- Temperature Superconducting (HTS) SQUID and an HTS magnetic shield: HTS-SQUID monitor has been installed in the beam transport line of the RIKEN Ring Cyclotron (RRC). Unlike other existing facilities, the HTS-SQUID monitor allows us to measure the DC of highenergy heavy-ion beams nondestructively in such a way that the beams are diagnosed in real - time and the beam current extracted from the RRC can be recorded without interrupting the beam user's experiments.

High-speed operation of SQUID–array-type interface circuits using a cryocooler

We have evaluated the operation of high-temperature superconducting SQUID–array- type interface circuits with normal-metal control lines. Transimpedance amplification with the operating speed up to 1 Gbps was demonstrated by using a cryocooler. The influence of the number of SQUIDs connected in series and parallel on the output level from the SQUID–array interface circuits was examined by Josephson circuit simulation and Monte-Carlo simulation. It was found that the configuration of two parallel SQUID–array with 64 SQUIDs gives the highest output signals when the junction characteristics in the arrays have a certain spread. We have fabricated the interface circuits by the conventional interface-engineered junction process. The process reproducibility was 100 μA ± 25% for the junction I c, 3.02 pH ± 5% and 2.57 pH ± 17% for the sheet inductance values of the counter- and base-electrodes, respectively. The transimpedance at low frequencies reached 20 and 4 V/A for the input levels of 20 and 100 μA, respectively. Output voltages as high as 4.4 mV at 4.2 K and 2.3 mV at 40 K were obtained. Furthermore, an output voltage of 600 μV was obtained for the 1 Gbps 2 15−1 pseudo random binary signal input at 40 K.

ＳＱＵＩＤと超伝導磁気シールドを用いた高感度ビーム電流モニターの開発

Beam diagnostics are an essential constituent of any accelerator. There are a large variety of beam parameters, and total current is one of the most important parameters for accelerators. A current monitor is used to operate an accelerator efficiently and to improve the performance of the machine. A Faraday cup is the most fundamental current detection process, in which charged particles are stopped in the cup. However, this destructive method cannot be applied for high-current or high-energy beams because the total energy carried by the beam can destroy and activate the intercepting material. Therefore, non-destructive beam current measurement requires the use of current transformers that detect the magnetic field produced by the pulsed or DC beam. On the other hand, a new type of beam current monitor using a low-temperature superconducting (LTS) magnetic shield and an LTS SQUID was developed to measure the faint ion beams that are below the lowest measurable limit of the DC current transformer (DCCT) for atomic-physics studies. Recently, a prototype of a highly sensitive SQUID current monitor for measuring the intensity of faint beams, such as radioisotope beams, was completed for the RIKEN RI beam factory. This monitor is composed of a high-temperature superconducting (HTS) magnetic shield and an HTS SQUID. The first measurements using ion beams were carried out in the CNS experimental hall and RIKEN Ring Cyclotron (RRC). This paper first describes the principle of the conventional current monitor such as the Faraday cup and current transformers. Second, the progress of the LTS SQUID current monitor is discussed, and finally, the present status of the prototype of the HTS SQUID current monitor at RIKEN and the results of the first beam measurement are given.

Non-destructive evaluation with a linear array of 11 HTS SQUIDs

A linear array of 11 High Temperature Superconducting (HTS) SQUIDs was used for non-destructive evaluation (NDE) applications. The array consists of 11 SQUID magnetometers arranged linearly along a single substrate with 0.75 mm spacing. The SQUIDs have 105 nT//spl Phi//sub 0/ field sensitivity and <20 /spl mu//spl Phi//sub 0///spl radic/Hz noise values at 1 kHz with DC bias current. We used an eddy current NDE technique. The eddy current induction coils were arranged such that there was a null in the induction field at the SQUIDs. Single Frequency and white-noise (frequency-continuum) induction schemes were used. Both SQUIDs and induction coils were placed at the bottom of a liquid nitrogen dewar with a 4 mm hot-to-cold distance. Flawed and unflawed samples were scanned beneath the array. The phase and amplitude of the SQUID's response relative to the induction signal were acquired. This paper presents experimental results and their interpretation.

Two methods for a first order hardware gradiometer using two HTS SQUIDs

Two types of first order electronic gradiometers have been developed using high temperature superconducting (HTS) SQUIDs. Gradiometry is accomplished in hardware by either 1) subtracting the output of the signal and background SQUIDs in a summing amplifier (parallel technique) or 2) converting the inverted background SQUID output to a magnetic field at the sensor SQUID (series technique). Balance levels achieved are 2000 and 1000 at 20 Hz for the parallel and series methods respectively. Balance level as a function of frequency is presented. Balance level for hardware gradiometry is limited by time delays from the electronics and how well the signal amplitudes are matched. A simple algorithm that allows one to estimate the limit on balance level from these factors is presented and compared with data.

Design and preliminary results from a high temperature superconducting SQUID milliscope used for non-destructive evaluation

We present the design and preliminary results from a SQUID 'milliscope'. The device was designed for nondestructive evaluation (NDE) as part of the Enhanced Surveillance Program at Los Alamos National Laboratory. A high temperature superconducting (RTS) SQUID sensor is used to map magnetic fields induced in the sample. Eddy currents are induced in the conducting sample by a wire coil designed to produce minimal magnetic field at the SQUID when no sample is present. The features of interest are characterized by anomalies in the induced magnetic field. The goal of the instrument is sensitivity to small features generally buried under several intervening layers (/spl sim/1-20 mm) of conducting and/or nonconducting materials and robustness of design (i.e. the ability to operate in a noisy, unshielded environment). The device has primarily focussed on specific NDE problems such as the ability to detect buried seams in conducting materials and quantify the width of these seams. We present the design of the instrument, and some data to demonstrate its capabilities.

High-temperature superconducting SQUID as calibrator of ac voltage dividers and LF voltmeters

The performance of a radio-frequency SQUID made of high-temperature superconducting ceramics as a calibrator of ac voltage dividers and LF voltmeters at frequencies up to 1 MHz is investigated. The measurement technique and errors are analyzed. The obtained results suggest that in principle the high-temperature superconducting device can reproduce the voltage divider scale with an error less than 10−3 dB within an input signal range from several microvolts to several volts.