Hall Probe Array Research Articles

Current distribution monitoring enables quench and damage detection in superconducting fusion magnets

Fusion magnets made from high temperature superconducting ReBCO CORC® cables are typically protected with quench detection systems that use voltage or temperature measurements to trigger current extraction processes. Although small coils with low inductances have been demonstrated, magnet protection remains a challenge and magnets are typically operated with little knowledge of the intrinsic performance parameters. We propose a protection framework based on current distribution monitoring in fusion cables with limited inter-cable current sharing. By employing inverse Biot-Savart techniques to distributed Hall probe arrays around CORC® Cable-In-Conduit-Conductor (CICC) terminations, individual cable currents are recreated and used to extract the parameters of a predictive model. These parameters are shown to be of value for detecting conductor damage and defining safe magnet operating limits. The trained model is then used to predict cable current distributions in real-time, and departures between predictions and inverse Biot-Savart recreated current distributions are used to generate quench triggers. The methodology shows promise for quality control, operational planning and real-time quench detection in bundled CORC® cables for compact fusion reactors.

Automatic Detection of Local Obstacles in a Long Length RE-123 Coated Conductor by Deep Learning Based Image Classification in Reel-to-Reel Magnetic Microscopy

We have succeeded in detecting local obstacles automatically in a 200-m-long RE-123 coated conductor (CC) by introducing deep-learning based image recognition in reel-to-reel scanning Hall probe microscopy (RTR-SHPM). Longitudinal critical current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> ) homogeneity in CCs is one of the most important requirements for practical applications. Usually, such properties as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> variation as a function of longitudinal coordinate is characterized by magnetization measurements adopting Hall probe array as a de facto standard characterization method for ensuring uniformity in long CCs. In this measurement, local <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> drop indicates the existence of current blocking obstacles. The group of authors also developed a magnetic microscopy applicable to reel-to-reel continuous measurements, RTR-SHPM, which makes it possible to visualize two-dimensional critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> ) in the tape plane because of its high-resolution imaging along the tape width. As a result, more elaborate defect detection has been enabled. However, in the conventional technique, the observation depends on the human eye in order for characterizing detailed features of the obstacles such as shape and size especially in case with a small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> drop, therefore, there was a limit to analyze the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> mapping with such detailed information extending to thousands of images in the long tape of several 100 of meters. In this study, the image classification based on the deep learning method was introduced in our magnetic microscopy. The analytical model classifies the input image into the defect position and the normal position, respectively, together with a heat map and a score of confidence in the recognition. As a result, we have succeeded in detecting obstacles automatically from more than 4,000 of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> images with a high accuracy of 98.5%. Furthermore, we revealed the existence of the obstacles which were not distinguishable by the local <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> criterion. This method allows us to clarify the origin of the instability of long CC wire and will have a strong impact as an evaluation technique for dramatically improving the reliability of the CCs.

A Noncontact &lt;italic&gt;T&lt;/italic&gt;c Evaluation Technique Using a Hall Probe Array

A noncontact superconducting transition temperature (T c ) measurement system using a Hall probe array was devised to measure T c of high-temperature superconductor (HTS) coated conductors (CCs) without physical contact to the surface, thereby avoiding contact with electrodes, which can cause damage to the conductor surface. The variations of the Hall voltage (V) distribution, which comes from the distribution of magnetic field on the surface of the CC, under a fixed applied field at different temperatures were used to estimate T c . The measured T c of the HTS GdBCO CC made by SuNAM was approximately 93.2 K by the noncontact T c method, as compared to approximately 93.5 K by the conventional four-probe-contact method in a commercial PPMS. Thus, the T c obtained by this system is comparable with that from the transport four-probe method. In addition, the noncontact T c measurement system devised and used in this study is simple because it does not need comparison and conversion with values measured by contact methods.

Anisotropy of the coherence length from critical currents in the stoichiometric superconductor LiFeAs

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, {\epsilon}_{\xi}. This allows extraction of {\epsilon}_{\xi} at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.

Critical Current and Stability of High-${\rm J}_{\rm c}$${\rm Nb}_{3}{\rm Sn}$ Rutherford Cables for Accelerator Magnets

In the framework of future LHC upgrades using Nb3Sn magnets, supported in part by the US LHC Accelerator Research Program (LARP) and the European EuCARD program, CERN is intensifying its research on Nb3Sn Rutherford cables. In the FRESCA cable test facility at CERN, two new Nb3Sn cable samples were investigated concerning their quench and critical current as well as stability performance. The two samples are based on RRP type strands with comparable layout. They have 27 strands with 0.7 mm diameter while the strands have 54 or 108 superconducting sub-elements. The cables are 10 mm wide and have a transposition pitch of about 75 mm. The cables are used in magnets built for LARP. Their performance is measured as a function of magnetic field up to 10 T with ramp rates of 10 to 1000 A/s and temperature of 1.9 and 4.3 K. A hall probe array is present to study the current distribution. Point heaters are used to study cable stability and current redistribution.

Enhancement of the field of first flux penetration due to Andreev bound states in [formula omitted

First flux penetration into single crystalline Bi2Sr2CaCu2O8+δ is investigated using magneto-optical imaging and Hall probe array magnetometry. Below 50K, a slight enhancement of the field of first flux penetration, Hp, is found when the edge normals are oriented at 45° with respect to the principal (ab) crystal axes, rather than parallel to them. We discuss this effect in terms of a role of Andreev bound states (Iniotakis et al., 2008 [1]).

Field dependence of critical current density in flat superconductor

Surface field of a thin superconductor YBa2Cu3O7-δ in mixed state is measured by a Hall probe array. To reproduce the measured field profiles, shielding current distributions are determined by numerical iterative calculations without supposing any models for field dependence of critical current density Jc (B). Utilizing the estimated local current density and local magnetic field for x – y coordinates, a field variation of current density is plotted. Though any model for Jc (B) is not used for numerical calculations, the field variation roughly shows a dependence like Kim model.

Current Redistribution Inside ITER Full-Size Conductors Well Before Any Transition Voltage Detection

It is well assessed that the cable in conduit conductors (CICC) performances are affected by the current distribution among their strands. In many experiments, during Tcs measurements, it has been evidenced that a current re-distribution among the cable sub-stage bundles appears just before the conductor transition. This is deduced mainly from the change of the magnetic field measured in the conductor proximity, even with a constant cable transport current. This paper presents the experimental evidence that the current begins to re-distribute well before any detectable voltage development. Such a phenomenon, repeatable and depending on the overall transport current, has been observed by means Hall probe arrays. These probes have been designed with suitable sensitivity and distribution, to permit also the reconstruction of the current distribution inside the CICC by means of the solution of the inverse magnetostatic problem. Some results from the experimental data taken from the NbTi BusBar III and the Poloidal Field Insert samples, tested respectively in the TOSKA facility at FZK, and in SULTAN at PSI in 2004, are discussed here.

Double transition in PrOs4Sb12 probed by local magnetization measurements using Hall probe array

Double superconducting transition in PrOs4Sb12 has been studied carefully by local magnetization measurements using Hall probe array. Local magnetization curves for each element of the Hall probe reveal that there exists a significant difference in the transition temperature at each point; two phases which give different Tc are stabilized in the crystal. The characteristics of superconducting double transition in PrOs4Sb12 are discussed based on the local magnetization measurements at multi-points.

Detecting and describing the inhomogeneity of critical current in practical long HTS tapes using contact-free method

The nondestructive and contact-free apparatus for measuring local critical current of long high temperature superconducting (HTS) tapes is presented. The local critical current of tape is acquired by using Hall probe array sensor to measure the remanent field after exposed to dc external magnetic field since the critical current is proportional to remanent field based on Bean critical state model. A detailed experiment on multifilamentary Bi2223/Ag tape is made to validate reproducibility, reliability, resolution, nondestructiveness and usefulness for manufacturer and user of tapes. The parameter COV (coefficient of critical current variation) is suggested for quantitatively describing the inhomogeneity and quality of practical long HTS tape based on Gaussian statistical analysis. The developed apparatus can detect HTS tape at velocity of 100 m/h with resolution smaller than 3 mm in liquid nitrogen.

Trapped Field Distribution within a Cylindrical Hole in Sm-Ba-Cu-O Bulk Superconductor

To study the possibility of HTS applications in the design of magnets for particle accelerators, we investigated the trapped field distribution in a bulk superconductor. An array of Hall probes, calibrated at liquid nitrogen temperature, was used to monitor the trapped field distribution in a hole (10 mm in diameter) in a rectangular bulk superconductor (15 mm thickness) as a function of time. The bulk superconductor was placed in an external field created by a dipole magnet, where the field strength was varied up to about 1.5 T. The decay process of the trapped field was monitored using the Hall probe array. The decay speed was observed to be dependent on the initial external field applied, the distance from the edge of the material and the trapped field strength. The results are summarized in this report

Tests and Field Map of LHCb Dipole Magnet

The LHCb experiment at the Large Hadron Collider (LHC) at CERN is aimed to study CP violation and to measure the rare decays of B-mesons with exceptionally high precision. A 4 Tm dipole magnet is required for particle separation and momentum measurements. The 1600 ton warm magnet with sloping poles was installed and fully commissioned by the end of 2004. It is the first detector magnet of the four LHC experiments to have been aligned and commissioned in its final position. In this paper, the magnet installation in the underground cavern of Point 8 and its alignment on the beam line are shortly reviewed. Results of a first magnetic field mapping in the region of the magnet poles and the fringe field in the location of the RICH detectors are presented. The mechanical equipment used for the automatic displacement of the Hall probe array is described, together with the precision of the measurements obtained which are compared with TOSCA finite element calculations

Vortex pumps in the crossing lattices regime of highly anisotropic layered superconductors

It is now well established that vortex dynamics in samples with a spatially asymmetric pinning potential can lead to rectifying vortex ‘diode’ behaviour. Spatial asymmetry is not a fundamental requirement for the control of vortex motion, however, and we demonstrate that vortex ‘lensing’ is possible in highly anisotropic layered superconductors simply under the action of non time-reversible trains of in-plane magnetic field pulses. Our devices depend crucially on the existence of ‘crossing’ pancake vortex (PV) and Josephson vortex (JV) lattices in Bi2Sr2CaCu2O8+δ (BSCCO) single crystals under tilted magnetic fields. An attractive interaction between these two sub-lattices makes it possible to indirectly manipulate the PV distribution by modifying the JV lattice, and a number of functional devices based on this principle have been proposed. In our experiments a BSCCO single crystal is placed on a Hall probe array, and cooled below Tc in a small out-of- plane magnetic field. Trains of sawtooth in-plane field pulses are then applied to the system and different elements of the Hall array used to demonstrate PV lensing or antilensing behaviour, depending on the pulse shape. The mechanism leading to lensing will be discussed and results compared with molecular dynamics simulations.

Reconstruction of Current Unbalance in Full-Size ITER NbTi CICC by Self-Field Measurements

Methods have been developed to study the distribution of the transport current among the strand bundles of cable-in-conduit conductors (CICC) by using self-field measurements with Hall probe arrays. The unbalance in the transport current is mainly caused by the unavoidable nonuniformity of the joints and can be a reason for a change in the voltage-temperature characteristic and consequently of the temperature margin. In the present study we focus on the reconstruction of the current unbalance in a full size NbTi CICC tested in the SULTAN test facility. The crucial point in the reconstruction procedure is the proper choice of the reference self-field profile corresponding to a uniform current distribution. To achieve this uniform distribution, the conductors were driven far into the current sharing regime. The self-field profile corresponding to the highest achieved voltage level was taken as a reference. This assumption is supported by the modeling of current-sharing runs in the CUDI-CICC network model. Furthermore, local redistribution effects were observed in the conductors at high currents. The current transfer associated with this redistribution is analyzed as well.

Dynamic Field Mapping for Obtaining the Current Distribution in High-Temperature Superconducting Tapes

The magnetic field profile on the surface of Bi-2223/Ag tapes has been dynamically measured across the width of the samples. The experimental technique uses a Hall-probe array with 7 sensors connected to a multiple channel lock-in amplifier especially programmed for fast and synchronous data acquisition measurements. The speed of the system is high enough to measure real-time profiles with 7 probes and 50 Hz sine current through the sample. A numerical method to estimate the current distribution inside the tapes using the measured field profile data is proposed. The inverse problem has been solved using certain assumptions on the current distribution in the superconductor. Validation of the results has been done by comparison with finite element method simulations.

Magnetic Profiles in Magnesium Diboride Single Crystals

Local vortex properties were studied using a Hall-probe array in magnesium diboride single crystals. We present the first field profiles in MgB2 and show the important role of surface barriers at 4.2 K and low field in the irreversible behavior.

Reel-to-reel critical current measurement of coated conductors

We introduce an apparatus for non-destructive, contact free, reel-to-reel critical currentmeasurement of coated conductor tape. Up to 100 m of tape can be passed in front of a sensor headfor the measurement in liquid nitrogen. The tape is transferred and characterized at a speed of10 mm s−1. After the liquid nitrogen passage it is actively heated up to room temperature to avoidmoisture condensation. Two different sensors in the probe head can be optionally activated:a linear Hall probe array in a field coil and an ac-measurement coil picking up thirdharmonic signals from the film. Once calibrated by transport measurement thisarrangement enables characterization of HTS films on magnetic as well as on non-magneticmetal tape substrates. In a 10 mm wide coated conductor, up to 400 A of critical current canbe measured.

Flux lenses in the crossing lattices regime of layered superconductors

We report here measurements of flux lensing behaviour which have been performed on Bi 2Sr 2CaCu 2O 8+ δ single crystals placed on top of linear Hall probe arrays. In the crossing lattices regime we observe significant flux focussing as the in-plane field is swept up with a small constant density of pancake vortices. We also observe a sharp discontinous lock-in feature in local magnetic induction when the net field is close to the a– b plane.

Magnetic imaging of YBCO coated conductors by Hall probes

To produce long YBCO superconducting coated conductors a contactless non-destructive characterization technique has been developed in order to determine the critical current distribution across the width of the tape point by point in a continuous way while reeling the tape between spools. These requirements have been satisfied by Hall probe arrays, which measure the magnetic field produced by the currents flowing in the superconductor in the presence of an external magnetic field. This technique has been used to map the critical current inhomogeneity by localizing macroscopic defects in the YBCO superconducting tape.

Investigation of the periodic magnetic field modulation in LHC superconducting dipoles

The windings of high-field accelerator magnets are usually made of Rutherford-type superconducting cables. The magnetic field distribution along the axis of such magnets exhibits a periodic modulation with a wavelength equal to the twist pitch length of the cable used in the winding. This effect, resulting from quasipersistent currents, was investigated with a Hall probe array inserted inside the aperture of the LHC superconducting dipoles, both in short models and full-scale prototypes. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. The origin and, the impact on the LHC dipole stability of the nonuniform current redistribution producing such a field modulation are discussed.