Normal-conducting State Research Articles

Magnetization Control of Zero-Field Intrinsic Superconducting Diode Effect.

The superconducting diode effect (SDE), which causes a superconducting state in one direction and a normal-conducting state in another, has significant potential for developing ultralow power consumption circuits and non-volatile memory. However, the practical control of the SDE necessities the precise tuning of current, temperature, magnetic field, or magnetism. Therefore, the mechanisms of the SDE must be understood to develop novel materials and devices capable of realizing the SDE under more controlled and robust conditions. This study demonstrates an intrinsic zero-field SDE with an efficiency of up to 40% in Fe/Pt-inserted non-centrosymmetric Nb/V/Ta superconducting artificial superlattices. The polarity and magnitude of the zero-field SDE are controllable by the direction of magnetization, indicating that the effective exchange field acts on Cooper pairs. Furthermore, the first-principles calculation indicates that the SDE can be enhanced by an asymmetric configuration of proximity induced magnetic moments in superconducting layers, which induces a magnetic toroidal moment. This study has important implications regarding the development of novel materials and devices that can effectively control the SDE. Moreover, the magnetization control of the SDE is expected to aid in the designing of superconducting quantum devices and establishing a material platform for topological superconductors. This article is protected by copyright. All rights reserved.

Probing the diffusive behaviour of beam-halo dynamics in circular accelerators

Circular particle accelerators at the energy frontier are based on superconducting magnets that are extremely sensitive to beam losses as these might induce quenches, i.e. transitions to the normal-conducting state. Furthermore, the energy stored in the circulating beam is so large that hardware integrity is put in serious danger, and machine protection becomes essential for reaching the nominal accelerator performance. In this challenging context, the beam halo becomes a potential source of performance limitations and its dynamics needs to be understood in detail to assess whether it could be an issue for the accelerator. In this paper, we discuss in detail a recent framework, based on a diffusive approach, to model beam-halo dynamics. The functional form of the optimal estimate of the perturbative series, as given by Nekhoroshev’s theorem, is used to provide the functional form of the action diffusion coefficient. The goal is to propose an effective model for the beam-halo dynamics and to devise an efficient experimental procedure to obtain an accurate measurement of the diffusion coefficient.

Corrosion rate and surface hardness optimization in wire electrical discharge machining of AISI A2 tool steel in various conductive states and mediums

The current work focuses on optimizing the material characteristics such as surface hardness and corrosion rate on the surface of the machined specimens of A2 Tool Steel (A2TS) under various conditions like normal conductive state (NCS) and superconductive state (SCS) in flushing and submergible medium. The machining parameters such as voltage, current, pulse-on, pulse-off, wire-feed, wire-tension, and fluid-pressure are considered for experimentation through orthogonal array L18 using the Taguchi method. The surface morphology of machined specimens surface is examined using energy dispersive x-ray analysis (EDAX), x-ray diffraction (XRD), and Scanning Electron Microscope (SEM). The experimental results reveal that voltage, pulse off, and fluid pressure increase while corrosion rate decreases and similarly, the increases in voltage, pulse-on, and wire feed the surface hardness increases in various states and mediums. The conductive states and mediums improved the material characteristics, such as corrosion rate and surface hardness of A2TS. The oxide layer (ferric oxides) formation reduces in SCS A2TS machined in submergible as elemental reaction with environment reduced with formation of chromium compounds. The bonding strength of the SCS A2TS material increased during re-solidification of machined specimens in submergible increases the surface hardness as a result, carbon content will increases. These results are noticed from EDAX, XRD and SEM analysis. An improvement of corrosion rate between NCS A2TS and SCS A2TS in flushing medium is 57.59% and in submergible medium is 59.35% respectively. The progress of surface hardness between NCS A2TS and SCS A2TS in flushing medium is 2.58% and in submergible medium is 4.93%, respectively.

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.

Quench Protection Studies for the High Luminosity LHC Nb$_3$Sn Quadrupole Magnets

Achieving the targets of the High Luminosity LHC project requires the installation of new inner triplet magnet circuits for the final focusing of the particle beams on each side of the two main interaction points. Each of the four circuits will include six 150 mm aperture, 132.2 T/m gradient, Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Sn quadrupole magnets to be installed in the LHC tunnel. The recently updated circuit topology is such that the protection of each magnet can be studied from a single magnet point-of-view. To limit the hot-spot temperature and the peak voltage-to-ground, a protection system was designed that quickly and reliably transfers voluminous parts of the coil to the normal-conducting state, hence distributing more homogeneously the magnets stored energy in the windings. This system is based on two elements: quench heaters attached to the outer layers of the magnet coils and CLIQ (Coupling-Loss Induced Quench). The performance of the protection system is investigated by simulating the electro-magnetic and thermal transients occurring after a quench with the program STEAM-LEDET, and by conducting dedicated experiments at the CERN and FNAL magnet test facilities. The effectiveness of the quench protection system is assessed at all representative operating current levels. Furthermore, the coils hot-spot temperature and peak voltage to ground are analyzed for various failure cases, conductor parameters, and parameter distribution among the four coils. It is concluded that the proposed design assures an effective, reliable, and fully redundant quench protection system.

Simulations and measurements of beam loss patterns at the CERN Large Hadron Collider

The CERN Large Hadron Collider (LHC) is designed to collide proton beams of unprecedented energy, in order to extend the frontiers of high-energy particle physics. During the first very successful running period in 2010--2013, the LHC was routinely storing protons at 3.5--4 TeV with a total beam energy of up to 146 MJ, and even higher stored energies are foreseen in the future. This puts extraordinary demands on the control of beam losses. An un-controlled loss of even a tiny fraction of the beam could cause a superconducting magnet to undergo a transition into a normal-conducting state, or in the worst case cause material damage. Hence a multi-stage collimation system has been installed in order to safely intercept high-amplitude beam protons before they are lost elsewhere. To guarantee adequate protection from the collimators, a detailed theoretical understanding is needed. This article presents results of numerical simulations of the distribution of beam losses around the LHC that have leaked out of the collimation system. The studies include tracking of protons through the fields of more than 5000 magnets in the 27 km LHC ring over hundreds of revolutions, and Monte-Carlo simulations of particle-matter interactions both in collimators and machine elements being hit by escaping particles. The simulation results agree typically within a factor 2 with measurements of beam loss distributions from the previous LHC run. Considering the complex simulation, which must account for a very large number of unknown imperfections, and in view of the total losses around the ring spanning over 7 orders of magnitude, we consider this an excellent agreement. Our results give confidence in the simulation tools, which are used also for the design of future accelerators.

&amp;beta;-Phase Instability in Binary Ti&amp;ndash;&lt;i&gt;x&lt;/i&gt;Nb Biomaterial Single Crystals

The 1⁄2-phase transformation and ¢-phase stability in Ti­xNb (28 x 40 at%) single crystals were investigated using electrical resistivity measurements, transmission electron microscopy (TEM) observations, and specific heat measurements. The crystal for x = 28 exhibits distinct anomalous negative temperature dependence of the resistivity coefficient and thermal hysteresis accompanied by the presence of the athermal 1⁄2phase and ¢-phase lattice modulation. Although the crystal for x = 30 appears in the ¢-phase lattice modulation, it does not exhibit a clear negative temperature dependence of the resistivity coefficient or the athermal 1⁄2-phase. The crystal of x = 30 also shows a relatively high absolute value of resistivity at 15K among the crystals for 28 x 40 and a low Debye temperature in a normal conductive state. The crystal for x = 30 that shows the lattice modulation, high resistivity, and low ¢-phase Debye temperature correspond to the low stability of the ¢-phase. Moreover, the stability strongly depends on the Nb content of the binary Ti­Nb crystal. [doi:10.2320/matertrans.MC201202]

生体用 Ti-Nb 二元系合金単結晶における &amp;beta; 相の不安定性とその基礎物性への影響

The ω-phase transformation and β-phase stability in Ti-xNb (28≤x≤40 at%) single crystals were investigated using electrical resistivity measurements, transmission electron microscopy (TEM) observations, and specific heat measurements. The crystal for x=28 exhibits distinct anomalous negative temperature dependence of the resistivity coefficient and thermal hysteresis accompanied by the presence of the athermal ω-phase and β-phase lattice modulation. Although the crystal for x=30 appears in the β-phase lattice modulation, it does not exhibit a clear negative temperature dependence of the resistivity coefficient or the athermal ω-phase. The crystal of x=30 also shows a relatively high absolute value of resistivity at 15 K among the crystals for 28≤x≤40 and a low Debye temperature in a normal conductive state. The crystal for x=30 that shows the lattice modulation, high resistivity, and low β-phase Debye temperature corresponds to the low stability of the β-phase. Moreover, the stability strongly depends on the Nb content in the binary Ti-Nb crystal.

Superconducting Phase and Pairing Fluctuations in the Half-Filled Two-Dimensional Hubbard Model

The two-dimensional Hubbard model exhibits superconductivity with d-wave symmetry even at half-filling in the presence of a next-nearest neighbor hopping. Using plaquette cluster dynamical mean-field theory with a continuous-time quantum Monte Carlo impurity solver, we reveal the non-Fermi liquid character of the metallic phase in proximity to the superconducting state. Specifically, the low-frequency scattering rate for momenta near (π, 0) varies nonmonotonically at low temperatures, and the dc conductivity is T linear at elevated temperatures with an upturn upon cooling. Evidence is provided that pairing fluctuations dominate the normal-conducting state even considerably above the superconducting transition temperature.

Current-assisted thermally activated flux liberation in ultrathin nanopatterned NbN superconducting meander structures

We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature $T\ensuremath{\approx}0.4{T}_{\text{c}}(0)$. The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one. Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal-conducting state, which can be registered as a voltage pulse of nanosecond duration. We considered three different models (vortex-antivortex pairs, vortex edge barriers, and phase-slip centers) to explain the experimental data. Only thermally excited vortices, either via unbinding of vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a satisfactory and consistent description for all measurements.

A Single-Stage SQUID Multiplexer for TES Array Readout

We have developed a single-stage SQUID multiplexer (SQMUX) for time-domain multiplexing of transition edge sensors (TESs). The individual channels in our SQMUX circuit are SQUID series arrays (SSAs). Several of these SSAs form one column, of which exactly one SSA is active at a time. The channel to be read out is selected by setting a superconducting-to-normalconducting (SN) switch into its normalconducting state. All other channels are short-circuited by SN-switches in their superconducting state. This SQMUX topology ensures that there are no noise contributions from off-channels. The SN-switches employed are SSAs, as well. The addressing is performed inductively by means of coupling magnetic flux into the switch. Our SQMUX can be operated at the TES operating temperature. It is directly read out by a room temperature preamplifier. The first realizations of this single- stage SQMUX are 4:1 and 2:2 multiplexers. We present the application of the new SQMUX to read out TES X-ray microcalorimeters. Furthermore, we discuss the application of SQMUXs to the readout of distributed power TES Detectors.

Progress on the Development of a Superconducting Connection for Niobium Cavities

The availability of a superconducting connection between adjacent niobium radio-frequency (RF) cavities with the capability to carry up to 30 mT of the magnetic flux would be particularly of great benefit to layouts of long accelerators like the International Linear Collider (ILC). It would shorten the distances between structures and therefore the total length of an accelerator with the associated cost reductions. In addition, the superconducting connection would be ideal for a superstructure-two multi-cell cavities connected through a half wavelength long beam pipe providing the coupling. Two single-cell niobium cavities have been designed with Nb-1Zr flanges welded to one of the irises to allow a connection between them with a niobium gasket. A transition to the normal-conducting state of the connection due to the applied RF field causes a reduction of the cavities' quality factor. The cavity design will be presented in this contribution along with possible choices of materials for the joint.

Detection of high energy cosmic rays with the resonant gravitational wave detectors NAUTILUS and EXPLORER

The cryogenic resonant gravitational wave detectors NAUTILUS and EXPLORER, made of an aluminum alloy bar, can detect cosmic ray showers. At temperatures above 1 K, when the material is in the normal-conducting state, the measured signals are in good agreement with the expected values based on the cosmic rays data and on the thermo-acoustic model. When NAUTILUS was operated at the temperature of 0.14 K, in superconductive state, large signals produced by cosmic ray interactions, more energetic than expected, were recorded. The NAUTILUS data in this case are in agreement with the measurements done by a dedicated experiment on a particle beam. The biggest recorded event was in EXPLORER and excited the first longitudinal mode to a vibrational energy of ∼670 K, corresponding to ∼360 TeV absorbed in the bar. Cosmic rays can be an important background in future acoustic detectors of improved sensitivity. At present, they represent a useful tool to verify the gravitational wave antenna performance.

A SQUID multiplexer with superconducting-to-normalconducting switches

We present a novel multiplexer scheme for time-division multiplexing in superconductingquantum interference device (SQUID) readout configurations. The channels of our SQUIDmultiplexer (SQMUX) are dc SQUID sensors which are connected in series to form acolumn. A superconducting-to-normalconducting switch (SN-switch) in parallel witheach channel is actuated in order to activate or deactivate an individual channelof the column. With a switch in the normalconducting state, its correspondingSQUID channel is active and sensitive to an input signal. When a switch is in thesuperconducting state its corresponding SQUID channel is short-circuited. It isthen insensitive to input signals and does not generate noise. The addressingsequence is chosen such that only one SQUID channel is active at a time. Wesuggest employing SN-switches which are based on dc SQUIDs themselves. Doing soenables the addressing signals of the SN-switches to be applied inductively, i.e.,galvanically isolated. The SQMUX operation is demonstrated in a configuration whereboth the SN-switches and the sensing SQUIDs are based on SQUID series arrays.

Quenching characteristics of electrical contacts using YBaCuO bulk superconductor

A persistent current switch (PCS) is used for various superconducting applications, such as superconducting magnetic energy storage (SMES) system. The authors have proposed a mechanical switch of Y–Ba–Cu–O (YBCO) bulk as a PCS. In previous study, in order to investigate the contact characteristics of the switch, 50 A current test was performed and the contact resistance between two contacts was measured by four terminal method. As a result, it became clear that polishing and metal depositing on YBCO surface was effective to reduce the residual resistance, and the authors achieved to reduce this resistance to 0.27 μΩ, which was less than our target value of 1 μΩ. At the same time, the greater current test (above 50 A) was performed in order to examine the transition to normal conductive state in the YBCO contacts. As a result, it could be found that the current value when a local quench occurred (local quenching current: LQC) had a strong relationship with the layer thickness of deposited silver on YBCO surface. This paper focused on the relationship between the transition current to the normal conductive state in the YBCO (quenching current) and the layer thickness of deposited silver on YBCO surface. And it was also observed whether the number of current cycles affected the value of quenching current.

Inductance Ratio of Flat Type Fault Current Limiter With High Tc Superconducting Disks

We have proposed a flat type of the superconducting fault current limiter (FCL) which is a modified version of a magnetic shielding type of the fault current limiter. The flat type FCL basically consists of a pancake coil and doughnut-like form high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconductor (HTS) disk, each of which works as a primary winding and a secondary one-turn winding, respectively. In order to clarify the static current-limiting performance of the flat type FCL, the magnetic field analyses were carried out for the small modules of the FCL. The inductance values of the FCL in the superconducting state and normal-conducting state of the HTS disk, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">limit</sub> , were calculated and their ratio <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">limit</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> was estimated. The high performance of the FCL must be brought about by the large inductance ratio. It was confirmed that lower <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> was derived by sandwiching the pancake primary winding with two HTS disks and by using thin spacer between them, and larger <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">limit</sub> a is produced by connecting the built-up pancake primary windings in series. This suggests that a full-scale FCL may be designed and produced by building up many modules whose pancake winding is sandwiched with the HTS disks.

Comparative Study of Transformer-Type Superconducting Fault Current Limiters Considering Magnetic Saturation of Iron Core

The paper reports on the transient performances of the transformer type superconducting fault current limiter (SCFCL). This type of fault current limiters (FCL) consists of a transformer in series with the transmission line and a superconducting current limiting device connected to the transformer. The performances are analysed through simulation during a short-circuit fault in the presence and absence of the shunt inductor. Magnetic saturation of the transformer iron core was compared to clarify its influence on current limiting characteristics of these SCFCLs. The magnetization curve of the transformer core is expressed by Froehlich's formula and the time dependence of resistance R(t) of the current limiting device during its S-N transition (transition from superconducting to normal-conducting state) is considered. It was shown that magnetic saturation deteriorates the current limiting characteristics of this FCL.

RAP — ACOUSTIC DETECTION OF PARTICLES: FIRST RESULTS AT 4.2 K

RAP (Rivelazione Acustica di Particelle) is a small cylindrical aluminum bar (l = 500 mm , d = 181.7 mm ) placed at the DAΦNE Beam Test Facility, where it is hit by a 510 MeV electron beam, coming from the DAΦNE Linac. Aim of the experiment is to measure the mechanical vibrations of the bar caused by the interaction with the beam. On June, 2004 RAP successfully collected data for the first time at cryogenic temperature. Several runs at different temperatures (4.5, 70 and 273 K) have been performed and a number of shots in normal-conducting state of the bar were detected. The preliminary results are in good agreement with the Thermo-Acoustic Model. In the next months the mounting of a dilution refrigerator and the data taking of the bar in the superconducting state (T = 100 mK ) have been planned.

Temperature dependence of the work function of Bi 2Sr 2CaCu 2O 8 single crystal cleaved at low temperature

We measured the anomalous change in the work function of Bi 2Sr 2CaCu 2O 8 around a critical temperature ( T c ≈ 85 K). The work function becomes a minimum at T c; the work function decreases in a normal-conductive state and then increases in a superconductive state as the temperature decreases. An increase in the work function for a transition from a normal-conductive state to a superconductive state at 0 K is about 9 meV. The contribution of the chemical potential and the surface dipole barrier to the work function are discussed.

Fabrication of meander shaped (Y,Yb)BCO LPE films for PCS materials

We have investigated the basic characteristics of the liquid phase epitaxy (LPE) grown (Y,Yb) Ba 2Cu 3O y film and have proposed a meander shaped film to realize a high critical current as well as a high electric resistance in the normal-conducting state due to a long current path for the persistent current switch (PCS) materials. The LPE film exhibited an excellent critical current density in the superconducting state and a high electric resistance in the normal state which are both required for PCS applications. The meander shape of the seed films could be maintained during the further LPE growth. This could be explained by decreasing the lateral growth rate due to the combined phenomena of the step bunching and the difficulty of the overhanging growth above the MgO valley which was formed by the partial dissolution of the MgO surface to the solution. The meander shaped LPE film with a long current path on the substrate of 2″ in diameter could be successfully grown, which is a strong candidate as the PCS materials.