Magnetic Field Ramp Research Articles

Calculation method for pulsed magnetic field energy supplied to Nb3Sn ITER CS conductors during SULTAN stability tests

Cable-In-Conduit Conductors (CICCs) for the ITER Central Solenoid (CS) magnets are designed to operate in the presence of fast changing current and magnetic field during the plasma-operating scenario. For ITER, the AC loss of several types of Nb3Sn CICCs was experimentally tested, but only very limited experimental data is available for quantitative analysis of the minimum quench energy (MQE). In the SULTAN testing facility (Swiss Plasma Centre) few CS conductors were tested on MQE, but the magnetic field amplitude and ramp rate settings are far from the actual ITER operating conditions. Nevertheless, such tests are needed as a basis to calibrate and benchmark the codes that describe the quench behavior. Moreover, during the stability tests in Sultan, the temperature measurements show severe fluctuations, which can introduce a large error for the energy calculation. An interpretation is given for the temperature fluctuation and a procedure is proposed to significantly reduce the error in the pulsed energy calculation.

Study of wave form compensation at CSNS/RCS magnets

A method of wave form compensation for magnets of the Rapid Cycling Synchrotron (RCS), which is based on transfer function between magnetic field and exciting current, was investigated on the magnets of RCS of Chinese Spallation Neutron Source (CSNS). By performing wave form compensation, the magnetic field ramping function for RCS magnets can be accurately controlled to the given wave form, which is not limited to sine function. The method of wave form compensation introduced in this paper can be used to reduce the magnetic field tracking errors, and can also be used to accurately control the betatron tune for RCS.

Thermal coupling effect on the vortex dynamics of superconducting thin films: time-dependent Ginzburg–Landau simulations

In this paper, vortex dynamics of superconducting thin films are numerically investigated by the generalized time-dependent Ginzburg–Landau (TDGL) theory. Interactions between vortex motion and the motion induced energy dissipation is considered by solving the coupled TDGL equation and the heat diffusion equation. It is found that thermal coupling has significant effects on the vortex dynamics of superconducting thin films. Branching in the vortex penetration path originates from the coupling between vortex motion and the motion induced energy dissipation. In addition, the environment temperature, the magnetic field ramp rate and the geometry of the superconducting film also greatly influence the vortex dynamic behaviors. Our results provide new insights into the dynamics of superconducting vortices, and give a mesoscopic understanding on the channeling and branching of vortex penetration paths during flux avalanches.

Dendritic Flux Avalanches of the Nonhomogeneous/Granular Superconducting Thin Films: Numerical Simulations

Superconductors can get unstable in the applications due to its thermomagnetic instability characteristics. Superconducting thin films, fabricated by either the pulsed laser deposition or chemical evaporation, usually show nonhomogeneous or granular microstructures. Thus, nonhomogeneous distribution of the superconducting properties is intrinsically introduced into the thin films. In this paper, we study the flux avalanche behaviors of the nonhomogeneous/granular MgB 2 thin films under applied magnetic field by numerically solving the coupled nonlocal and nonlinear thermomagnetic equations. The influences of the grain sizes and thickness of the channels between islands on the propagation paths of the dendritic flux avalanches are considered in the simulation. It is found that the flux avalanches preferentially propagate along the channels, and the smaller the critical current density inside the channel the more avalanches propagate along it. The numerical simulation results are consistent with the experimental results. In addition, we have investigated the effect of environment temperature and magnetic field ramp rate on the flux avalanche behaviors of nonhomogeneous MgB 2 thin films. Our results may be beneficial to the design and real application of the thin film superconducting devices.

Reversible Martensitic Transformation under Low Magnetic Fields in Magnetic Shape Memory Alloys

Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The samples with two distinct microstructures were tested along the [001] austenite crystallographic direction using a custom designed magneto-thermo-mechanical characterization device while carefully controlling their thermodynamic states through isothermal constant stress and stress-varying magnetic field ramping. Measurements revealed that these meta-magnetic shape memory alloys were capable of generating entropy changes of 14 J kg−1 K−1 or 22 J kg −1 K−1, and corresponding magnetocaloric cooling with reversible shape changes as high as 5.6% under only 1.3 T, or 3 T applied magnetic fields, respectively. Thus, we demonstrate that this alloy is suitable as an active component in near room temperature devices, such as magnetocaloric regenerators, and that the field levels generated by permanent magnets can be sufficient to completely transform the alloy between its martensitic and austenitic states if the loading sequence developed, herein, is employed.

Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile.

Ferromagnetic and antiferromagnetic orders of a phase-separated manganite probed throughout the B−T phase diagram

We employ resonant soft x-ray diffraction (RSXD) to isolate the signal from the CE-type antiferromagnetic phase of ${{(\mathrm{La},\mathrm{Pr})}_{1\ensuremath{-}}}_{x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (with $x\ensuremath{\approx}3/8$), and follow only this phase through the known phases of the material in the $B\text{\ensuremath{-}}T$ phase diagram. This material is known to exhibit a range of electronic ordering phenomena, most notably a metal-insulator transition (associated with colossal magnetoresistance) and phase separation between the antiferromagnetic phase and a ferromagnetic phase. Bulk magnetization measurements under the same $B\text{\ensuremath{-}}T$ conditions were also conducted, giving a full picture of both phases for direct side-by-side comparison. The comparison specifically focuses on the metal-insulator transition. Upon magnetic field ramping to this transition, we find that the CE-type order undergoes a sharp quench at high temperatures (above phase coexistence temperatures) but that at lower temperatures, where the CE order is metastable, the transition broadens significantly. At the lowest temperatures, where a spin glass-type phase is expected, a slow annihilation of remanent CE domains is observed. Finally, a refined phase diagram is presented.

Flux avalanche in a superconducting film with non-uniform critical current density.

The flux avalanche in type-II superconducting thin film is numerically simulated in this paper. We mainly consider the effect of non-uniform critical current density on the thermomagnetic stability. The nonlinear electromagnetic constitutive relation of the superconductor is adopted. Then, Maxwell's equations and heat diffusion equation are numerically solved by the fast Fourier transform technique. We find that the non-uniform critical current density can remarkably affect the behaviour of the flux avalanche. The external magnetic field ramp rate and the environmental temperature have been taken into account. The results are compared with a film with uniform critical current density. The flux avalanche first appears at the interface where the critical current density is discontinuous. Under the same environmental temperature or magnetic field, the flux avalanche occurs more easily for the film with the non-uniform critical current density. The avalanche structure is a finger-like pattern rather than a dendritic structure at low environmental temperatures.

Creation of p-wave Feshbach molecules in selected angular momentum states using an optical lattice

We selectively create p-wave Feshbach molecules in the orbital angular momentum projection state of 6Li. We use an optical lattice potential to restrict the relative momentum of the atoms such that only the molecular state couples to the atoms at the Feshbach resonance. We observe the hollow-centered dissociation profile, which is a clear indication of the selective creation of p-wave molecules in the states. We also measure the dissociation energy of the p-wave molecules created in the optical lattice and develop a theoretical formulation to explain the dissociation energy as a function of the magnetic field ramp rate for dissociation. The capability of selecting one of the two closely-residing p-wave Feshbach resonances is useful for the precise characterization of the p-wave Feshbach resonances.

Modeling and Characterization of Electroformed MEMS Variable Capacitors for Cobalt Iron Magnetostriction Measurements

Electroplated CoFe alloys demonstrating Joule magnetostriction (i.e., a large change in material shape induced by an applied magnetic field) have been recently developed for the creation of microfabricated magnetoelastic resonators for tagging applications. This development requires a measurement technique for evaluating film magnetostriction performance to provide feedback for optimization of the electrodeposition process. This technique must be suitable for electrodeposited films in terms of thickness and ease of processing, have the required sensitivity and dynamic range for accurate microstrain measurements in low magnetic fields up to saturation, and be cost competitive with commercial instruments. A new MEMS variable capacitor has been developed for this purpose that incorporates electroformed mechanical and electrical features as well as the electrodeposited film under test. This capacitor has a modeled 3 pF variable range covering microstrains up to 200 ppm. This presentation will cover the design, modeling, and characterization of these capacitors for magnetostriction measurements of CoFeB and CoFe electrodeposited films. This will also include a discussion of a custom printed circuit board for extracting capacitance signals during the magnetic field ramp inside of a superconducting quantum interference device (SQUID) magnetometer.

Preliminary Magnetic Design of a Superconducting Dipole for Future Compact Scanning Gantries for Proton Therapy

Proton therapy is a rapidly developing technique in cancer treatment since the radiation dose delivered to the target volume is maximized and the dose to surrounding healthy tissue is minimized. The 3-D scanning-irradiation method is performed by means of fast sweeper magnets for transverse scanning, and the scanning in depth of the tumor is performed by fast energy changes in steps of 1% in less than 100 ms. Therefore, high magnetic field ramping speed is necessary. To aim the scanning beam from all directions to the tumor in the patient, the last part of the beam transport and scanning system are mounted on a rotatable gantry. The last bending magnet in the gantry is a major component that drives the weight, footprint, and costs of the whole machine. Superconducting magnets have the potential to reduce the facility weight and costs, maintaining at the same time a large scanning field size. The preliminary magnetic design of the superconducting dipole for a proton therapy gantry is presented in this paper. The design consists in four Nb 3Sn racetrack coils and considers the requirement of large aperture for the transverse scanning and low field at the patient location using an active shielding system.

Dendritic flux avalanches and the accompanied thermal strain in type-II superconducting films: effect of magnetic field ramp rate

Dendritic flux avalanches and the accompanying thermal stress and strain in type-II superconducting thin films under transverse magnetic fields are numerically simulated in this paper. The influence of the magnetic field ramp rate, edge defects, and the temperature of the surrounding coolant are considered. Maxwell's equations and the highly nonlinear E–J power-law characteristics of superconductors, coupled with the heat diffusion equation, are adopted to formulate these phenomena. The fast Fourier transform-based iteration scheme is used to track the evolution of the magnetic flux and the temperature in the superconducting film. The finite element method is used to analyze the thermal stress and strain induced in the superconducting film. It is found that the ramp rate has a significant effect on the flux avalanche process. The avalanches nucleate more easily for a film under a large magnetic field ramp rate than for a film under a small one. In addition, the avalanches always initiate from edge defects or areas that experience larger magnetic fields. The superconducting films experience large thermal strain induced by the large temperature gradient during the avalanche process, which may even lead to the failure of the sample.

Electric-field assisted depinning and nucleation of magnetic domain walls in FePt/Al2O3/liquid gate structures

We present a magneto-optical Kerr effect study of the magnetization reversal in a FePt/Al2O3 structure under electric (E) fields generated in a liquid electrolyte environment. The FePt film was partially covered with a thick Al2O3 layer that allowed for the study of a pinned domain wall between two regions of different coercive field. Depinning of the trapped domain wall into the region of higher coercivity was achieved by applying positive gate voltages during the magnetic field ramp and prevented in the presence of negative gate voltages. Moving from positive to negative gate voltages produced, in addition, an increase (decrease) in the number (size) of reverse domains in the high anisotropy region. This effect has been associated to an E-field induced decrease of the saturation field. Using a liquid gate to assist domain wall depinning as presented here can be used for the control of multiple pinning structures in parallel.

Secondary magnetic field harmonics dependence on vacuum beam chamber geometry

The harmonic magnetic field properties due to eddy currents have been studied with respect to the geometry of the vacuum beam chamber. We derived a generalized formula enabling the precise prediction of any field harmonics generated by eddy currents in beam tubes with different cross-sectional geometries. Applying our model to study the properties of field harmonics in beam tubes with linear dipole magnetic field ramping clearly proved that the circular cross section tube generates only a dipole field from eddy currents. The elliptic tube showed noticeable magnitudes of sextupole and dipole fields. We demonstrate theoretically that it is feasible to suppress the generation of the sextupole field component by appropriately varying the tube wall thickness as a function of angle around the tube circumference. This result indicates that it is possible to design an elliptical-shaped beam tube that generates a dipole field component with zero magnitude of sextupole. In a rectangular-shaped beam tube, one of the selected harmonic fields can be prevented if an appropriate wall thickness ratio between the horizontal and vertical tube walls is properly chosen. Our generalized formalism can be used for optimization of arbitrarily complex-shaped beam tubes, with respect to suppression of detrimental field harmonics.

Many-body dynamics ofp-wave Feshbach-molecule production: A mean-field approach

We study the mean-field dynamics of $p$-wave Feshbach-molecule production in an ultracold gas of Fermi atoms in the same internal state. We derive a separable potential to describe the low-energy scattering properties of such atoms, and use this potential to solve the mean-field dynamics during a magnetic-field sweep. Initially, on the negative scattering length side of a Feshbach resonance the gas is described by the BCS theory. We adapt the method by Szyma\ifmmode \acute{n}\else \'{n}\fi{}ska et al. [Phys. Rev. Lett. 94, 170402 (2005)] to $p$-wave interacting Fermi gases and model the conversion dynamics of the gas into a Bose-Einstein condensate of molecules on the other side of the resonance under the influence of a linearly varying magnetic field. We have analyzed the dependence of the molecule production efficiency on the density of the gas, temperature, initial value of the magnetic field, and magnetic-field ramp speed. Our results show that in this approximation molecule production by a linear magnetic-field sweep is highly dependent on the initial state.

수소 재액화용 단열 탈자 냉동기의 설계

Adiabatic demagnetization refrigerator (ADR) for hydrogen re-liquefaction operating between 24 K and 20 K has been designed. <TEX>$Dy_{0.9}Gd_{0.1}Ni_2$</TEX>, whose Curie temperature is 24 K, is selected as a magnetic refrigerant. The magnetic refrigerant powder is sintered with oxygen-free high purity copper (OFHC) powder to enhance its effective thermal conductivity as well as to achieve relatively high frequency. A perforated plate heat exchanger (PPHE) operated with forced convection is utilized as a heat switch. The forced convection heat switch is expected to have fast response relative to a conventional gas-gap heat switch. A conduction-cooled high Tc superconducting (HTS) magnet is employed to apply external magnetic field variation on a magnetic refrigerant. <TEX>$2^{nd}$</TEX> generation GdBCO coated conductor HTS tape with Kapton<TEX>$^{(R)}$</TEX> insulation (SUNAM Inc.) will be utilized for the HTS magnet. The magnetization and demagnetization processes are to be achieved by the AC operation of the HTS magnet. The designed magnetic field and target ramp rate of the HTS magnet are over 4 T with 180 A and 0.4 T/s, respectively. AC loss distribution on HTS magnet is theoretically estimated.

Direct observation of magnetocaloric effect by differential thermal analysis: Influence of experimental parameters

The magnetocaloric effect is the isothermal change of magnetic entropy and the adiabatic temperature change induced in a magnetic material when an external magnetic field is applied. In this work, we present an experimental setup to study this effect in metamagnetic transitions, using the differential thermal analysis technique, which consists in measuring simultaneously the temperatures of the sample of interest and a reference one while an external magnetic field ramp is applied. We have tested our system to measure the magnetocaloric effect in La0.305Pr0.32Ca0.375MnO3, which presents phase separation effects at low temperatures (T<200K). We obtain ΔT vs H curves, and analyze how the effect varies by changing the rate of the magnetic field ramp. Our results show that the intensity of the effect increases with the magnetic field change rate. We also have obtained the effective heat capacity of the system without the sample by performing calorimetric measurements using a pulse heat method, fitting the temperature change with a two tau description. With this analysis, we are able to describe the influence of the environment and subtract it to calculate the adiabatic temperature change of the sample.

Cryogenic Test of the Full-Size Superconducting Magnet for the Booster Synchrotron of the NICA Project

Abstract A full-size prototype dipole magnet has been designed, fabricated and tested for the Booster synchrotron of a new collider facility NICA at JINR in Dubna. The magnet is based on a cold window frame iron yoke and a curved single-layered superconducting winding made from a hollow NbTi composite superconductor cable cooled with a forced two-phase helium flow at T=4.5 K. The maximal operating magnetic field in the elliptical aperture of 128 mm x 64 mm is 1.8 T. The magnetic field ramp rate of 1.2 T/s should be achievable. The design features of the magnet 2.2 m long as well as the test results are presented.

Evolution of interference patterns of strongly interacting Fermi gases in a harmonic trap

We study the evolution of the interference patterns of strongly interacting Fermi gases in a harmonic trap after removal of the optical lattice, by numerically solving the superfluid order-parameter equation. We find that for the strongly interacting Fermi gas elastic collisions during the expansion blur the interference peaks. In order to obtain a nearly ballistic expansion, the fast magnetic field ramp technique is applied in experiment. We simulate the fast magnetic field ramp process before expansions of strongly interacting Fermi gases. We find that clear interference patterns are formed, and oscillate for a long time in the harmonic trap. We also calculate the interference patterns in different superfluid regimes, which accord with the experimental observations.

Microstructure of the heliospheric termination shock: Full particle electrodynamic simulations

[1] A particle-in-cell code is used to examine kinetic properties of pickup ions at the heliospheric termination shock. The code treats the pickup ions self-consistently as a third component. The simulations are one-dimensional in spatial variations. We use a relative pickup ion density of 30% and two different values of the magnetic field–shock normal angle: ΘBn = 90° and ΘBn = 87°. The oblique shock is chosen in order to allow for wave vectors parallel to the magnetic field due to instabilities in the foot. In addition, a run is presented with a 60% relative pickup ion density to investigate a pickup ion–dominated shock. Upstream of the ramp is an extended foot due to reflected pickup ions. In this foot the magnetic field continuously increases, and the bulk speed of the pickup ions as well as the bulk speed of the solar wind ions decrease and reach at the magnetic field ramp the downstream value. The positive bulk velocity of the pickup ions in the extended foot perpendicular to the magnetic field and to the shock normal causes an electric field in the shock normal direction. This leads to a large increase of the shock potential well upstream of the magnetic field ramp. The maximum value of the potential is ∼0.35 the shock ram energy and is by a factor 5 larger than expected for a weak shock without pickup ions. Pickup ion reflection at the shock is almost 100%; part of the pickup ions are essentially specularly reflected by the magnetic field force term of the Lorentz force in the overshoot and part of the pickup ions are reflected in the extended foot due to a combination of the magnetic force term and the cross-shock potential. In the 30% pickup ion case, about 90% of the total thermal energy gain in the shock is gained by pickup ions, a little under 10% by the solar wind ions. The thermal energy gain by pickup ions increases as the pickup ion relative density increases. The pickup ion temperature increases continuously from the upstream edge of the extended foot to the shock ramp and then stays constant through the overshoot and downstream.