Transient Magnetic Field Research Articles

Low-level structure ofGe70from lifetime andg-factor measurements following α transfer to aZn66ion beam

The $g$ factor of the ${2}_{1}^{+}$ state in $^{70}\mathrm{Ge}$ was remeasured using a different experimental approach. Furthermore, for the first time an experimental value (although with a large uncertainty) was obtained for the $g$ factor of the ${2}_{2}^{+}$ state in $^{70}\mathrm{Ge}$. All this was accomplished by employing the technique of \ensuremath{\alpha} transfer to an energetic $^{66}\mathrm{Zn}$ ion beam in inverse kinematics combined with transient magnetic fields in ferromagnetic gadolinium. The value of the $g({2}_{1}^{+})$ factor obtained ranges from +0.32(11) to +0.43(12), subject to certain assumptions. This range of values is in general agreement with the range of values in the literature, where Coulomb excitation and different IMPAC techniques were used. Lifetimes of several low-lying states were redetermined using the Doppler-Shift-Attenuation-Method. The deduced $B(E2)$ values and the $g({2}_{1}^{+})$ factor are discussed within the framework of large-scale full $\mathit{fp}$ shell model calculations with a closed $^{40}\mathrm{Ca}$ core and including excitations from the ${f}_{7/2}$ orbital. The results are compared with recent data for $^{68}\mathrm{Ge}$ and $^{68}\mathrm{Zn}$.

Ultrafast all-optical control of the magnetization in magnetic dielectrics

The purpose of this review is to summarize the recent progress on laser-induced magnetization dynamics in magnetic dielectrics. Due to the slow phonon-magnon interaction in these materials, direct thermal effects of the laser excitation can only be seen on the time scale of almost a nanosecond and thus are clearly distinguished from the ultrafast nonthermal effects. However, laser pulses are shown to indirectly modify the magnetic anisotropy in rare-earth orthoferrites via the crystal field, and to bring about spin reorientation within a few picoseconds. More interesting, however, are the direct nonthermal effects of light on spin systems. We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on a femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites. In addition, circularly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. An all-optical scheme of excitation and detection of different antiferromagnetic resonance modes with frequencies of up to 500GHz will be discussed as well. The reported effects open new and exciting possibilities for ultrafast manipulation of spins by light and provide new insight into the physics of magnetism on ultrafast time scales.

Measurement of the magnetic field radiating by electrostatic discharges during the verification of the ESD generators

This work aims to investigate the transient magnetic field radiating by two commercial generators of electrostatic discharges. Near field measurements have been conducted, a few centimetres away from the discharge point (Pellegrini target). The Pellegrini target is mounted in the centre of a grounded metal plane, inside an anechoic chamber. The measurements are performed in three different directions in relation to the discharge direction. The experimental data show that each ESD generator produces a different transient magnetic field. Furthermore, additional differences in the magnetic field produced by each generator are noted, depending on the direction of the measurement. Finally, comparisons concerning the magnetic field produced by each generator as well as useful conclusions for the decrease of the magnetic field are presented.

Experimentalgfactor andB(E2)value of the41+state in Coulomb-excitedZn66compared to shell-model predictions

The $g$ factor of the ${4}_{1}^{+}$ state in $^{66}\mathrm{Zn}$ has been measured for the first time employing the technique of projectile Coulomb excitation in inverse kinematics combined with transient magnetic fields. The lifetime of this state, $\ensuremath{\tau}({4}_{1}^{+})=1.1(2)$ ps, which has been remeasured by the Doppler shift attenuation method, is twice as large as a previously determined value. Both the deduced $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})=133(24)$ ${e}^{2}\phantom{\rule{0.3em}{0ex}}{\mathrm{fm}}^{4}$ and the $g$ factor of the ${4}_{1}^{+}$ state, $g({4}_{1}^{+})=+0.65(20)$, were interpreted, together with results for the ${2}_{1}^{+}$ state, in the framework of shell-model calculations based on a $^{56}\mathrm{Ni}$ closed-shell core. The present results are also compared with those for neighboring $^{64}\mathrm{Zn}$ and $^{68}\mathrm{Zn}$ that were obtained in previous measurements by the same technique.

“Flux Jumps” in Cable-in-Conduit Conductors Induced by Transient Magnetic Field

A jump-like temperature increase that can not be explained only by AC losses was observed during the tests of Nb-Ti CICC model samples in transient magnetic fields. The samples were placed between two racetracks that generate background transient magnetic field (TMF). The flux jumps occurred in CICC samples even during the slow discharge of racetrack coils. A physical model was developed to explain this phenomenon. According to this model, transient magnet field generates the screening currents in the loops formed by the superconducting strands and normal matrix. In a real winding the additional currents can be either added to or subtracted from the transport current in superconducting strands depending on a mutual orientation of the varying magnet flux density and the transport current vectors. The critical current of the strands can be exceeded as result of a simultaneous increase of the strands temperature due to AC losses and the screening current. After the sample reaches its critical state the energy stored in the loops quickly dissipates in the normal matrix of the strands. This results in a jump-like increase of the conductor temperature

Transient finite element magnetic field calculation method in the anisotropic magnetic material based on the measured magnetization curves

A lot of magnetic materials are anisotropic. In the 3D finite element method calculation, anisotropy of the material is taken into account. Anisotropic magnetic material is described with magnetization curves for different magnetization directions. The 3D transient calculation of the rotational magnetic field in the sample of the round rotational single sheet tester with circular sample considering eddy currents is made and compared with the measurement to verify the correctness of the method and to analyze the magnetic field in the sample.

Investigation of MR signal modulation due to magnetic fields from neuronal currents in the adult human optic nerve and visual cortex

Neuronal currents produce weak transient magnetic fields, and the hypothesis being investigated here is that the components of these parallel to the B0 field can potentially modulate the MR signal, thus providing a means of direct detection of nerve impulses. A theory for the phase and amplitude changes of the MR signal over time due to an external magnetic field has been developed to predict this modulation. Experimentally, a fast gradient-echo EPI sequence (TR = 158 ms, TE = 32.4 ms) was employed in an attempt to directly detect these neuronal currents in the adult human optic nerve and visual cortex using a 280-mm quadrature head coil at 1.5 T. A symmetrical intravoxel field distribution, which can be plausibly hypothesized for the axonal fields in the optic nerve and visual cortex, would result in phase cancellation within a voxel, and hence, only amplitude changes would be expected. On the other hand, an asymmetrical intravoxel field distribution would produce both phase and amplitude changes. The in vivo magnitude image data sets show a significant nerve firing detection rate of 56%, with zero detection using the phase image data sets. The percentage magnitude signal changes relative to the fully relaxed equilibrium signal fall within a predicted RMS field range of 1.2-2.1 nT in the optic nerve and 0.4-0.6 nT in the visual cortex, according to the hypothesis that the axonal fields create a symmetrical Lorentzian field distribution within the voxel.

Nonthermal ultrafast optical control of the magnetization in garnet films

We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on the femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites while circularly polarized pulses additionally act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. Due to the slow phonon-magnon interaction in these dielectrics, thermal effects of the laser excitation are clearly distinguished from the ultrafast nonthermal effects and can be seen only on the time scale of nanoseconds for sample temperatures near the Curie point. The reported effects open exciting possibilities for ultrafast manipulation of spins by light, and provide insight into the physics of magnetism on ultrafast time scales.

Low-level structure of 52Ti based on g factor and lifetime measurements

The g factors of the 21+ and 41+ states in radioactive 52Ti (T1/2=1.7m) have been measured for the first time using the technique of α transfer to 48Ca beams in inverse kinematics, in combination with transient magnetic fields. In addition, employing the Doppler-shift-attenuation method, the lifetime of the 21+ state has been re-determined with much higher precision while the lifetime values of the 41+ and the 23+ states were obtained for the first time. The B(E2) and g factor results were compared with the results obtained within the framework of full fp spherical shell model calculations using several recent effective interactions commonly utilized in this mass region. The two measured g factors were accounted for by none of the calculations except for the FPD6 calculation with modified input single-particle energies. The new data were also discussed in relation to the neighbouring isotopes, 48,50Ti, isotones, 50,52,54Cr, and the magic N=28 shell closure. Our calculations generally reproduce well the trends of the data in terms of their dependence on neutron number.

Measurement of the magnetic field radiating by electrostatic discharges using commercial ESD generators

The aim of this work is the investigation of the transient magnetic field radiating by two different commercial generators of electrostatic discharges. Measurements of the magnetic field generated by contact electrostatic discharges have been conducted a few centimeters away from the discharge point. In this paper the current transducer, which is used for the measurement of the discharge current is not mounted on a grounded metal plate, but instead it is on an insulating material. With this aberration to the Standard a closer simulation to the magnetic field produced by the electrostatic discharge generators on the equipment under test is obtained. It is proved by the measurements that each generator produces a different transient magnetic field, which results in a different way on the equipment that is tested. Also, comparisons of the magnetic field between the two generators and useful conclusions for the decrease of the magnetic field are presented.

A vector Preisach model combined with a Newton–Raphson method for transient magnetic field computations

This paper presents a computational approach for transient magnetic field problems with vector hysteresis. A nonlinear update scheme is given to improve the convergence of the nonlinear iteration in the particular hysteresis case, in combination with the finite integration implicit time domain formulation. A simplified vector Preisach model is coupled with the Newton–Raphson method for handling magnetic nonlinearities. The Newton–Raphson method equipped with a relaxation strategy is used for linearization. The proposed computational approach is validated by means of a transformer model three-limbed ferromagnetic core exhibiting alternating and rotating fluxes.

Experimental g factors and [formula omitted] values in Ar isotopes: Crossing the [formula omitted] semi-magic divide

Measurements of g factors and lifetimes of the 21+ states in 36,38Ar have been performed via α-transfer reactions using 32S and 34S ions in inverse kinematics combined with the techniques of transient magnetic fields and Doppler-shift-attenuation. The variations, with the neutron number, of the measured g factors and B(E2) values of 36,38,40Ar have been explained by large-scale shell model calculations. The good agreement achieved between experiment and theory demonstrates the impact of the effective nucleon–nucleon interactions and the magic N=20 shell closure. The present new results are a very good show-case for the universal difference in sensitivity of excitation energies, magnetic moments and electromagnetic transition probabilities to nuclear wave functions behaviour as closed shells are approached, crossed, and left behind. Aspects of the collectivity of these nuclei are also discussed from the perspective of a simple vibrational model.

Switching characteristics of MgB2 wires subjected to transient application of magnetic field

We investigated current transport properties in MgB2 wires subjected to transient application of magnetic field, and then discussed their applicability to magnetically controlled switching elements. These wires were fabricated by powder-in-tube (PIT) technique, and Cu–Ni as well as stainless was utilized for their sheath material. A short sample of the wire was installed in a Gifford–McMahon (GM) cryocooler, and was impregnated with solid nitrogen for the sake of thermal stabilization. After stabilizing the temperature of the sample at a certain bias current, the voltage and the temperature in between potential taps were monitored with a transient application of magnetic field. Such measurements were repeated systematically as functions of bias current, operating temperature and magnetic field intensity. It was shown that the sample quickly generated resistance without temperature increment by applying the transient and weak magnetic field. This indicates the applicability of MgB2 wires to persistent current switches (PCS) with quick response.

Solution of transient hysteretic magnetic field problems with hybrid Newton-polarization methods

This paper presents two hybrid Newton-polarization methods for transient magnetoquasistatic field simulation, including hysteretic ferromagnetic materials, in conjunction with the finite integration implicit time-domain formulation. The hybrid techniques are based on the combination of the polarization method and the Newton method. The first hybrid technique uses the magnetic polarization obtained from the polarization method instead of the modified reluctivity for hysteretic material in the Newton method at each iteration. The second hybrid technique combines the solutions from the overrelaxed polarization method and the underrelaxed Newton method at each iteration. The purpose of the two hybrid algorithms is to increase the robustness of the nonlinear iteration, without loosing the quadratic speed of convergence in the vicinity of the solution.

Experimentalgfactors andB(E2)values of21+,41+,22+, and31−states inZn64andZn68compared to shell model predictions

The g factors of the short-lived 2{sub 1}{sup +},4{sub 1}{sup +}, and 3{sub 1}{sup -} states in {sup 64}Zn and the 2{sub 1}{sup +},4{sub 1}{sup +},2{sub 2}{sup +}, and 3{sub 1}{sup -} states in {sup 68}Zn have been measured using the combined technique of projectile Coulomb excitation in inverse kinematics and transient magnetic fields. In addition, B(E2)'s have been deduced from remeasured lifetimes of several excited states employing the Doppler-shift-attenuation method. Whereas the present data for the 2{sub 1}{sup +} states agree very well with our previous results, the g factors of the other states have been determined for the first time. All experimental data were compared with results from large-scale shell model calculations based on proton and neutron configurations in the fp and fpg model spaces with {sup 40}Ca and {sup 56}Ni as inert cores, respectively.

The Dynamic Performance Analysis Model of EMS-MAGLEV System Utilizing Coupled Field-Circuit-Movement Finite Element Method

Many electro-mechanical motional systems include parts that can move relative to the others. EMS-Maglev system is one of the examples, which combines the devices for propulsion, levitation and on-board electric power transfer in a single electromagnetic structure. The analysis of its mechanical dynamic characteristics is very important for the design of the configuration and the control system. Because of the complexity of the time-varying magnetic configuration, analytical method can hardly obtain the dynamic performance. Therefore, analysis of the electromagnetic field is necessary. Moreover, with ordinarily transient finite element method, it is also difficult to consider the external power supply of linear synchronous motors and the working status of linear generator. In this paper, a modified transient finite element algorithm for the performance analysis of magnetically levitated vehicles of electromagnetic type is presented. The algorithm incorporates external power system and vehicles movement equations into FE model of transient magnetic field computation directly. Sliding interface between stationary and moving region is used during the transient analysis. The periodic boundaries are implemented in an easy way to reduce the computation scale. Unfortunately, this directly coupled FE analysis is very time-consuming. To overcome this problem, a fast solving technique for the mechanical dynamic characteristic of electromechanical motional systems is also proposed in this paper. Based on a sequence of finite element analysis, and a set of equivalent electrical circuit parameters extracted, the method incorporates electrical equation and vehicles movement equation into state equations. It is proved that this method can be used for both electromotional static and dynamic cases. Through test of a transformer and an EMS-MAGLEV system, it reveals that the method gives reasonable results at very low computational costs comparing with transient finite element analysis.

Magnetization dynamics of interacting iron nanocrystals inSiO2

We find that ensembles of iron nanocrystals embedded within a matrix of $\mathrm{Si}{\mathrm{O}}_{2}$ exhibit both a large Faraday rotation and ultrafast magnetic relaxation. The Verdet constant is $6.3\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.4\ifmmode^\circ\else\textdegree\fi{}∕\mathrm{cm}∕\mathrm{Oe}$ and a still-unsaturated Faraday rotation of $3.8\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.2\ifmmode^\circ\else\textdegree\fi{}∕\ensuremath{\mu}\mathrm{m}$ was observed at a wavelength of $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for a specimen containing nanocrystals ranging up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter. The dynamic response of the nanocrystals to a transient magnetic field produced by a current pulse propagating through a lithographically patterned wire was probed using time-resolved magneto-optical Kerr-effect microscopy. The rise time of the magnetic response to a transient out-of-plane field was observed to be as fast as $26\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. Magnetostatic interactions between nanoparticles play a significant role in determining the observed static and dynamic properties of the nanocomposite.

Substorm injection modeling with nondipolar, time‐dependent background field

We model energetic particle injections during substorms by investigating the particle interaction with an earthward propagating electromagnetic pulse of spatially localized transient electric (E) and magnetic (B) fields, superposed over a background B field. The current work extends our previous model by considering the background field to be nondipolar (stretched) before the arrival of the pulse (i.e., during the substorm growth phase), changing in a time‐dependent manner into a dipole field in the wake of the pulse. The particle motion still conserves the first adiabatic invariant, even in the stretched B field, and both protons and electrons are convected earthward by the E × B drift to regions of higher field, undergoing betatron acceleration. As in the previous model, we find fully analytical solutions for the gyrocenter motion of the 90° pitch angle particles, and we use them to compute the injected particle flux. We discuss how the model can explain several injection features such as the low/high energy cutoffs, and finally we apply the solutions to a simulation of an actual injection event, obtaining good agreement with observations. The current results with the more realistic background field show significant increase in particle flux for “substorm energies” (tens to hundreds of keVs) compared with the case of a dipole background, leading to the conclusion that the particles have to arrive from closer to Earth than before in order to explain the observed injected flux levels. The new model provides a better fit to observations than the previous one, since it requires lower transient E fields (more realistic of a typical substorm), and thus better explains the ubiquity of particle injections associated with substorms.

The HERMES transversely polarized hydrogen target

The HERMES transversely polarized hydrogen internal target is operational since the end of 2001 in the HERA lepton storage ring. A problem which has to be faced is the nucleon depolarization which can take place when the transient magnetic field generated by the beam interacts with the polarized nucleons and change their spin state. The performance of the target is described and compared with the previous running periods.

First g factor measurement using a radioactive 76Kr beam

The g factor of the first 2+ state of radioactive 76Kr (T1/2=14.8 h) has been measured, extending the systematics of the previously measured stable 78,80,82,84,86Kr isotopes. The measurement was performed with a radioactive 76Kr ion beam applying projectile Coulomb excitation in inverse kinematics combined with the transient magnetic field technique. The 76Kr beam was produced and accelerated in batch mode (re-cyclotron method) at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. A total of three production and acceleration cycles yielded six hours of beam on target with peak rates of 108 particles/s. About 5.6×104 particle-γ coincidence events were recorded. The g factor g(76Kr;2+1)=+0.37(11) was obtained by direct comparison to the known g(78Kr;2+1) value re-measured immediately after the 76Kr runs with the same setup and under almost identical kinematic conditions.