High Field Amplitudes Research Articles

Structural transformation of carbon-encapsulated core-shell CoNi nanoparticles during magnetically induced CO2 reduction into CO

Controlling product distribution in CO2 hydrogenation is of great scientific interest, the selective CO production through the reverse water-gas shift reaction (RWGS) being one of the most investigated processes. Herein, we report the synthesis of new core-shell Co@Ni NPs encapsulated in carbon (Co@Ni@C) to prevent their aggregation at the high-temperatures reached during magnetically induced catalysis. This bifunctional system has been simultaneously used as heating agent and catalyst for the magnetically induced hydrogenation of CO2. While at low magnetic fields Co@Ni@C produces CH4:CO mixtures, at higher field amplitudes it selectively generates carbon monoxide. Indeed, Co@Ni@C has shown to be one of the most active catalysts reported to date, which reaches a maximum conversion of 74.2% with complete selectivity towards CO at 53 mT and 300 kHz. In addition, recycling and cyclability experiments have demonstrated that Co@Ni@C becomes fully selective for CO after being exposed to high field amplitudes (i.e. reaction temperature above 500 °C), even when it exposed to low magnetic fields again. This change in the selectivity is due to an atomic rearrangement of the core-shell structure, as was confirmed by EDX, XAS, TPR and TPD analysis.

High-Temperature Superconducting Undulator Prototype Coils for Compact Free-Electron Lasers

Short-period and high-field undulators are crucial for the production of coherent light up to X-rays in compact free-electron lasers (FELs). Besides, future colliders like CLIC or FCC-ee demand high-field damping wigglers to reach a low beam emittance. Both applications may benefit from the use of high-temperature superconductors (HTS), e.g. by providing high magnetic field amplitudes or higher operating temperatures. This paper presents and summarizes our HTS undulator prototype coils, designed and manufactured at CERN, made from coated REBCO tape superconductor. The coil design, described and already powered at 77K in previous works, is based on no-insulation (NI) vertical racetracks with a period length of 13 mm, assembled with iron poles. Powering tests of several HTS undulator prototype coils at 4.2 K, performed at KIT, revealed safe operations at high engineering current densities of 2.3 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with produced magnetic fields of up to 1.5 T at 3.5mm distance from the magnetic iron poles. Reasonable effective time constants of around 100 s were obtained and further powering tests in regions beyond <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> c with 3.6 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> showed the stability of our NI design and created magnetic fields in the region of 2 T.

Lagrangian descriptor and escape time as tools to investigate the dynamics of laser-driven polar molecules.

We consider the nonlinear dynamics of a diatomic polar molecule under a linearly polarized laser field. We assume a model in which the molecule dipole is coupled with a time-dependent electric field. This system presents a bound energy region where the atoms are bound, and a free-energy region where the atoms are dissociated. Due to the nonalignment between the dipole axis and the laser direction, and the time dependence of the external field, this system presents two and a half degrees of freedom, namely the vibrational degree, the rotation degree, and the time. To investigate the system dynamics, instead of using the Poincaré surface-of-section technique, we propose the use of the Lagrangian descriptor associated with the escape times. The Lagrangian descriptor is a quantity that reveals complex structures in the phase space, whereas the escape times are the time span in which a trajectory is initially in the bound region before escaping to the unbound region. The combination of these two quantities allows us to distinguish between real stability regions from other complex structures, including stickiness regions, and a different formation, which we call escape islands. With the help of these tools, we find that for high-field amplitudes the inclusion of rotation leads to an increase of the stability regions, which implies a decrease of the dissociation in comparison with the one-dimensional case.

Propagation of an Ultrashort Terahertz Pulse with a High Electric Field Amplitude in a Silicon Sample

Propagation of an Ultrashort Terahertz Pulse with a High Electric Field Amplitude in a Silicon Sample

An Easily Used Phenomenological Magnetization Model and Its Empirical Expressions Based on Jiles-Atherton Parameters.

In this paper, a simple magnetization model convenient for engineering applications is presented based on the expressions of the first-order LTI system model. Considering the trade-off between the nonlinearity of anhysteretic magnetization and the hysteresis width, the proposed model employs two different equations with different magnetic field amplitudes. Furthermore, the proposed model utilizes the first-order LTI system model with a low magnetic field amplitude and a simple nonlinear function, based on the amplitude-frequency function, with a high magnetic field amplitude. Two important characteristic parameters for engineering applications, namely, amplitude and the equivalent phase lag, were exacted and analyzed to validate the computation precision of the proposed model. Then, the model was verified through comparisons to the validated Jiles-Atherton model. For easy use, similar to a physics-based model instead of a fitting method, empirical expressions for the model parameters were given, and applicable ranges of these equations were determined using the parameters of the Jiles-Atherton model. Finally, an example of the magnetization model applied to an on/off type device was computed to further verify the effectiveness of the proposed model with quite a simple expression.

Reducing cross-field demagnetization of superconducting stacks by soldering in pairs

Superconducting stacks can be used as strong permanent magnets in several applications. One of their uses is to build light and compact superconducting motors for aviation, where these magnets can be used in the rotor, but they can demagnetize quickly in the presence of cross fields. In this article, we propose a new configuration of soldered stacks face-to-face, which can be constructed by relatively simple joining techniques. Based on numerical modeling of the cross-field demagnetization of stacks of two and 16 tapes, we show that such a sample can withstand around twice as high ripple field amplitudes than isolated stacks. This is due to the increase in the parallel penetration field by around a factor 2. For cross-field amplitudes below this value, a soldered stack can retain higher permanent magnetization than isolated stacks. This method of reducing cross-field demagnetization does not decrease the power or torque rating of a motor, compared to other strategies like the increase in the gap between rotor and stator.

Thermo-magnetic loading effects on high-frequency dynamic behaviour of magnetic shape memory alloys

This paper theoretically studies the thermo-magnetic loading effects on the long-term (>100 s, reaching the steady state) high-frequency (>100 Hz) dynamic behaviour of magnetic shape memory alloys actuated by cyclic magnetic fields. The material's dynamic behaviour at different levels of ambient heat-transfer, ambient temperature, applied magnetic field frequency and amplitude are simulated by a dynamic model incorporating both magnetic-field-induced martensite reorientation and temperature-driven martensitic phase transformation. Analytical expressions of the material's long-term steady-state behaviour (i.e., stable strain amplitude and temperature) as a function of ambient thermal conditions and magnetic loading conditions are further derived. It is found from model simulations and analytical calculations that weak ambient heat-transfer, high ambient temperature, and high magnetic field amplitude (to trigger martensite reorientation) lead to large net heat generation from dissipative martensite reorientation and thus increase material's stable temperature, which results in reduced twinning stress and increased stable strain amplitude. It is also found that the material's stable temperature can reach the characteristic phase transformation temperature triggering the martensite-to-austenite transformation. In this case, weaker ambient heat transfer, higher ambient temperature and higher magnetic field frequency result in larger heat generation rate accelerating the martensite-to-austenite transformation. Therefore, less martensite remains in the material and the material's stable strain amplitude becomes smaller.

Ultra-broadband THz pulses with electric field amplitude exceeding 100 kV/cm at a 200 kHz repetition rate.

We demonstrate a table-top source delivering ultra-broadband THz pulses with electric field strength exceeding 100 kV/cm at a repetition rate of 200 kHz. The source is based on optical rectification of 23 fs pulses at 1030 nm delivered by a ytterbium-doped fiber laser followed by a nonlinear temporal compression stage. We generate THz pulses with a conversion efficiency of up to 0.11 % with a spectrum extending to 11 THz using a 1 mm thick GaP crystal and a conversion efficiency of 0.016 % with a spectrum extending to 30 THz using a 30 µm thick GaSe crystal. The essential features of the emitted THz pulse spectra are well captured by simulations of the optical rectification process relying on coupled nonlinear equations. Our ultrafast laser-based source uniquely satisfies an important requirement of nonlinear THz experiments, namely the emission of ultra-broadband THz pulses with high electric field amplitudes at high repetition rates, opening a route towards nonlinear time-resolved THz experiments with high signal-to-noise ratios.

Dynamic magnetic susceptibility of a ferrofluid: The influence of interparticle interactions and ac field amplitude.

Based on numerical results of dynamic susceptibility, a simple theory of the dynamic response of a ferrofluid to an ac magnetic field is obtained that includes both the effects of interparticle dipole-dipole interactions and the dependence on field amplitude. Interparticle interactions are incorporated in the theory using the so-called modified mean-field approach. The new theory has the following important characteristics: in the noninteracting regime at a weak ac field, it gives the correct single-particle Debye theory results; it expands the applicability of known theories valid for high concentrations [Ivanov, Zverev, and Kantorovich, Soft Matter 12, 3507 (2016)10.1039/C5SM02679B] or large values of ac field amplitudes [Yoshida and Enpuku, Jpn. J. Appl. Phys. 48, 127002 (2009)10.1143/JJAP.48.127002], in accordance with their applicability. The susceptibility spectra are analyzed in detail. It is demonstrated that interparticle dipole-dipole interactions and an increase in field amplitude have an opposite effect on the dynamic response of ferrofluids, so that at certain field amplitudes, relaxation processes in the system of interacting particles are determined by the characteristic relaxation times for an ideal paramagnetic gas. The new theory correctly predicts the dynamic susceptibility and characteristic relaxation times of a ferrofluid at high ac field amplitudes as long as the Langevin susceptibility χ_{L}≲1, which is a complex characteristic of ferrofluid density and the intensity of interparticle dipole-dipole interactions.

Resonance magnetoelectric effect in a composite ferromagnet–dielectric–piezoelectric Langevin-type resonator

The direct and converse magnetoelectric (ME) effects in a Langevin-type resonator are observed and investigated for the first time. The resonator comprised a disk made of lead–zirconate titanate ceramic, located between two sapphire cylinders, with layers of amorphous ferromagnet glued to its side surfaces. ME effects were observed at frequencies of bending (∼70 kHz) and longitudinal (∼200 kHz) oscillations of the resonator upon its excitation by ac magnetic and electric fields. The field conversion efficiency for the direct ME effect was α E ≈ 31 V (Oe−1 cm−1). The magnetization modulation efficiency for the converse ME effect reached α B ≈ 0.72 G cm−1 V−1. The nonlinear ME effects such as frequency doubling, frequency mixing, and resonance frequency shift are observed at high field amplitudes. The magnitude of both the direct and converse ME effects in the resonator is controlled by an external dc magnetic field. The Langevin-type ME resonators can be used in highly sensitive magnetic field sensors, controlled resonators and transformers, and other components of electronics and microsystem technology.

New experimental approach to nonlinear dielectric effects in the static limit

We present a nonlinear dielectric technique that measures the real and imaginary component of permittivity in a time resolved fashion for a sequence of high field amplitudes. Data is recorded within a few milliseconds of exposing the sample to sinusoidal fields without dc-bias. This short exposure time and a sample thickness of only 10 μm greatly reduces heating effects. The Piekara factor quantifying the nonlinear dielectric effect can be derived from analyzing the response at both the fundamental and the third harmonic frequency, and for both quantities the near quadratic field dependence is verified. The small deviations from the expected field dependence are ascribed to a resolved fifth harmonic response. In this manner, the nonlinear dielectric effect of the polar glass-forming liquid propylene glycol is determined in the static limit, and it is found that the Piekara factor changes from −2.9 × 10−16 V−2 m2 to −1.4 × 10−16 V−2 m2 when the temperature is raised from 204 K to 235 K.

Interposed pulsed power system for fast separation of radioactive beams

This paper presents the novel design of an interposed pulsed power system, developed to drive a fast-switching magnet with a large multi-mH load inductance, and high field amplitude (> 1 T). This modulator can produce variable flat-top pulses from 1 to 30 ms with rise and fall times of less than 0.5 ms at a variable duty cycle of 3–91% into a heavily inductive load. The system employs a novel over-voltage topology to overcome the inherent inductance and achieve the fast rise and fall times, switching to a precision DC supply to efficiently maintain the flattop without requiring many-kV voltage. We present a power source design consideration, including the results of computer modeling as well as the first experimental results of a modulator scaled test model will also be presented.

Kinetic Ising model under sinusoidal oscillating external magnetic field: hysteresis and dynamic phase transition

Kinetic Ising ferromagnet, a bistable system, is studied in two dimensions on large systems under a time-varying sinusoidal external magnetic field of a fixed period P and different amplitudes $$h_0$$ . Far below the critical temperature $$T_c$$ of ferro to paramagnet transition of the equilibrium zero-field Ising model, another critical temperature $$\theta _c$$ is found to exist below which the system is in dynamic ordered phase (DOP) and above which the system is in dynamic disordered phase (DDP). Two important aspects of a bistable system under external time varying field, hysteresis and dynamic phase transition (DPT) are studied performing extensive numerical simulations. The hysteresis loops are asymmetric in DOP whereas they are symmetric in DDP. The hysteresis loop properties are estimated, developing a new methodology which provides information on loop orientation $$\alpha $$ and its aspect ratio $$\varPhi $$ over the usual properties such as area A and dynamic correlation C. The DPT occurs at a temperature $$\theta _c$$ ( $$\ll T_c$$ ) at which the mean metastable lifetime $$\langle \tau \rangle $$ becomes comparable to the half period $$P_{1/2}$$ of the applied filed. Though the smaller systems at higher field amplitudes are found to be in the stochastic resonance regime, the larger systems at all field amplitudes are found to display a continuous transition. The DPT is characterized by the critical exponents of 2d equilibrium Ising model. Discontinuous transition due to stochastic resonance at high field regime seems to be a finite size effect for the DPT under a sinusoidal oscillating field.

Ambient Pressure Ion Funnel: Concepts, Simulations, and Analytical Performance.

In mass spectrometry (MS), a major loss of ions can readily occur during their transfer from atmospheric pressure to a lower pressure, which limits performance. Here, we report an ion funnel that can be used to effectively focus ions at ambient pressure (∼777 Torr) to significantly enhance performance in electrospray ionization (ESI) MS. For seven singly charged test ions (m/z 124-1131), the ambient pressure ion funnel (APIF) is demonstrated to improve ion abundances, sensitivity, and detection limits by up to factors of ∼17, ∼16, and ∼3, respectively, compared to the operation of conventional ESI-MS. Simulated ion trajectories were used to rationalize the enhanced performance of the APIF, which is attributed primarily to using a relatively high RF field amplitude to radially confine ions, a high DC field, and a wide exit ring electrode. The effective focusing of ions at ambient pressures should be beneficial in the future for improving the performance of (i) additional methods that ionize molecules at atmospheric pressure, (ii) ambient pressure ion mobility-based instruments, and (iii) high flow rate liquid chromatography mass spectrometry platforms.

Development and characterization of novel piezoceramic material based stack actuator

The present research investigated the performance characteristics of multilayer stack actuator fabricated using the novel lead-free BaZr0.05Ti0.95O3 (BZT) piezoceramic material in the high field amplitude and low frequency dynamic regime. A systematic approach was undertaken, firstly, to experimentally study the electromechanical strain and degree of hysteresis of a single layer, and evaluate the dependency on the field amplitude and frequency. Based on the obtained characteristics of the single layer, a careful choice of the combination of field amplitude and frequency was implemented to study the free stroke of a multilayer stack actuator. The novel material based stack actuator fabricated at the laboratory scale yielded encouraging results for high precision actuatoric applications.

Assessing hyperthermia performance of hybrid textile filaments: The impact of different heating agents

The application of advanced textile materials functionalized with magnetic nanoparticles (MNP) has become increasingly important in tumor therapy. In this regard, the incorporation of MNP into polymeric filaments, so-called hybrid filaments, enables the development of magnetic heatable stents that, once implanted, are used to deliver therapeutic heat to the tumor site under the influence of an external alternating magnetic field (AMF). In this study, we investigate the hyperthermia performance of different hybrid filaments functionalized with MNP and discuss the relevant parameters influencing the heating efficiency such as MNP immobilization, agglomeration and magnetization as well as AMF settings. For this, three different MNP types (self-synthesized single domain MNP and two commercially available multi-domain MNP, BAYFERROX® and BNF-Starch) were incorporated in polypropylene filaments by melt spinning. Their heating efficiency was assessed in dependence of the AMF amplitude. Because of the blocking of Brownian rotation and bigger effective anisotropy constant caused by MNP agglomeration inside the filaments, the heating efficiency of all hybrid filaments was smaller than that of the corresponding MNP dispersed in water. Nevertheless, in all cases the heating efficiency increased with magnetic field amplitude. The highest specific loss power (SLP) values were reached for the self-synthesized particles at lower magnetic field amplitudes (<25 kA/m) and for BNF-Starch particles at high magnetic field amplitudes (>25 kA/m). This is caused by the higher anisotropy constant of the BNF-Starch particles and their collective magnetic response at higher magnetic field amplitudes.

Electric field dependent polarization switching of tetramethylammonium bromotrichloroferrate(III) ferroelectric plastic crystals

Tetramethylammonium bromotrichloroferrate(III) ([N(CH3)4][FeBrCl3]) is a plastic crystal ferroelectric with small dielectric constant &amp;lt;20 and piezoelectric coefficient as high as 110 pC/N. Here, super-coercive hysteresis and dielectric properties under direct current (DC) bias fields up to 260 and 120 kV/cm, respectively, were studied to shed light on the polarization switching [N(CH3)4][FeBrCl3] and the related family of plastic crystal and supramolecular ferroelectrics. [N(CH3)4][FeBrCl3] exhibited peak-to-peak strains of 0.1% and saturated ferroelastic switching at fields of 170 kV/cm. Above 170 kV/cm, rates of field increase were too fast for domain switching, resulting in reduced strain rates during the switching cycle. Leakage currents had larger contributions at higher field amplitudes. This was also reflected in the switching behavior at higher frequencies, 100 Hz, in which hysteresis was asymmetric and switching incomplete. The dielectric constant and loss exhibited a butterfly-like shape during application of DC bias electric fields indicative of domain switching, but showed a small dielectric tunability of 0.038 and no signs of dielectric stiffening, with the relative permittivity from 16.9 to 17.3 at fields from 0 to 120 kV/cm. The present findings provide insight into the domain switching kinetics and dielectric properties of [N(CH3)4][FeBrCl3] that will assist with further development of plastic crystal ferroelectrics.

Measuring the Output Voltage of a Linear-Motor Type Flux Pump With an Insulated HTS Coil

High-temperature superconducting (HTS) flux pump can supply DC current into the superconducting magnet coil in a contactless manner. The flux pump works similarly as a DC power supply, with low output voltage and high output current. In this paper, based on the linear-motor type flux pump previously introduced from our laboratory (W. Wang et al. 2018), we miniaturized it and measured its the output DC voltage. By using different peak values and frequencies of the applied AC travelling magnetic wave, we obtained the pumping characteristics of this linear-motor type flux pump. With the same applied magnetic strength, it was found that the pumped current is not directly proportional to the frequency, we interpret that the critical current of the HTS stator was likely deteriorated by the heat generated in the stator due to high magnetic field amplitude and frequency. From the pumping current curve, a voltage-current relationship of the HTS flux pump was obtained. This method mentioned in this paper can directly derive the output voltage of the HTS flux pump, and could also be applicable for all other types of flux pumps. The method helps further optimizing HTS flux pumps which will be applied in the dedicated MRI and the brushless superconducting motor under developing.

Ultrafast strong-field absorption in gapped graphene

We study theoretically the strong-field absorption of an ultrafast optical pulse by a gapped graphene monolayer. At low field amplitudes, the absorbance in the pristine graphene is equal to the universal value of $2.3$ percent. Although the ultrafast optical absorption for low field amplitudes is independent of the polarization, linear or circular, of the applied optical pulse, for high field amplitudes, the absorption strongly depends on the pulse polarization. For a linearly polarized pulse, the optical absorbance is saturated at the value of $\approx 1.4$ percent for the pulse's amplitude of $\geq 0.4~\mathrm{V/\AA}$, but no such saturation is observed for a circularly polarized pulse. For the gapped graphene, the absorption of a linearly polarized pulse has a weak dependence on the bandgap, while for a circularly polarized pulse, the absorption is very sensitive to the bandgap. %Opening a bandgap in graphene by placing in on, for example, SiC substrate strongly modify the ultrafast absorption at small field amplitudes.

Experimental and theoretical study of bumped characteristics obtained with cylindrical Langmuir probe in magnetized helium plasma

Cylindrical Langmuir probe measurements in a helium plasma were performed and analysed in the presence of a magnetic field. The plasma is generated in the ALINE device, a cylindrical vessel 1 m long and 30 cm in diameter using a direct coupled RF antenna (νRF = 25 MHz). The density and temperature are of the order of 1016 m−3 and 1.5 eV, respectively, for 1.2 Pa helium pressure and 200 W RF power. The axial magnetic field can be set from 0 up to 0.1 T, and the plasma diagnostic is a RF compensated Langmuir probe, which can be tilted with respect to the magnetic field lines. In the presence of a magnetic field, I(V) characteristics look like asymmetrical double probe ones (tanh-shape), which is due to the trapping of charged particles inside a flux tube connected to the probe on one side and to the wall on the other side. At low tilting angle, high magnetic field amplitude, power magnitude and low He pressure, which are the parameters scanned in our study, a bump can appear on the I(V) in the plasma potential range. We then compare different models for deducing plasma parameters from such unusual bumped curves. Finally, using a fluid model, the bump rising on the characteristics can be explained, assuming a density depletion in the flux tube, and emphasizing the role of the perpendicular transport of ions.