External Pulsed Field Research Articles

Fractal analysis of magneto-optical visualization of the remagnetization of a permanent magnet in a pulsed field

The results of fractal analysis of images of the surface of a permanent magnet KS37, obtained by the method of the polar Kerr effect using an indicator bismuth-containing ferrite-garnet film after magnetization reversal by a pulsed field of 0.1–1.5 T, are presented. The obtained dependences of the residual magnetization on the magnitude of the external pulsed field are compared with the fractal dimension of the magneto-optical images of the magnet surface after exposure to the pulsed field. It is shown that the maximum value of the fractal dimension corresponds to the demagnetized state of the magnet.

The mechanical stresses reduction in a thin-walled quasi-force-free magnetic system inserted into external crossed magnetic fields

The mechanical stresses in the thin-walled quasi-force-free winding of the solenoid, which arise when a strong pulsed magnetic field is produced, can be significantly reduced if the winding is inserted into the space between two magnets creating crossed fields. It is shown that, with a rational choice of the amplitude and shape of the external field pulses, the stresses can be reduced to values less than one tenth of the magnetic pressure of the generated field. Keywords: strong magnetic fields, quasi-force-free field, force-balanced coil.

Pulsed field induced excitation in impurity doped quantum dot: Interplay with Gaussian white noise

Current study strives to realizing the role of Gaussian white noise (GWN) and external pulsed field (PF) on the time-average excitation rate (TAER) of impurity doped quantum dot (QD). The PF is characterized by a pulse shape function which may be continuous or discontinuous with time. In the present study we invoke sinusoidal and triangular shape functions as continuous ones and regular telegraphic and random telegraphic shape functions as discontinuous ones. GWN links with the system additively and multiplicatively. The study emphasizes on analyzing the role of various physical parameters, application of noise, pathway of application of noise and the attributes of the PF (amplitude, phase, shape and the number of pulses) in designing the TAER profiles. The TAER profiles come out to be comprising of features like persistent rise, persistent fall, maximization (useful for attaining large nonlinear optical response), minimization and saturation (the dynamic freezing).

Electron multiplication and avalanching in nanovoids at the initial stage of nanosecond discharge in liquid water

This work presents a study on electron multiplication in cylindrical nanovoids formed in liquid water due to strong pulsed external electric fields. The state-of-the-art simulation toolkit Geant4-DNA is used to describe electron interactions in liquid water. Scaling laws for electron propagation are introduced, concluding that the product of the electric field strength E and the cavity radius R is a suitable parameter to compare simulation results for different values of E and R, and 1/R scaling is proposed for space and time variables. It is shown that the electron avalanche can grow inside of the nanovoid if E ⋅ R > 19.4 eV. The avalanche growth is fed by the emission of the secondary electrons from the surface of the nanovoid. This electron avalanche can also provide a higher total ionization yield along the nanovoid than the simple direct flight of electrons through the nanovoid length. Characteristic electron multiplication timescale and avalanche velocity are determined. Multiplication timescale is shown to be of the order of 1 ps or shorter for electron-multiplying conditions. The velocity of the electron avalanche is about 2.8 × 106 m s−1. The multiplication timescale and the avalanche velocity are consistent with recent experimental observations on nanosecond discharge initiation in liquid water.

Ordered Solvents and Ionic Liquids Can Be Harnessed for Electrostatic Catalysis.

Herein, we employ classical molecular dynamics simulations using the Drude oscillator-based polarizable force field, quantum chemical calculations, and ONIOM multiscale calculations to study (a) how an external field orders the solvent environment in a chemical reaction and then (b) whether in the absence of this same applied field the ordered solvent environment alone can electrostatically catalyze a chemical reaction when compared with the corresponding disordered solvent. Our results show that a 0.2 V/Å external electric field, which is below the threshold for bond breaking of solvent molecules, leads to significant ordering of bulk methanol solvent and the ionic liquid [EMIM][BF4]. Importantly, in the absence of this same field, the ordered solvent lowers the activation energy of the hydrogen-transfer reaction of o-alkylphenyl ketones in excess of 20 kcal/mol when the solvent is methanol and by over 30 kcal/mol for [EMIM][BF4]. Even a 0.1 V/Å external field has effects of ca. 10 and 20 kcal/mol, respectively. This work suggests a possible strategy for scaling electrostatic catalysis by applying a pulsed external field to the reaction medium to maintain solvent ordering while allowing the reaction to proceed largely in the absence of an external field.

Entropic quantum machine

We study nanomachines whose relevant (effective) degrees of freedom f >> 1 but smaller than f of proteins. In these machines, both the entropic and the quantum effects over the whole system play the essential roles in producing nontrivial functions. We therefore call them entropic quantum machines (EQMs). We propose a systematic protocol for designing the EQMs, which enables a rough sketch, accurate design of equilibrium states, and accurate estimate of response time. As an illustration, we design a novel EQM, which shows two characteristic shapes. One can switch from one shape to the other by changing temperature or by applying a pulsed external field. We discuss two potential applications of this example of an EQM.

Exploring the Crystallinity of Mold Fluxes by Pulsed Electric Current

Herein, the effect of a pulsed external field on the crystallinity of CaO–SiO2–Al2O3‐based mold fluxes with different basicities during continuous cooling is investigated by means of X‐ray diffraction and scanning electron microscope analysis. It is well known that an increase in basicity is advantageous for increasing the crystallinity of a mold flux, which relates closely to the horizontal heat transfer and lubrication during casting. However, this rule has changed under electrical pulse processing. In this study, by comparing the crystallinity and crystal phase morphology of the different basicities of mold fluxes after pulsed current treatment, it is found that a pulsed current increases the crystallinity of mold fluxes with low basicity but inhibits the formation of the crystalline phase of mold fluxes with high basicity. It is seen that an electric pulse can play a significant role in regulating the crystallinity of mold fluxes, thus providing a new pathway to solve the contradiction between horizontal heat transfer and lubrication during solidification.

Accurate control quantum transition in a nonlinear two-level system

We investigate an accurate control of quantum transition dynamics in a nonlinear two-level system with interparticle interaction. We design an external field pulse train to manipulate a quantum system from an initial state to a desired final state via adjusting the durations of each field pulse. We obtain the analytic expressions for the durations of each field pulse, which depend on the interparticle interactions, the coupling strength, the amplitude of the external field, and the desired transition probability value of the final state. We show the transition probability can be controlled accurately to any desired value, including the arbitrary superposition state and complete population transfer. With the aid of phase space approach and energy conservation law, an theoretical interpretation of such control technique is given. We also discuss the superiority of the control technique by calculating the infidelity of control process.

The continuous control of output power of a CuBr laser by a pulsed external magnetic field

Abstract An axial pulsed external magnetic field (EMF), instead of continuous one, has been applied on the operation of a CuBr laser with small-bore tube. A double synchronize triggering system has been designed and used for the laser pumping and simultaneously applying a magnetic field pulses up to 1100 Gauss. By applying an EMF along the tube axis, the laser output power has been decreased. Through adjusting the delay timing between triggers of magnetic field and laser discharge circuits, the laser output power can be changed, continuously. The maximum decrease of the laser output power has been obviously observed at a delay time of 120 ns between triggers.

A two-terminal perpendicular spin-transfer torque based artificial neuron

A two-terminal perpendicular spin-transfer torque (p-STT) based neuron with a 100 × 100 nm device size was fabricated with a p-STT magnetic tunneling junction (MTJ) spin valve. It performed well in the integrate-and-fire event of the input voltage pulse (spike) cycles, which originate from the imperfect structure of the MgO tunneling barrier in a p-STT MTJ spin valve (i.e. a nano-scale poly-grain size distribution of the MgO tunneling barrier). In particular, the integrate-and-fire cycles rapidly increase as the input voltage pulse (spike) amplitude, width, and external assistant perpendicular magnetic field decrease.

Nonlinear Stimulated Raman Exact Passage by Resonance-Locked Inverse Engineering.

We derive an exact and robust stimulated Raman process for nonlinear quantum systems driven by pulsed external fields. The external fields are designed with closed-form expressions from the inverse engineering of a given efficient and stable dynamics. This technique allows one to induce a controlled population inversion which surpasses the usual nonlinear stimulated Raman adiabatic passage efficiency.

Residual currents generated from vacuum by an electric field pulse in 2+1 dimensional QED models

In the framework of strong field QED, the generation of a residual alternating polarization current is demonstrated, which remains after switching off an external field pulse. This effect is stipulated by inertial properties of the physical vacuum. In the standard vacuum D = 2+1 QED, this current is rapidly damped fast but can be available, apparently, for observation in the graphene, where the Fermi velocity vF ≪ c plays an analogous role as the light velocity.

Polarizabilities of Impurity Doped Quantum Dots Under Pulsed Field: Role of Multiplicative White Noise

We perform a rigorous analysis of the profiles of a few diagonal and off-diagonal components of linear (α xx , α yy , α xy , and α yx ), first nonlinear (β xxx , β yyy , β xyy , and β yxx ), and second nonlinear (γ xxxx , γ yyyy , γ xxyy , and γ yyxx ) polarizabilities of quantum dots exposed to an external pulsed field. Simultaneous presence of multiplicative white noise has also been taken into account. The quantum dot contains a dopant represented by a Gaussian potential. The number of pulse and the dopant location have been found to fabricate the said profiles through their interplay. Moreover, a variation in the noise strength also contributes evidently in designing the profiles of above polarizability components. In general, the off-diagonal components have been found to be somewhat more responsive to a variation of noise strength. However, we have found some exception to the above fact for the off-diagonal β yxx component. The study projects some pathways of achieving stable, enhanced, and often maximized output of linear and nonlinear polarizabilities of doped quantum dots driven by multiplicative noise.

Analytical solution of precessional switching in nanomagnets driven by hard-axis field pulses

The precessional switching process of a magnetic nanoparticle subject to external field pulses applied along the hard-axis is considered. The critical field pulse amplitude necessary to realize the switching is determined. Then, the analytical solution of magnetization switching dynamics is derived in the lossless limit by using elliptic functions. Moreover, expressions for the field pulse duration tolerances which guarantee successful switching are also obtained. The theoretical predictions are verified by macrospin numerical simulations of ultra-fast magnetization switching.

Transient eddy current flow metering

Measuring local velocities or entire flow rates in liquid metals or semiconductor melts is a notorious problem in many industrial applications, including metal casting and silicon crystal growth. We present a new variant of an old technique which relies on the continuous tracking of a flow-advected transient eddy current that is induced by a pulsed external magnetic field. This calibration-free method is validated by applying it to the velocity of a spinning disk made of aluminum. First tests at a rig with a flow of liquid GaInSn are also presented.

Polarizabilities of Impurity Doped Quantum Dots under Pulsed Field: Role of Additive White Noise

We make a rigorous exploration of the profiles of a few diagonal and off-diagonal components of linear (αxx, αyy, αxy andαyx), first nonlinear (βxxx, βyyy, βxyy andβyxx), and second nonlinear (γxxxx, γyyyy, γxxyyandγyyxx) polarizabilities of quantum dots under the influence of external pulsed field. Simultaneous presence of additive white noise has also been considered. The quantum dot contains dopant described by a Gaussian potential. The numbers of pulse and the dopant location have been found to fabricate the said profiles jointly. The β components display greater complexity in their profiles in comparison with the α and γ counterparts. The presence of noise prominently enhances the influence of dopant coordinate on the polarizability profiles, particularly for α and γ components. However, for β components, the said influence becomes quite evident both in the presence and absence of additive noise. The study reveals some means of achieving stable, enhanced, and often maximized output of noise-driven linear and nonlinear polarizabilities.

Ultrafast magnetization switching by spin-orbit torques

Spin-orbit torques induced by spin Hall and interfacial effects in heavy metal/ferromagnetic bilayers allow for a switching geometry based on in-plane current injection. Using this geometry, we demonstrate deterministic magnetization reversal by current pulses ranging from 180 ps to ms in Pt/Co/AlOx dots with lateral dimensions of 90 nm. We characterize the switching probability and critical current Ic as a function of pulse length, amplitude, and external field. Our data evidence two distinct regimes: a short-time intrinsic regime, where Ic scales linearly with the inverse of the pulse length, and a long-time thermally assisted regime, where Ic varies weakly. Both regimes are consistent with magnetization reversal proceeding by nucleation and fast propagation of domains. We find that Ic is a factor 3–4 smaller compared to a single domain model and that the incubation time is negligibly small, which is a hallmark feature of spin-orbit torques.

State transfer of nS ultracold Rydberg atoms in external electric fields

The state transfer from cesium nS (n = 45−53) states to Stark (product) states induced by a weak electric field (WEF) pulse was investigated using the state-selective field ionization method in a standard magneto-optic trap (MOT). The WEF pulse shifts the nS Rydberg states that anticross with the (n − 4) hydrogen-like manifolds, causing state transitions from the initial excited nS state to the Stark states. The mechanism of transfer was investigated by changing the rising and falling time of the WEF pulse and the switching off time of the external field pulse had an important role during the evolution process of product states. The population of the product states is also measured as a function of the principal quantum number and Rydberg densities.

Hybrid surface-flux method for extraction of the ionization amplitude from the calculated wave function

A method is proposed for extracting ionization amplitudes from the solution of the time-dependent Schr\"odinger equation (TDSE) describing a system in a time-dependent external field. The method is a hybrid of two earlier developed methods, the time-dependent surface flux (t-SURFF) method and the method using the propagated wavepacket as the source term in a time-independent driven Schr\"odinger equation with the field-free Hamiltonian. It is demonstrated that the method combines the advantages of the parent ones and allows the extraction of ionization amplitudes by solving the TDSE within a small spatial domain (with the boundary conditions provided by exterior complex scaling) and a time interval, not exceeding the external field pulse duration.

Production of large volume, strongly magnetized laser-produced plasmas by use of pulsed external magnetic fields

The production of strongly magnetized laser plasmas, of interest for laboratory astrophysics and inertial confinement fusion studies, is presented. This is achieved by coupling a 16 kV pulse-power system. This is achieved by coupling a 16 kV pulse-power system, which generates a magnetic field by means of a split coil, with the ELFIE laser facility at Ecole Polytechnique. In order to influence the plasma dynamics in a significant manner, the system can generate, repetitively and without debris, high amplitude magnetic fields (40 T) in a manner compatible with a high-energy laser environment. A description of the system and preliminary results demonstrating the possibility to magnetically collimate plasma jets are given.