Realistic Electric Fields Research Articles

Molecular dynamics simulations of the effect of static electric field on progressive ice formation.

Ice accumulation under static electric fields presents a significant hazard to transmission lines and power grids. Contemporary computational studies of electrofreezing predominantly probed excessive electric fields (109 V/m) that are significantly higher than those typically encountered in proximity to transmission lines. To elucidate the influence of realistic electric fields (105 V/m) on ice crystallization, we run extensive molecular dynamics (MD) simulations across dual ice-water coexistence systems. Three aspects of work were accordingly examined. First, we investigated the influence of the effect of static electric fields, with a strength of 105 V/m, along three orthogonal axes on the phase transition during the encountered freezing and melting processes. Second, we established the mechanism of how the direction of an electric field, the initial ice crystallography, and the adjacent crystal planes influence the solidification process. Third, the results of our MD simulations were further post-processed to determine the dipole moment, radial distribution, and angle distribution resulting from the static electric field. Our results indicate that while weak electric fields do not cause complete polarization of liquid water molecules, they can induce a transition to a more structured ice-like geometry of the water molecules at the ice-water interphase region, particularly when applied perpendicular to the ice-water interphase. Notably, the interface adjacent to cubic ice exhibits a greater response to the electric fields than that adjacent to hexagonal ice. This is attributable to the intrinsic differences in their original hydrogen bonding networks.

Transcranial electric stimulation modulates firing rate at clinically relevant intensities

BackgroundNotwithstanding advances with low-intensity transcranial electrical stimulation (tES), there remain questions about the efficacy of clinically realistic electric fields on neuronal function. ObjectiveTo measure electric fields magnitude and their effects on neuronal firing rate of hippocampal neurons in freely moving rats, and to establish calibrated computational models of current flow. MethodsCurrent flow models were calibrated on electric field measures in the motor cortex (n = 2 anesthetized rats) and hippocampus. A Neuropixels 2.0 probe with 384 channels was used in an in-vivo rat model of tES (n = 4 freely moving and 2 urethane anesthetized rats) to detect effects of weak fields on neuronal firing rate. High-density field mapping and computational models verified field intensity (1 V/m in hippocampus per 50 μA of applied skull currents). ResultsElectric fields of as low as 0.35 V/m (0.25–0.47) acutely modulated average firing rate in the hippocampus. At these intensities, firing rate effects increased monotonically with electric field intensity at a rate of 11.5 % per V/m (7.2–18.3). For the majority of excitatory neurons, firing increased for soma-depolarizing stimulation and diminished for soma-hyperpolarizing stimulation. While more diverse, the response of inhibitory neurons followed a similar pattern on average, likely as a result of excitatory drive. ConclusionIn awake animals, electric fields modulate spiking rate above levels previously observed in vitro. Firing rate effects are likely mediated by somatic polarization of pyramidal neurons. We recommend that all future rodent experiments directly measure electric fields to insure rigor and reproducibility.

Exploiting Atomic Control to Show When Atoms Become Molecules

AbstractPrecision‐placed atom qubits in silicon offer a unique means to confine electrons and control their spins with extreme accuracy, which can be leveraged to construct powerful quantum computers. To date atom qubits in silicon have been successfully realized using electrons hosted either on a single phosphorus atom or on a multi‐donor quantum dot. Here, a novel molecular regime is explored in which electrons are bound to two donor dots separated by ≈8 nm in a natural silicon substrate. The molecular state, provided by these spatially separated donors, is used to study with exquisite precision the impact of confinement potential on the electronic and spin properties of qubits. Unique spin filling measurements, performed on up to five electrons, confirm how electrons are shared between both sites of the molecule, forming hybridized molecular states. The precise atomic locations of the donor atoms in the silicon lattice are determined by combining the experimental electron spin resonance spectra and the state‐of‐the‐art atomistic modeling of multi‐electron wave‐functions in presence of realistic electric fields. The donor molecule studied in this work exhibits excellent qubit properties and addresses the impact that the confinement potential has, at the atomic scale, on the desired properties of electron spin qubits.

Electric field control of electromagnon frequency in multiferroics

Electromagnons, which are coupled polar and magnetic excitations in magnetoelectric materials, are of large interest for electronic and computing technological devices. Using molecular dynamics simulations based on an ab initio effective Hamiltonian, we predict that the frequency of several electromagnons can be tuned by the application of electric fields in the model multiferroic ${\mathrm{BiFeO}}_{3}$, with this frequency either increasing or decreasing depending on the selected electromagnon. In particular, we show that the frequency of electromagnons localized at ferroelectric domain walls can be tuned over a 200 GHz range by realistic dc electric fields. We interpret the realized frequency increase (decrease) by local hardening (softening) of the associated polar phonons which couples to the applied electric field. The increase versus decrease in the electromagnon frequency is further found to be correlated with the real-space localization of such phonons.

Analysis of electric fields in mass spectrometry

Analysis of two-dimensional and, in particular, three-dimensional fringing electric fields in quadrupole mass spectrometers is a challenging task. In this work, we consider an analytical approach for such analysis. We show how to obtain precise amplitudes of multipole components of 2D and 3D electric fields. We describe some mathematical methods for accurate evaluation of realistic three-dimensional electric fields generated by quadrupole mass filters. This technique can be applied to obtain precise amplitudes of multipole components of 3D electric fields as distributions along some axis. All methods described here, are implemented in the PAnalyser application, which we use to analyse electric fields in different quadrupole systems. We provide some practical examples, which include analysis of quadrupole mass filters with hyperbolic and circular electrodes, a 3D Paul ion trap, and a linear ion trap with triangular electrodes. We also investigate 3D fringing electric fields: a DC and Quadrupole fringing fields within quadrupole mass filters with circular electrodes. Another example, interesting both theoretically and practically, is the analysis of the fringing field between two adjacent quadrupole systems – electrodes of a Brubaker pre-filter and an analytical mass filter. We also show how to synthesize an electric field from a given axial expansion of multipole components.

Modelling acute and lasting effects of tDCS on epileptic activity.

Transcranial Direct brain stimulation (tDCS) is commonly used in order to modulate cortical networks activity during physiological processes through the application of weak electrical fields with scalp electrodes. Cathodal stimulation has been shown to decrease brain excitability in the context of epilepsy, with variable success. However, the cellular mechanisms responsible for the acute and the long-lasting effect of tDCS remain elusive. Using a novel approach of computational modeling that combines detailed but functionally integrated neurons we built a physiologically-based thalamocortical column. This model comprises 10,000 individual neurons made of pyramidal cells, and 3 types of gamma-aminobutyric acid (GABA) -ergic cells (VIP, PV, and SST) respecting the anatomy, layers, projection, connectivity and neurites orientation. Simulating realistic electric fields in term of intensity, main results showed that 1) tDCS effects are best explained by modulation of the presynaptic probability of release 2) tDCS affects the dynamic of cortical network only if a sufficient number of neurons are modulated 3)VIP GABAergic interneurons of the superficial layer of the cortex are especially affected by tDCS 4) Long lasting effect depends on glutamatergic synaptic plasticity.

Relation between the electric field and activation of cortical neurons in transcranial electrical stimulation

BackgroundTo address the brain areas and circuits affected by transcranial electrical stimulation (tES), which had been used widely to treat psychiatric and neurological diseases, the stimulus-induced electric field in the cortex was calculated using a head model that reflects anatomical information. To obtain detailed information at the macroscopic and microscopic levels, multi-scale modeling was proposed that integrates the head model with multi-compartmental models of cortical neurons. ObjectiveOur goal was to use multi-scale modeling to describe the relation between the stimulus-induced electric field and neuronal responses during tES. MethodsWe simulated sub- and supra-threshold neuronal responses to stimulus-induced uniform and realistic electric fields. For the realistic electric field, multi-scale models that combined the head model derived from structural MRIs and multi-compartmental models of neurons were constructed. Then, we simulated the steady-state membrane polarization for sub-threshold stimulation and the excitation threshold for supra-threshold stimulation by varying the tES montages. The electric field calculated was decomposed into two orthogonal components, the radial and tangential fields, which were compared to the neuronal responses. ResultsThe stimulus-induced electric field depended strongly on stimulus parameters, and neuronal excitability showed a higher correlation with the radial field. We demonstrated that neurons exhibited linear polarization during sub-threshold stimulation depending on the local radial field intensity that resulted in a significant relation regardless of the tES montage. Supra-threshold stimulation showed a stronger relation with the radial field, but rather complex patterns of excitation thresholds depending on neurons’ morphological features. ConclusionOur results indicated that cortical neurons are affected greatly by the relative direction of the stimulus-induced electric field, which may be a necessary step toward a detailed understanding of tES’ potential mechanisms.

The role of electron scattering from registration detector in the “Troitsk nu-mass” MAC-E type spectrometer

There is a proposal to search for a sterile neutrino in a few keV mass range by the “Troitsk nu-mass” facility. In order to estimate sterile neutrino mixing one needs to make precision spectrum measurements well below the endpoint using the existing electrostatic spectrometer with a magnetic adiabatic collimation, or MAC-E filter. The expected signature will be a kink in the electron energy spectrum in tritium beta-decay. In this paper we consider the systematic effect of electron backscattering on the detector used in the spectrometer. For this purpose we provide a set of Monte-Carlo simulation results of electron backscattering on a silicon detector with a thin golden window with realistic electric and magnetic fields in the spectrometer. We have found that the probability of such an effect reaches up to 20–30%. The scattered electron could be reflected backwards to the detector by electrostatic field or by magnetic mirror. There is also a few percent probability to escape from the spectrometer through its entrance. A time delay between the scattering on the detector and the return of the reflected electron can reach a couple of microseconds in the Troitsk spectrometer. Such estimations are critical for the planning upgrades of the detector and the registration electronics. All considered effects are relevant to any MAC-E type spectrometer with solid detector.

The effects of transient, localized electric fields on equatorial electron acceleration and transport toward the inner magnetosphere

Motivated by recent observations of intense electric fields and elevated energetic particle fluxes within flow bursts beyond geosynchronous altitude (Runov et al., 2009, 2011), we apply modeling of particle guiding centers in prescribed but realistic electric fields to improve our understanding of energetic particle acceleration and transport toward the inner magnetosphere through model‐data comparisons. Representing the vortical nature of an earthward traveling flow burst, a localized, westward‐directed transient electric field flanked on either side by eastward fields related to tailward flow is superimposed on a nominal steady state electric field. We simulate particle spectra observed at multiple THEMIS spacecraft located throughout the magnetotail and fit the modeled spectra to observations, thus constraining properties of the electric field model. We find that a simple potential electric field model is capable of explaining the presence and spectral properties of both geosynchronous altitude and “trans‐geosynchronous” injections at higher L‐shells (L > 6.6 RE) in a manner self‐consistent with the injections' inward penetration. In particular, despite the neglect of the magnetic field changes imparted by dipolarization and the inductive electric field associated with them, such a model can adequately describe the physics of both dispersed injections and depletions (“dips”) in energy flux in terms of convective fields associated with earthward flow channels and their return flow. The transient (impulsive), localized, and vortical nature of the earthward‐propagating electric field pulse is what makes this model particularly effective.

Method for full Bloch sphere control of a localized spin via a single electrical gate

We calculate the dependence on an applied electric field of the g tensor of a single electron in a self-assembled InAs∕GaAs quantum dot. We identify dot sizes and shapes for which one in-plane component of the g tensor changes sign for realistic electric fields, and show that this should permit full Bloch sphere control of the electron spin in the quantum dot using only a static magnetic field and a single vertical electric gate.

Two-dimensional electronic spectroscopy of an excitonically coupled dimer

The two-dimensional three-pulse photon echo signals from a dimer system are investigated in detail. A perturbative approach is used to calculate the response of the system via a numerical propagation of the density matrix in exciton state representation. Exciton vibrational coupling is modeled by Redfield relaxation theory. The main goal of this paper is to dissect the two-dimensional spectrum of the dimer to give better understanding of how the dynamics influence the spectral features. We will show how different Liouville pathways lead to the appearance/disappearance of diagonal and cross peaks. The inclusion of a Gaussian shaped electric field will be contrasted to the use of delta-pulses in the impulsive limit. The impulsive limit is found to be a satisfactory approximation at long population times, while at shorter times, in the pulse-overlap region, more realistic electric fields are called for.

Electric-field induced exponential enhancement of electromagnetic absorption in semiconductors at a strong magnetic field

The influence is investigated of a dc (electric) field F perpendicular to a strong uniform static magnetic field H on the absorption coefficient of a semiconductor for high-frequency electromagnetic waves polarized along the electric field. The absorption coefficient for some (e.g., phonon-assisted) cyclotron resonances is predicted to exhibit a strong, almost exponential increase by orders of magnitude with increasing F up to a relatively weak F c , the upper limit of the range in which the considered absorption mechanism holds true. The predicted effect is suggested to be observable in typical semiconductors at realistic electric and strong magnetic fields and low temperatures.

Quasi‐steady drift paths in a model magnetosphere with AMIE electric field: Implications for ring current formation

Recent measurements show that magnetospheric convection electric fields during the main phases of magnetic storms are much more complicated in spatial structure than the electric fields that have generally been used to model formation of the stormtime ring current. To investigate the transport effects of such more realistic stormtime electric fields on magnetospheric charged particles, we map the Assimilative Model of Ionospheric Electrodynamics (AMIE) electric potential functions analytically with a simple magnetic field model at selected times of interest during several magnetic storms from 1997–1998 and during the extremely large storm of July 2000. We calculated corresponding contours of constant Hamiltonian (kinetic plus potential energy) for first invariant values that range from 0 to 30 MeV/G (representative of equatorially mirroring cold plasma, ring current, and radiation‐belt ions and electrons). These equatorial contours would constitute drift paths if the electric field were truly constant in time. We thus calculated how far along such quasi‐drift paths the corresponding particles would have drifted after specific amounts of time, and we compare these quasi‐drift characteristics with those obtained from a simple semiempirical model of the convection electric field. We find considerable variability among stormtime equatorial quasi‐drift paths, reflecting the known variability of AMIE equipotentials. Patterns of equatorial quasi‐drift in the simplified electric field model are (by construction) symmetric about the dawn‐dusk meridian. During the main phases of storms, the equatorial electric field derived from AMIE tends to be strongest in an MLT sector several hours wide on the night side. This concentration of AMIE equipotentials provides a channel for rapid transport (requiring ∼20–30 min) for ions with first invariant values representative of the ring current population from the nightside neutral line to low L values (∼3–4) near dusk, where the partial ring current forms. During the extremely large “Bastille Day” storm of 15 July 2000 (minimum Dst = −300 nT) the drift patterns derived from AMIE show penetration of ions to as low as L ∼ 2. This deep penetration of ring current ions could help to account for the very strong ring current that was observed during this storm. The quasi‐steady state drift properties help us anticipate the implications of a more realistic electric field model for the particle drifts that lead to the formation of the stormtime ring current.

Zener Breakdown in the Optical Absorption of Semiconductor Superlattices

Electrical breakdown in the optical absorption of strongly coupled superlattices takes place at high, but realistic electric fields. The Zener breakdown is characterized by a strong inhomogeneous line broadening and, eventually, the disappearance of sharp transitions. We show that the Zener breakdown is unambiguously related to the continuous nature of the spectrum and the infinite extension of the wave functions.

Modelling geomagnetically induced currents produced by realistic and uniform electric fields

The methods used to model geomagnetically induced currents (GIC) on power systems depend on the nature of the electric field used as input. A uniform electric field, often used to simplify the modelling, is shown to have different properties from realistic nonuniform fields. Realistic fields which go to zero at infinity can be uniquely represented by the sum of the gradient of a scalar potential and the curl of a vector function. The scalar potential term is conservative and irrotational, while the vector term is nonconservative and solenoidal. In contrast, a uniform electric field can be represented simply by the gradient of a scalar potential. These different mathematical properties mean that modelling techniques derived for uniform fields may not work for realistic fields. This is examined using, as an example, the modelling of GIC produced in a conducting network at the surface of the Earth. It is shown that uniform electric fields can be represented by voltage sources in the transmission lines or at power system ground points. However, realistic electric fields, such as those produced by the auroral electrojet, cannot be represented by voltage sources at ground points and have to be represented by a voltage source in the transmission lines.

Cusp ion injection and number density modeling in realistic electric and magnetic fields

A quantitative model of ion injection and number density in the magnetospheric cusp is developed, incorporating mutually consistent magnetic and electric fields. This work extends the method of Onsager et al., who calculated precipitating particle fluxes from quantitative models of magnetosheath flow and ion acceleration at the magnetopause. We have simulated cusp ion energy-latitude spectrograms at mid-altitude. Both the large-scale energy-latitude dispersion and the imbedded small-scale energy-pitch-angle V signatures are clearly evident in these simulated spectrograms. Our results show that a much finer V microsignature is obtained when the ion injection source is restricted to a small region. However, the cutoff of the plasma injection at the magnetosheath sonic line also yields relatively narrow V's, even without restricting the injection region to a small locus on the magnetopause. This effect is most noticeable in winter conditions. By incorporating temporal effects of separate bursts of reconnection which last 1.4 min and are 3.6 and 4.6 mins apart, our simulation has also reproduced a meso-scale cusp ion injection structure, whose distinguishing feature is the overlap of two independent injections on the same field lines. This cusp injection model which returns precipitating particle flux also allows us to calculate the number density profile in the cusp. Our preliminary result along the noon-meridian cusp is presented.

Physical modelling and experimental investigation of the abrupt afternoon decrease of Cosmic Noise Absorption

The amplitude of the substorm-related Cosmic Noise Absorption (CNA) bay often displays significant reduction near the noon MLT meridian in the auroral zone. Statistically the Local Time position of the afternoon CNA depression shows clear dependence on the IMF B Z values during preceding hour. We modelled the drift motion of charged particles (having different energies and pitch angles) in realistic magnetic and electric fields to investigate whether the convection electric field may cause considerable reduction of energetic electron precipitation. We found that the outward convection and deceleration lead to considerable decrease of particle fluxes and changes in their characteristics during particle drift over the noon sector. As a result, we found this effect alone may reduce precipitation by at least 30% over a 3 hour MLT segment of particle drift trajectories close to noon.

Particle tracing in the magnetosphere: New algorithms and results

We use a fast, efficient method to trace charged particles through realistic magnetospheric electric and magnetic fields, greatly reducing computer simulation times. The method works for particles having arbitrary charge, energy, or pitch angle but which conserve the first two adiabatic invariants. We also apply an efficient method of classifying drift orbits, which greatly simplifies the task of identifying the last closed drift path or other drift boundaries. Finally, we calculate the time‐independent evolution of the bounce‐averaged phase space density along convective drift orbits. With these three tools, convective evolution of the particle distribution from the tail can now be described quantitatively, an essential step in understanding the production of unstable distributions in the magnetosphere. One can also categorize topologically different drift orbits, which is necessary to understand the unique particle signatures of the convecting plasma such as Alfvén layers and the plasmapause. These signatures can then be used to extract the electric and magnetic fields or to test the validity of the model fields. The method is particularly appropriate for particles in the energy range 0.01<E<100 keV, which are influenced by both electric and magnetic fields, and for time periods without invariant destroying waves.

Model studies of the latitudinal extent of the equatorial anomaly during equinoctial conditions

The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between ±20° magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high‐frequency propagation and to transionospheric propagation effects. During the daytime, upward E × B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately ±15° dip latitude. The E × B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of ƒ0F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.

Extraction Efficiency Improvement through Central Region Studies

To enhance Crocker Nuclear Laboratory's ability to supply large beam currents to experiments in MFE, medical, and basic nuclear physics, it is necessary to increase the extraction efficiency of its cyclotron. Adding axial slits to the central region (as done at SIN) has increased deuteron extraction efficiency from 55% to 75%. Another improvement planned will decrease the large radial phase space presented to the extractor. A radial slit system is being designed to limit the radial momentum and displacement from early equilibrium orbits. To optimize this, studies were made with a 3-dimensional electrolytic tank system in which realistic central electric fields can be simulated. Central orbits were calculated using an adaptation of the Michigan State University code PINWHEEL, which allows placement of radial slits. Using a general orbit code the ions are then accelerated to the extractor where radial phase space and extraction efficiency are determined.