Low-frequency Field Research Articles

The Influence of the Surface Density of Thermally Expanded Graphite Sheets on the Acoustic Wave Transmission

The paper presents the results of experimental and theoretical studies of the influence of the surface density of a thin porous sheet of thermally expanded graphite on the transmission coefficient of the acoustic wave. The possibility of using the theory of thin films to describe the processes of transmission of acoustic waves through porous sheet in the field of low frequencies and small thicknesses has been proven. The influence of the operating frequency on the sensitivity of the transmission coefficient to the surface density of the sheet was assessed.

Measurement-Based Neural Network Technique for Modeling the Low-Frequency Electric Field Radiated Behavior of Satellite Units

In this work, a machine learning technique employing a measurement-driven artificial neural network (ANN) architecture is proposed as a solution to the precise determination of the position and the moment of equivalent electric dipoles for unit characterization. These dipoles are used to match the generated electric field from various sources inside the spacecraft, during space exploration missions. Various methodologies for unit characterization have been proposed in the literature, the most common being the heuristic approaches, least squares variants, method of auxiliary sources, etc. Contrary to the previous time-consuming post-process methodologies, the proposed electric dipole neural network (EDMnet) can offer a real-time characterization of the measured unit (Device Under Test) after a proper training stage, especially as a fast pre-compliance method. The network uses the electric field vector, measured at 14 discrete locations, as input and reports the position and moment of the electric dipole that best matches the measured fields. In this work, various ANN architectures are tested and compared in order to select the optimal EDMnet parameters for accurate source identification. It is shown that the size of the artificial training data affects the performance of the network. The proposed EDMnet can provide accuracy in mm-scale, with respect to dipole positioning, greater than 99% in dipole moment prediction.

Novel Volume Integral Equation Approach for Low-Frequency E-Field Dosimetry of Transcranial Magnetic Stimulation.

A new volume integral equation (VIE) approach is introduced to study transcranial magnetic stimulation (TMS) and high-contrast media at low frequencies. This new integral equation offers a simple solution to the high-contrast breakdown observed in low-frequency electric field (E-field) dosimetry of conductive media. Specifically, we employ appropriate approximations that are valid for low frequencies and stabilize the VIE by introducing a basis expansion set that removes solutions associated with high eigenvalues in the equation. The new equation is devoid of high-contrast breakdown and does not require the use of auxiliary surface variables or projectors, providing a straightforward practical solution for the VIE analysis of TMS. Our results indicate that the novel VIE formulation matches boundary element, finite element, and analytical solutions. This new VIE represents a first step towards including anisotropy in integral equation E-field dosimetry for brain stimulation.

Pulsed electric field at resonance frequency combat Klebsiella pneumoniabiofilms.

Healtcare-associated infections have increased due to the development of antimicrobial resistance (AMR) of Gram-negative pathogens (GNPs) and the development of outbreacks over the past two decades. In this work, we investigated how exposure to positive electric pulses affects the growth characteristics of Klebsiella pneumonia (K. pneumonia), a common cause of pneumonia. We explored the impact of varying exposure frequencies (0.2-2Hz) and time (15-90min, at resonance frequency) on bioelectric signals produced during cell division, biofilm formation, and bacterial antibiotic susceptibility. Our research found that an extremely low-frequency pulsed electric field (ELF-PEF) significantly inhibited K. pneumonia growth. Specifically, exposure to 0.8Hz for one hour increased the antibiotic susceptibility of K. pneumonia to inhibitors of cell wall formation, proteins, β-lactamase, DNA, and other substances. We also noticed a notable decrease in K. pneumonia biofilm development exposed to ELF-PEF. Our results suggest that the interaction of K. pneumonia cells with ELF-PEF at the specified frequency and time alters cellular activity and bacterial structure. This technique may be used in the future to treat K. pneumonia infections both in vitro and in vivo.

Numerical and analytical inspection of magnetic field effects in the radical pair mechanism by a simplified rate equation model.

The radical pair mechanism is by now the most prominent candidate for a biologically relevant quantum effect of magnetic fields. Recently, N. Ikeya and J. R. Woodward demonstrated a magnetic field effect for sub-extremely low frequency (ELF) fields in the mT range by investigating the autofluorescence spectrum of flavin adenine dinucleotide in living HeLa cells. We apply a simple rate equation model to show numerically and analytically that magnetic field effects can be expected to exist in the whole ELF range.

Molecular dynamics of deformation and fragmentation processes in liquid droplets under pulsed electric field

The deformation and breakup of droplets are widely observed in various aspects of production and daily life. Using molecular dynamics simulations, this study primarily investigated the deformation and breakup of droplets in a nitrogen environment under pulsed electric fields of different frequencies. The results indicate that the droplet deformation undergoes periodic changes under the influence of the pulsed electric field. At equivalent electric field strengths, droplets are more likely to break under a 6.25 GHz pulsed electric field compared to a 25 GHz pulsed electric field, where droplet deformation remains stable without breaking. At every frequency, the bond energy of water molecules consistently decreases. The solvent-accessible surface area changes with the electric field at 6.25 GHz. Furthermore, the minimum value of hydrogen bond quantity reached under high-frequency pulsed fields is higher than that under low-frequency fields. The change in hydrogen bond quantity is inversely proportional to the solvent-accessible surface area. These findings provide insights into the microscopic mechanisms of droplet deformation and breakup, offering a reference for future studies.

Fluorescence by incoherently pumped polar quantum dot placed in a low-frequency monochromatic field

Abstract We studied spectral properties of high-frequency fluorescent radiation from a two-level quantum system with broken inversion spatial symmetry, which can be implemented as a model of a one-electron two-level asymmetric polar semiconductor quantum dot whose electric dipole moment operator has permanent unequal diagonal matrix elements. The case of the excitation of this system by incoherent pumping of some sort supplemented with an additional driving monochromatic field, which frequency is much lower than the optical transition frequency of the quantum dot, was considered. An analytical expression for the fluorescence spectrum as a function of the amplitude and frequency of the monochromatic driving field, as well as of the pumping intensity, was derived. We also discussed feasible practical applications of the effects under study in the field of quantum optics and optoelectronics for the generation of high-frequency radiation, possessing stationary equidistant spectrum, and for the control of the radiative properties of such nanodevices as spasers (dipole nanolasers).

Decreases in Sympathetic Activity Due to Low-Intensity Extremely Low-Frequency Electric Field Treatment Revealed by Measurement of Spontaneous Fluctuations in Skin Conductance in Healthy Subjects

Decreases in Sympathetic Activity Due to Low-Intensity Extremely Low-Frequency Electric Field Treatment Revealed by Measurement of Spontaneous Fluctuations in Skin Conductance in Healthy Subjects

High sensitivity measurement of ULF, VLF, and LF fields with a Rydberg-atom sensor.

Fields with frequencies below megahertz are challenging for Rydberg-atom-based measurements, due to the low-frequency electric field screening effect caused by the alkali-metal atoms adsorbed on the inner surface of the container. In this paper, we investigate electric field measurements in the ultralow frequency (ULF), very low frequency (VLF), and low frequency (LF) bands in a Cs vapor cell with built-in parallel electrodes. With optimization of the applied DC field, we achieve high-sensitive detection of the electric field at frequencies of 1 kHz, 10 kHz, and 100 kHz based on the Rydberg-atom sensor, with the minimum electric field strength down to 18.0 μV/cm, 6.9 μV/cm, and 3.0 μV/cm, respectively. The corresponding sensitivity is 5.7 μV/cm/Hz1/2, 2.2 μV/cm/Hz1/2, and 0.95 μV/cm/Hz1/2 for the ULF, VLF, and LF fields, which is better than a 1-cm dipole antenna. Besides, the linear dynamic range of the Rydberg-atom sensor is over 50 dB. This work presents the potential to enable more applications that utilize atomic sensing technology in the ULF, VLF, and LF fields.

Electrostatic Waves and Electron Holes in Simulations of Low-Mach Quasi-perpendicular Shocks

Collisionless low-Mach-number shocks are abundant in astrophysical and space plasma environments, exhibiting complex wave activity and wave–particle interactions. In this paper, we present 2D Particle-in-Cell (PIC) simulations of quasi-perpendicular nonrelativistic (v sh ≈ (5500–22000) km s−1) low-Mach-number shocks, with a specific focus on studying electrostatic waves in the shock ramp and precursor regions. In these shocks, an ion-scale oblique whistler wave creates a configuration with two hot counterstreaming electron beams, which drive unstable electron acoustic waves (EAWs) that can turn into electrostatic solitary waves (ESWs) at the late stage of their evolution. By conducting simulations with periodic boundaries, we show that the EAW properties agree with linear dispersion analysis. The characteristics of ESWs in shock simulations, including their wavelength and amplitude, depend on the shock velocity. When extrapolated to shocks with realistic velocities (v sh ≈ 300 km s−1), the ESW wavelength is reduced to one-tenth of the electron skin depth and the ESW amplitude is anticipated to surpass that of the quasi-static electric field by more than a factor of 100. These theoretical predictions may explain a discrepancy, between PIC and satellite measurements, in the relative amplitude of high- and low-frequency electric field fluctuations.

Construction of Chiral-Magnetic-Dielectric Trinity Structures with Different Magnetic Systems for Efficient Low-Frequency Microwave Absorption.

The fabrication of carbon nanocoil (CNC)-based chiral-dielectric-magnetic trinity composites holds great significance in low-frequency microwave absorption fields. However, it is not clear that how the different magnetic systems affect the magnetic and frequency responses of the composites. Herein, four types of magnetic metals, FeCo, CoNi, FeNi, and FeCoNi, are selected to be combined with the chiral templates respectively, resulting in four types of chiral-dielectric-magnetic composites with similar morphology. The CNC templates endow all the composites with excellent dielectric loss. Further permeability analysis and the micro-magnetic simulation confirm that the frequency response region can be well adjusted by changing the magnetic systems with specific magnetic resonance modes and magnetic domain motion. Due to the synergistic effect between magnetism, chirality, and dielectricity, the FeNi-based composites exhibit the best low-frequency microwave absorption performance. The minimum RL of -60.7 dB is achieved at 6.7 GHz with an ultra-low filling ratio of 10%, and the EAB value in low-frequency region is extended to 3.7 GHz. This study provides further guidelines for the design of the chiral-dielectric-magnetic trinity composites in low-frequency microwave absorption.

Analytically controlling the laser-induced electron phase in sub-cycle motion

We develop an approach to directly control the phase of an electron's sub-cycle motion in an intense laser field. The method is established by tuning a low-frequency electric field applied on a centrosymmetric gaseous target during its interaction with a few-cycle infrared laser pulse. We find a universal analytical relation between the low-frequency electric field and its induced harmonic frequency shift, derived by the strong-field approximation. This simple relation and its universality are confirmed numerically by directly solving the time-dependent Schrödinger equation. Moreover, we demonstrate the benefits of the discovered relation in applications, including continuously and precisely tuning XUV waves and developing a method of comprehensively sampling the THz pulse. Published by the American Physical Society 2024

Stimulus effects of extremely low-frequency electric field exposure on calcium oscillations in a human cortical spheroid.

High-intensity, low-frequency (1 Hz to 100 kHz) electric and magnetic fields (EF and MF) cause electrical excitation of the nervous system via an induced EF (iEF) in living tissue. However, the biological properties and thresholds of stimulus effects on synchronized activity in a three-dimensional (3D) neuronal network remain uncertain. In this study, we evaluated changes in neuronal network activity during extremely low-frequency EF (ELF-EF) exposure by measuring intracellular calcium ([Ca2+]i) oscillations, which reflect neuronal network activity. For ELF-EF exposure experiments, we used a human cortical spheroid (hCS), a 3D-cultured neuronal network generated from human induced pluripotent stem cell (hiPSC)-derived cortical neurons. A 50 Hz sinusoidal ELF-EF exposure modulated [Ca2+]i oscillations with dependencies on exposure intensity and duration. Based on the experimental setup and results, the iEF distribution inside the hCS was estimated using high-resolution numerical dosimetry. The numerical estimation revealed threshold values ranging between 255-510 V/m (peak) and 131-261 V/m (average). This indicates that thresholds of neuronal excitation in the hCS were equivalent to those of a thin nerve fiber.

Engineering the apparent quantum yield and emission rate of fluorophore molecules by coupling fluorophore dipoles with plasmon modes of gold using low frequency electric fields

Abstract Localized surface plasmons produced by gold and silver nanostructures have been utilized to enhance the intensity of fluorophore molecules. The issue with using nanostructure plasmons for fluorescence enhancement is their short-range nature (5–50 nm from the nanostructures), which limits accessibility to a few molecules. In addition, fluorophore dipoles needed to be aligned with the plasmon electric fields to maximize the fluorescence enhancement. To address these issues, we used low-frequency electric fields (<5 MHz) and commercially available nanorod and nanosphere samples and studied their effectiveness in enhancing the fluorescence of fluorophore-labeled short single-stranded DNA molecules (22 bases). We demonstrated that DNA molecules and nanorod particles can effectively be manipulated around the charging frequency of DNA molecules (∼3 MHz). Nanorod particles enhanced the fluorescence emission rate by ∼50-fold. When the 3 MHz electric field was introduced, the emission rate increased to over 700-fold. We also found that the introduction of a 3 MHz electric field aided the enhancement of the intrinsic quantum yield fluorophore molecules, which resulted in over a 1000-fold fluorescence enhancement. This enhancement was due to the very high electric produced by polarized DNA dipoles at 3 MHz, which resulted in a torque on fluorophore dipoles and subsequently aligning the fluorophore dipole axis with the plasmon electric field. At a fundamental level, our results demonstrate the role of the low-frequency electric field in the fluorophore–plasmon coupling. These findings can directly be applied to many fluorescence detection systems, including the development of biosensors.

Modern technologies for medical rehabilitation of children with knee joint injuries

Pathology of the knee joint in children is a pressing problem in pediatric traumatology, orthopedics and pediatrics. The frequency of injuries to the knee joint is determined by the complexity of its structure, biomechanical features with a high probability of overload. At the same time, intra-articular injuries in children and adolescents, accounting for up to 7 % in the structure of inpatient pathology, are the cause of pronounced functional disorders of the lower limb and the risk of developing post-traumatic osteoarthritis Currently, physical factors are widely used at various stages of medical rehabilitation of children with damage to the capsular-ligamentous apparatus of the knee joint. The conducted studies have established the effectiveness of using local air cryotherapy in the early postoperative period, characterized by a pronounced anti-edematous, analgesic effect. Early regression of pain and elimination of microcirculatory disorders in the area of injury as a result of a course of pulsed low-frequency magnetic therapy at various stages of medical rehabilitation of children with damage to the capsular-ligamentous apparatus of the knee joint have been recorded. The beneficial effect of a pulsed low-frequency electrostatic field on indicators of orthopedic status, and thermography indicators in the area of injury in children with knee joint injuries has been established. Data were obtained on more pronounced regression of pain, earlier restoration of range of motion in the damaged joint and relief of signs of post-traumatic synovitis with the complex use of a pulsed low-frequency electrostatic field and robotic mechanotherapy in children with damage to the capsular ligamentous apparatus of the knee joint. The obtained clinical and functional results are of great importance for injuries of the knee joint with the risk of contractures in the postoperative period.

Low-frequency electric field sensing via superposition orbital angular momentum light.

The polarization and orbital angular momentum (OAM) degrees of freedom carried by light have important applications in precision optical measurement and optical sensing. Here we show that the electro-optic Pockels effect of a magnesium-doped lithium niobate (MgO:LiNbO3) crystal can be used to measure a low-frequency electric field. By exploiting the rotation property of superposition OAM light, we experimentally observe that the minimum measured precision of electric field intensity is about 0.18 V/m. This study offers a method to perform low-frequency electric field sensing.

Effect of low-frequency electric field assisted freezing on ice crystals of tilapia fish protein

The mechanism by which low-frequency alternating electric fields affect ice crystals and quality of aquatic products remain unclear. To study the effect of low-frequency electric field freezing (LF-EFAF) on product ice crystals, differences between LF-EFAF treated samples and air freezing (AF) were compared. The results showed that LF-EFAF increased the subcooling of fish protein by 0.3 °C and doubled the freezing time compared to AF. The equivalent diameter and cross-sectional area of LF-EFAF samples were reduced by up to 20% and 35% respectively, compared to AF samples. Moisture distribution of results showed that LF-EFAF restrained the increase in free water content of fish proteins. The LF-EFAF had a positive effect on the size and morphology. This resulted in less mechanical damage to the muscle fibres, which led to better water retention and inhibited moisture migration. Therefore, LF-EFAF can be an effective means of fish proteins preservation.

Multiphoton parallel transmission (MP-pTx): Pulse design methods and numerical validation.

We propose and evaluate multiphoton parallel transmission (MP-pTx) to mitigate flip angle inhomogeneities in high-field MRI. MP-pTx is an excitation method that utilizes a single, conventional birdcage coil supplemented with low-frequency (kHz) irradiation from a multichannel shim array and/or gradient channels. SAR analysis is simplified to that of a conventional birdcage coil, because only the radiofrequency (RF) field from the birdcage coil produces significant SAR. MP-pTx employs an off-resonance RF pulse from a conventional birdcage coil supplemented with oscillating -directed fields from a multichannel shim array and/or the gradient coils. We simulate the ability of MP-pTx to create uniform nonselective brain excitations at 7 T using realistic and field maps. The RF, shim array, and gradient waveform's amplitudes and phases are optimized using a genetic algorithm followed by sequential quadratic programming. A 1 ms MP-pTx excitation using a 32-channel shim array with current constrained to less than 50 Amp-turns reduced the transverse magnetization's normalized root-mean-squared error from 29% for a conventional birdcage excitation to 6.6% and was nearly 40% better than a 1 ms birdcage coil 5 kT-point excitation with optimized kT-point locations and comparable pulse power. The MP-pTx method resembles conventional pTx in its goals and approach but replaces the parallel RF channels with cheaper, low-frequency shim channels. The method mitigates high-field flip angle inhomogeneities to a level better than 3 T CP-mode and comparable to 7 T pTx while retaining the straightforward SAR characteristics of conventional birdcage excitations, as low-frequency shim array fields produce negligible SAR.

Low-frequency weak electric field measurement based on Rydberg atoms using cavity-enhanced three photon system

Introduction: Rydberg atoms are ideal for measuring electric fields due to their unique physical properties. However, low-frequency electric fields below MHz can be challenging due to the accumulation of ionized free electrons on the atomic vapor cell’s surface, acting as a shield.Method: This paper proposes a Cavity-enhanced three-photon system (CETPS) measurement scheme, which uses a long-wavelength laser to excite the Rydberg state, reducing atomic ionization and enhancing detection spectrum resolution. A theoretical model is proposed to explain the quantum coherence effect of the light field, measured electric field, and the atomic system.Result: The results show that the proposed scheme significantly increases the electromagnetically induced transparency (EIT) spectral peak and narrows the spectral width, resulting in the maximum slope increasing by more than an order of magnitude.Discussion: The paper also discusses the impact of the Rabi frequency of the two laser fields and the coupling coefficient of the optical cavity on the transmission spectrum amplitude and linewidth, along with the optimal configuration of these parameters in the CEPTS scheme.

The SF and remanence evaluation of magnetic shields based on SST for low-frequency and degaussing situation

As the dominant shielding functional material, the permalloy sheet primarily determines the static and low-frequency shielding performances of magnetically shielded room (MSR). However, the lack of measurement of shielding sheets for practical use would lead to a non-negligible evaluation error in MSR performance. Therefore, an estimation technique of shielding factor (SF) and the remanence of MSR is proposed in this paper while considering the nonlinear magnetic characteristics of the permalloy sheet tested by a single sheet tester for low-frequency field and degaussing situation (L-D-SST). First, a high-accuracy measurement system, comprising L-D-SST (for exciting magnetic field and sensing the corresponding B and H) and the control system (for applying excitation to L-D-SST and amplifying, filtering, and collecting B and H signals), is established. Weak magnetic fields at low frequencies and decaying alternating demagnetizing field excitations are separately applied to the L-D-SST for basic magnetization curves (BM curves) and an anhysteretic magnetization curve (AM curve) tests. Furthermore, the BM and AM curves are respectively integrated with the FEM algorithm for accurate and reliable estimations of the SF and remanence of MSR in an operational state. In addition, the tested magnetic properties are applied for the optimization of MSR shielding quality.