Threshold Field Strength Research Articles

A low-cost high-power microwave receiver

The simple receiver described monitors the incident power density close to sensitive electronics and provides a record of the occurrence of potentially damaging RF exposure. This receiver measures the peak value of the incident field strength over a broad instantaneous bandwidth (1-6 GHz), identifies time periods during which threshold field strengths (2, 10, and 50 V/m) are exceeded, covers a +or-45 degrees field of view, and detects a single pulse whose duration exceeds 250 ns. This receiver purposely uses a simple design to permit low-cost fabrication, and a unique microstrip antenna was developed to fulfil design objectives. >

Sensitive multiphoton ionization.

We present comprehensive results for the ionization of the one-dimensional model atom with one electron bound by an attractive \ensuremath{\delta}-function potential in time-dependent external fields. We employ a high-order numerical method, allowing for high-resolution scans of the (essentially exact) ionization probability as a function of the external field parameters within reasonable computer time. We show that, for a sinusoidal external field with small frequency (multiphoton regime), the ionization depends sensitively on the field strength. This phenomenon is a generic multiphoton effect: The dominating peaks in the ionization rate as a function of the ponderomotive energy are separated by the photon energy. This structure is not sensitive to the pulse turn-on time, and, contrary to earlier beliefs, it also persists in the regime where tunneling is the dominant ionization process. (We obtain the tunneling background as the cycle-averaged dc rate.) The sensitive intensity dependence should be observable in photodetachment measurements with high resolution. A comparison with results recently presented by Greenwood and Eberly [Phys. Rev. A 43, 525 (1991)] for a one-dimensional finite-range model potential shows detailed agreement, demonstrating a surprising independence of the ionization on the short-range binding potential. Concerning time dependence, we report drastic deviations from the (usually excellently fulfilled) exponential decay law for the bound-state probability for cases where the latter becomes sufficiently small. In certain regions this function shows a sensitive dependence on time. In addition, we present evidence for exact zeros of the bound-state probability. Finally, we find the ionization rates in dc fields to agree excellently with those resulting from time-independent methods. Moreover, they yield a threshold field strength of some 80% of the experimental value for ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$.

Electric field and stress induced phase transitions in Cr-Cl boracite

By means of an optical investigation of Cr-Cl boracite crystals in polarized light it has been found that an orthorhombic mm2 phase is induced by stress, generated at the mechanically non-matching cubic (43m)/tetragonal (42m) interface, during the first order phase transition at T1 = 264 K. Electric field studies under simultaneous optical control have shown that the orthorhombic mm2 phase can also be induced in the entire temperature range of the non-polar 42m phase above an electric threshold field strength of about 90 kV · cm-1. On the basis of optical domain observations and electric measurements, a revised electric field strength/temperature phase diagram is proposed and discussed.

Negative ion formation by field ionization—studied by field ion microscopy and appearance energy measurements

The formation of negative ions by field ionization (NFI) is difficult even for molecules of high electron affinity because of a small difference in threshold field strength between ion formation by NFI and the onset of electron emission. For tetracyanoethylene and dichlorodicyanobenzoquinone we have studied the negative ion formation on a field cathode tip by field ion microscopy using the negative ions for imaging. The experiments reveal the growth of a conducting polymer layer under NFI conditions and the decisive role of this layer in the prevention of electron emission. The polymer layer has no significant effect on the field ion appearance energies of the molecular ions because the electron affinity obtained from measurements of the appearance energy of a number of molecules agrees quite well with those reported in the literature. The appearance energies of iodide and iodine cluster ions are also determined and interpreted by ion-forming field reactions.

Stationary states of the surface stabilized ferroelectric liquid crystal layers in electric field

Abstract Stationary states of surface stabilized ferroelectric liquid crystal layers in an electric field are analysed by use of the Taylor expansion method based on catastrophe theory. Two kinds of director distribution within the flat smectic layers are taken into account: the uniform and the presplayed one. The butterfly catastrophe describes the properties of the cells correctly. The results have a qualitative character. Two categories of transitions can be predicted: switching between stable states characterized by opposite uniform orientations of the polarization vectors, and deformation of the director field which relaxes after removing the field. The threshold field strengths are found and the role of the system parameters is investigated.

Rhabdomyolysis Due to Pulsed Electric Fields

High-voltage electrical trauma frequently results in extensive and scattered destruction of skeletal muscle along the current path. The damage is commonly believed to be mediated by heating. Recent experimental and theoretical evidence suggests, however, that the rhabdomyolysis and secondary myoglobin release that occur also can result from electroporation, a purely nonthermal mechanism. Based on the results of a computer simulation of a typical high-voltage electric shock, we have postulated that electroporation contributes substantially to skeletal muscle damage and could be the primary mechanism of damage in some cases of electrical injury. In this study, we determined the threshold field strength and exposure duration required to produce rhabdomyolysis by the electroporation mechanism. The change in the electrical impedance of intact skeletal muscle tissue following the application of short-duration, high-intensity electric field pulses is used as an indicator of membrane damage. Our experiments show that a decrease in impedance magnitude occurs following electric field pulses that exceed threshold values of 60 V/cm magnitude and 1-ms duration. The field strength, pulse duration, and number of pulses are factors that determine the extent of damage. The effect does not depend on excitation-contraction coupling. Electron micrographs confirm structural defects created in the membranes by the applied electric field pulses, and these represent the first clear demonstration of rhabdomyolysis in intact muscle due to electroporation. These results provide compelling evidence in support of our postulate.

Ballistic transport and electroluminescence in IIB–VI and IIA–VI compounds

The first commercially successful electroluminescence display of the thin film MISIM type (metal-insulator-semiconductor-insulator- metal) with S= ZnS: Mn rely completely on a special high field electronic transport mode called “travelling spike transport”. By this ballistic (loss-free with respect to phonons) acceleration process, optimum excitation efficiency and avalanching are achieved. Its threshold field strength is about 10 6 V/ cm in ZnS, and is low enough for the resulting electric breakdown to be made reversible via charge control, which can be achieved using a sandwich with insulating films. Very recently, ZnS like electrical behaviour of MISIM samples with S- SrS: Ce and CaS: Eu has been demonstrated. The achievement of the same transport process a fields even slightly lower in these IIA–VI compounds is highly probable. It gives rise to questions about the significance of band structure and a potentially marked difference between ZnS and the latter materials in high field transport. Some speculations about the optimum doping of SrS with rare earths are presented.

Electric-field-induced aggregation in dilute colloidal suspensions

Electric-field-induced chain formation in dilute, non-aqueous suspensions of sterically stabilized, 1 µm poly(methyl methacrylate)(PMMA) latices are investigated. Optical microscopy and digital image analysis provide the chain-length distribution. We find that the particles carry a charge sufficient to inhibit field-induced aggregation. Equilibrium predictions of chain aggregation incorporating a screened Coulombic repulsion and field-induced dipole attraction agree well with experimental observations near the onset of aggregation; chain formation becomes diffusion limited above the threshold field strength.

A Method for Determination of Elastic Constants K1, K2, K3 of a Nematic Liquid Crystal Only Using a Homogeneously Aligned Cell

It is shown that three Frank elastic constants K1, K2 and K3 of a nematic liquid crystal can be determined by measuring the capacitance change with a magnetic field of a homogeneously aligned cell. The splay elastic constant K1 is determined from the threshold magnetic field strength Hc1 when the magnetic field is applied normal to the cell. The bend elastic constant K3 is determined from the slope of the linear dependence between the inverse of the capacitance 1/C and the inverse of the magnetic field strength 1/H in the high field region H/Hc1> 3. The twist elastic constant K2 is determined from the threshold field strength when the magnetic field is applied oblique to the cell normal but perpendicular to the initial director. In these procedures, principal dielectric constants ε//, ε⊥, dielectric anisotropy Δε, anisotropy of the magnetic susceptibility Δχ can be determined simultaneously.

Influence of Oxygen and Metal Oxide Impurities in ZnS:Mn Film on Characteristics of Electroluminescent Devices

The effects of an oxygen and some metal oxide impurities introduced into the ZnS:Mn film at its postannealing process were examined with respect to the characteristics of an electroluminescent device. Depending on the oxygen pressure at the postannealing, the oxygen either decreases the emission threshold field strength in the ZnS:Mn film or forms an additional insulating film such as ZnSO4. The metal oxide impurities Fe, V, and Co caused a decrease in luminance via a killer effect on the Mn emission center. On the other hand, Ti, In, Na, and V decreased the emission threshold voltage by lowering the emission threshold field strength in the ZnS:Mn film.

Electric field effects on the virus M13, detected by electro-optical measurements

We have carried out transient electric birefringence experiments with M13 in aqueous solution. A normal effect with a negative sign is superimposed by anomalous positive effects. These prevail more and more upon raising the field strength. They apparently reflect electrically induced structural transitions. The threshold field strength decreases when buffer and/or electrolyte is removed.

Driven Morse oscillator: Classical chaos, quantum theory, and photodissociation.

We compare the classical and quantum theories of a Morse oscillator driven by a sinusoidal field, focusing attention on multiple-photon excitation and dissociation. In both the classical and quantum theories the threshold field strength for dissociation may be estimated fairly accurately on the basis of classical resonance overlap, and the classical and quantum results for the threshold are in good agreement except near higher-order classical resonances and quantum multiphoton resonances. We discuss the possibility of ``quantum chaos'' in such driven molecular systems and use the Morse oscillator to test the manifestations of classical resonance overlap suggested semiclassically.

Effects of static magnetic fields on diffusion in solutions.

Static magnetic fields affect the diffusion of biological particles in solutions through the Lorentz force and Maxwell stress. These effects were analyzed theoretically to estimate the threshold field strength for these effects. Our results show that the Lorentz force suppresses the diffusion of charged particles such as Na+, K+, Ca2+, Cl-, and plasma proteins. However, the threshold is so high, i.e., more than 10(4) T, that the Lorentz force does not affect the ion diffusion at typical field strengths (a few Tesla at most). Since the threshold of gradient fields for producing a change in ion diffusion through the Maxwell stress is more than 10(5) T2/m for paramagnetic molecules (FeCl3, O2) and plasma proteins, their diffusion would be unaffected by typical gradient fields (100 T2/m at most) and even by high gradient fields (less than 10(5) T2/m) used in magnetic separation techniques. In contrast, movement of deoxygenated erythrocytes and FeCl3 colloids (more than 10(3) molecules) is influenced by the usual gradient fields due to a volume effect.

Physiological Electric Fields can Influence Cell Motility, Growth, and Polarity

The revival of interest in the role of electric fields in guiding cell motility, growth, and polarization that has occurred over the past five years has led to several important discoveries that can be used to stimulate further investigations in this field and lead to useful clinical applications. The chapter discusses the most significant of these findings because they can be used to guide future research. Physiological electric-field strengths fall into the range of 1–200mV/mm and the application of much larger field strengths should be avoided by those interested in natural cellular behavior. The threshold level for galvanotaxis and galvanotropism may be as much as an order of magnitude lower than this after cells have been sensitized to the field, or when they are exposed to non-uniform fields. These are clearly very important areas for further research because the lower the threshold-field strength, the more likely it is that the cells will encounter those fields in vivo and would be using them to guide cellular functions.

Stochastic dynamics of hydrogenic atoms in the microwave field: modelling by maps and quantum description

The motion of an electron of a classical hydrogenic atom in an oscillating electric field is studied theoretically. An analysis is provided, based on the iterative (mapping) forms of the classical equations of motion in perturbation theory and the adiabatic approximation. This greatly facilitates the numerical investigation of stochasticity and the ionisation process and allows the approximate analytical estimation of the threshold field strengths for the onset of chaos and of the diffusion coefficient of the electron in energy space. The method is asymptotically exact at high field frequencies and gives a good approximation for medium and low frequencies. The adiabatic approximation describes well the approach of the stochastic ionisation threshold field strength to the static field ionisation threshold. From the quantum mechanical point of view the ionisation is a result of the great number of one-photon transitions in the strongly perturbed spectrum of the atom. This results in the diffusion of the electron in energy space identical to the diffusion due to stochastic classical motion. The estimation of the mean time of diffusive ionisation is also given.

Field adsorption and desorption of hydrogen on W(110) — an atom-probe study

An atom-probe study is presented of the electric field dependence of adsorption and desorption of hydrogen on tungsten (110). A distinct threshold field strength of approximately 3.8 V/Å was found for the desorption of hydrogen at 90 K. A simple theoretical model based on the Newns-Anderson model is presented to give a qualitative understanding of the experimental observations. Within this model a direct proportionality is derived between the reduction in binding energy of the adsorbate and the field strength.

Effects of low frequency electric fields on the sedimentation rate of human blood. Preliminary observations.

The erythrocyte sedimentation rate (ESR) of human blood samples exposed to electric fields at frequencies ranging from 1 Hz to 100 kHz was measured. Statistically significant differences in ESR were found between the exposed and the control samples at most of the frequencies used. The results suggest a possible nonlinear dependence of the effect on frequency, the threshold field strength varying from about 25 V/m to about 5 kV/m.

RF Susceptibility of an EEG and Considerations for Attenuating RFI in Hospitals

This paper discusses the testing accomplished to determine the radio frequency (RF) susceptibility of a typical electroencephalograph (EEG) and to determine the effectiveness of lead X-ray shielding as an RF shield. New data are presented on threshold field strengths and frequencies which just begin to cause interference on a typical EEG brain scan. Results are combined with data from other studies to show that a typical EEG machine would be immune to the RF environment expected in the vicinity of most hospitals. Therefore, expensive RF shielding in a hospital EEG suite is usually not required. Finally, an inexpensive shielding system utilizing X-ray shielding which was developed by the U.S. Veterans Administration is presented.

Alignment of microscopic particles in electric fields and its biological implications

It is well known that electromagnetic fields cause mechanical forces. If one applies an electrical field to a suspension of microscopic particles, these particles realign themselves along the direction of the field and form pearl-chain-like aggregates. These chains are mostly single stranded but they are frequently multistranded. This phenomenon has been investigated by a number of groups. Here we discuss the dependence of threshold field strength on particle size and frequency. Also, pulsed fields have been thought to be more effective than continuous fields of the same average power in evoking biological effects. Our measurement of the threshold power requirement for the pearl-chain formation indicates that pulsed fields require as much power as continuous fields. The biological significance of pearl-chain formation is briefly discussed.

Embryonic fibroblast motility and orientation can be influenced by physiological electric fields.

Epithelial layers in developing embryos are known to drive ion currents through themselves that will, in turn, generate small electric fields within the embryo. We hypothesized that the movement of migratory embryonic cells might be guided by such fields, and report here that embryonic quail somite fibroblast motility can be strongly influenced by small DC electric fields. These cells responded to such fields in three ways: (a) The cells migrated towards the cathodal end of the field by extending lamellipodia in that direction. The threshold field strength for this galvanotaxis was between 1 and 10 mV/mm when the cells were cultured in plasma. (b) The cells oriented their long axes perpendicular to the field lines. The threshold field strength for this response for a 90-min interval in the field was 150 mV/mm in F12 medium and between 50 and 100 mV/mm in plasma. (c) The cells elongated under the influence of field strengths of 400 mV/mm and greater. These fibroblasts were therefore able to detect a voltage gradient at least as low as 0.2 mV across their width. Electric fields of at least 10-fold larger in magnitude than this threshold field have been detected in vivo in at least one vertebrate thus far, so we believe that these field effects encompass a physiological range.