Electric Fields Of Order Research Articles

Low‐amplitude, low‐frequency electric field‐stimulated bone cell proliferation may in part be mediated by increased IGF‐II release

We have developed an in vitro model incorporating a low-amplitude (10(-7) V/cm), low frequency (f less than 100 Hz), capacitively coupled electric field in order to study the mechanism through which an electric field may increase bone cell proliferation. Utilizing this model we have previously shown that electric field-stimulated bone cell proliferation was dependent on release of mitogen activity into the culture medium from exposed cells. The current studies were intended to characterize this mitogen activity. In these studies we found that electric field-stimulated human bone cell proliferation was associated with increased IGF-II mRNA accumulation and IGF-II secretion suggesting that IGF-II may in part mediate the increase in bone cell proliferation following electric field exposure.

Plasma waves observed in the near vicinity of the space shuttle

The OSS 1 and Spacelab 2 missions found intense broadband waves in the near vicinity of the space shuttle. This paper contains a detailed observational characterization and theoretical investigation of the plasma waves observed within about 10 m of the space shuttle during the XPOP roll period of the Spacelab 2 mission. High wave levels are found from 31 Hz to 10 kHz (near the lower hybrid frequency). Above 10 kHz the wave levels decrease with frequency, reaching the background level near 56 kHz. The frequency distribution of wave electric fields is best interpreted in terms of three components below about 10 kHz and a high‐frequency tail. The primary component is a fairly uniform, high level of waves covering the frequency range from 31 Hz to 10 kHz. The two superposed components in this frequency range have electric fields of order twice the uniform level. The second component corresponds to a low‐frequency peak in the range 100–178 Hz. The third component is found near, and follows the trend of, the lower hybrid frequency. No evidence is found for a high‐frequency component localized above about 10 kHz. The waves show a pronounced amplitude and frequency variation with the quantity V∥/VT which measures the angle between the ionospheric magnetic field and the shuttle's velocity vector. Very low wave levels and small frequency extents are observed when V∥/VT ∼ 1 and the shuttle is moving primarily along the magnetic field. This implies that the waves are probably driven by water pickup ions. Observations of the waves below about 20 kHz during the free‐flight mission imply that the near‐zone waves have wave vectors oriented perpendicular to the magnetic field. The nulls observed in the wave data as the PDP spacecraft moves through the orbiter's wake imply that wavenumber increases with wave frequency. Hwang et al.'s theory for the near‐zone waves is shown to be inconsistent with the frequency distribution and wave vector orientations of the observed waves. A new theory involving Doppler‐shifted lower hybrid waves driven by beamlike distributions of water ions near the space shuttle is developed using linear theory. This linear theory can explain generation of waves with (1) frequencies ranging from near zero frequency to the lower hybrid frequency, (2) wave vectors essentially perpendicular to the magnetic field, (3) wavenumbers increasing with wave frequency, and (4) wavelengths observable by the PDP antenna. These characteristics are all consistent with the observed properties of the waves, thereby providing strong support for the theory. In addition, the theory permits two qualitative explanations for the V∥/VT effect. However, the linear theory cannot explain the details of the observed frequency spectrum of the waves. Strong, qualitative arguments that the effects of spatial inhomogeneity and nonlinear effects should modify the predicted linear spectrum are presented. In particular, the observed ratio of wave energy to thermal plasma energy is of order 10−5, sufficient for nonlinear and strong turbulence effects to be potentially important. Lastly, the observed V∥/VT effect implies optimum conditions for use of the space shuttle as a base for observing plasma waves generated by active experiments or natural ionospheric waves. Ideally, the shuttle's orbit should be designed so that V∥/VT exceeds about 0.7, thereby favoring polar orbits and arguing against equatorial orbits. Alternatively, free‐flying experiments should be used.

Low-frequency magnetic response in valence fluctuating and heavy fermion systems

By starting from an Anderson impurity Hamiltonian, a simple approximation for the spectral densities of the occupied and the empty f state is suggested. With it such thermodynamic quantities like the partial f occupancies go over, in the zero temperature limit, into the results of variational calculations. The approximation is applied in the presence of a crystalline electric field in order to calculate χ″( w,T). A comparison with previous, more elaborate calculations is provided for the case of vanishing CEF splittings.

Photocurrent measurements of electric-field-induced carrier energy shifts and tunnelling in In1-xGaxAs/InP quantum wells

The authors have measured photocurrent in In1-xGaxAs/InP quantum well samples as a function of temperature and applied electric field in order to study the escape of carriers initially photo-excited into the wells. For fields 8*104 V cm-1, there is suppression of the excitonic features in the spectrum which is attributed to a low exciton ionisation rate which causes carriers to recombine within the well rather than to undergo perpendicular transport. At higher fields, the authors observe strong Stark shifts of the excitons. They observe clear evidence of carrier tunnelling at low temperatures, and at higher temperatures they detect phonon-assisted tunnelling. The experimental results are shown to be in good agreement with the predictions of an exact theoretical calculation.

Cascade Fluorescences from Laser-Excited Lithium Atoms Immersed in a Static Electric Field for Plasma Diagnostics

The Stark effect on a laser-pumped Li atom to the 42D or 42F level was studied by the observation of two cascade fluorescences of 610.4 nm (32D→22P) and 323.3 nm (32P→22S) under a static electric field in order to develop a spectroscopic measurement of the static or quasi-static electric fields in plasmas with a combined technique of Li-beam probing and laser-induced fluorescence (LIF). The excitation spectra and field dependencies of both lines were obtained and found to agree with the theoretical values obtained by solving the rate equations. It was also shown that a method for exciting the forbidden transition from 22P to 42F is so sensitive as to make it possible to measure an electric field as weak as ∼ 100 V/cm. A practical method that is applicable to plasma devices is described, and the effect of the electron density on LIF is also discussed.

The effect of a combined static and microwave field on an excited hydrogen atom

The authors examine the behaviour of extremal Stark states of excited hydrogen under the influence of parallel static and microwave electric fields in order to assess when the system can be approximated by a Hamiltonian with one degree of freedom. The authors show that without a static field the Runge-Lenz vector precesses, so that only for short times does the electron remain approximately aligned along the field direction. In this case they provide estimates of the time during which the motion is nearly one-dimensional; this has a complicated dependence upon the microwave frequency. The amplitude of the Runge-Lenz oscillations decreases with increasing static field strength Fs; there is a critical static field, depending upon the microwave field frequency Omega and amplitude Fm, at which these oscillations are small enough for the electron motion to be considered one-dimensional. The authors provide estimates of this critical field for various microwave frequencies and show that it is largest when the microwave field is in resonance with the electron motion, in which case it is necessary that Fs0.22Fm.

Analytic theory of electric field depinning in the one-dimensional Fukuyama-Lee-Rice hamiltonian

Using Feigel'man's method in the theory of one-dimensional random systems we have evaluated analytically the depinning electric fieldE T and the static dielectric constant e0 for the Fukuyama-Lee-Rice hamiltonian in the weak pinning limit and for low temperatures. This is accomplished by solving a Fokker-Planck equation for finitedc electric fields in order to determine the field dependent pinning energy. The dielectric constant is found to remain independent of the electric field up to the threshold. The product e0 E T is also evaluated and compared with other theories.

Photoluminescence from GaAs quantum wells under high electric fields

The room-temperature photoluminescence spectra of n-modulation doped GaAs–AlGaAs multiple quantum wells have been studied at high parallel electric fields in order to obtain the distribution function of the two-dimensional electron system under high field conditions. The spectra show thermalized energy distributions with electron temperatures of ∼600 K in the range where current instabilities indicate negative differential conductivity. This temperature is well below the value expected for bulk GaAs, and can be understood on the basis of real-space transfer of hot electrons into AlGaAs.

Plasma Radius Control System for Tandem Mirrors

The operation of a tandem mirror reactor (TMR) at fractions of full power is an important part of the power plant operation. It has been shown that fractional power operation can best be achieved by controlling the plasma radius, leaving the rest of plasma parameters unchanged. The physics of magnetic confinement in a TMR can be used to design the desired radius control (RC) system. It is proposed to bias the field lines through the ends of a tandem mirror device and control the radial electric field in order to guard against rotational instabilities and ensure nonstochastic radial transport. In this scheme, plasma on the unbiased field lines suffers rapid transport and is lost. As such, the potential control of the direct converter plates as well as the location of the grounded halo dumps requires the flux tube containing the core plasma to match the direct converter area at all times. Based on this effect, an RC system can be devised that uses the conservation of the magnetic flux to adjust the field near the direct converter such that a flux tube with a smaller radius in the device matches the direct converter. Field lines outside this flux tube aremore » not properly biased and cannot support core plasma. A practical design of such an RC system for the Mirror Advanced Reactor Study conceptual TMR is presented. The power consumption by the RC coil system is modest and no engineering difficulty in the design and construction of such a system is expected. Utilization of such an RC system in advanced tandem mirror experiments can also be of great use in the study of such various phenomena as radial transport, control of the radial electric field, halo plasma production, radio-frequency absorption, etc.« less

Hypervirial-perturbational treatment of a particle in a box in the presence of an electric field

The hypervirial-perturbational method given by the present authors 3,4,5,8) is applied to the particle-in-a-box model subjected to an electric field in order to obtain the energy eigenvalues. The corrections up to the fourth-order are calculated in an analytical way, and higher-order corrections are determined by means of a computer. Numerical values are compared with previous results.

Distortion waves and phase slippage in nematics

It is shown that in the infinite nematic liquid, the distortion waves can propagate when submitted to the elliptically polarized shear deformation. These waves are helical and their propagation can also be seen as a rotation of the helical distortion. The analogy with the shear wave propagation in the rotating newtonian fluid is indicated. The rotation of the distortion is observed in a finite nematic slab submitted to an electric field in order to prevent the relaxation of the distortion. The analogy between the distortion and the two-dimensional superfluid order parameter is developed. The phase winding and slippage is observed experimentally.

Electric fields, Joule and particle heating in the high latitude thermosphere

A short review of the recent high latitude measurements of ionospheric electric fields is given. The importance of investigating large-scale and slowly-varying electric fields in order to study magnetospheric convection is stressed. The motion of such high energetic phenomena as auroral forms and spread E-region echoes must be treated by extreme caution when interpreted as a manifestation of convection motion. The relationship between the ionospheric source and polarization field is still an unanswered problem. It is indicated that progress can be made in this respect when electric fields and conductivities are measured simultaneously in the ionosphere. Evidence is shown at one occasion that the meridional component during an auroral substorm might be mainly a polarization field. The height-integrated Joule heating rate is occasionally found to be far larger than the solar radiation input at auroral altitudes. The presence of this additional heat source at any time of day is expected to have a strong impact on the global-scale atmospheric dynamics. From comparisons made between Joule and particle heating it appears that the two components are comparable. It is expected that high latitude incoherent radars will contribute substantially to our understanding of these phenomena in the near future.

Analysis of self-trapping using the wave equation with high-order nonlinear electric permittivity

The self-trapping of light pulses is considered using the classical nonlinear wave equation of Chiao, Garmire, and Townes, modified to include nonlinearities up to sixth order in electric field. A phenomenological approach is adopted in which the nonlinear coefficients are considered without reference to a specific molecular self-trapping mechanism. It is found that filament formation can occur only over a narrow range of values of the nonlinear coefficients. The method can be used as a criterion to test the validity of various self-trapping mechanisms.

Electromagnetic Backscattering from a Random Plasma Slab

The computer method developed by the authors for calculating the electromagnetic scattering from a random, plane parallel plasma slab is applied to the near-overdense and overdense regimes. The mean reflection coefficient and incoherent reflected power are calculated for an ensemble of random plasma slabs. The results are compared with the analytic solutions using a perturbation expansion of the wave equation to the second order in electric field. Choosing the correlation length of the random amplitudes fluctuation to be equal to that of the random telegraph signal fluctuation results in identical two-point correlation functions for each, but different higher-order correlations. Hence, the computed differences in the scattering calculated for the plasma slabs having the two types of fluctuations are due to the higher-order correlations.

High Electric Fields in II–VI Materials

Conventional electrophotographic processes operate with blocking contacts. This limits the available gain at low fields: one electron discharged per absorbed photon, at the most, during the photoconductive step of the process. A straightforward way to increase the gain is to increase the applied field until carriers multiply by impact ionization during their transit through the material. This requires high electric fields of order 10(6) V/cm, and in such high fields there is inevitably an increase in the dark current. In this competition between dark current and multiplication the irreducible source of increased dark current is Zener tunneling of carriers from valence band to conduction band. Examination of the form of current-voltage relations for tunneling and impact ionization shows that specimen dimensions have an important effect in shifting the balance in favor of multiplication. The multiplication properties have recently been studied for three II-VI materials with high band-gaps: ZnSe, CdS, and ZnO. High fields are produced with electrolyte blocking contacts to n type conducting crystals. Multiplication is seen in fields around 10(6) V/cm in ZnSe and ZnO, but not in CdS where the ultimate breakdown is by tunneling. Since the band gaps and low field mobilities are almost the same for ZnSe and CdS, this difference indicates that rather subtle features can be decisive in determining whether or not impact ionization takes place. Measurements have been made of charging and discharging of the free surface on ZnO single crystals. High fields can be achieved under these conditions, and there is tentative evidence that electron multiplication can contribute to the discharge.

CONTINUUM ASPECTS OF RF GRADIENT ACCELERATION OF PLASMA

A quasi-one-dimensional model of acceleration of a continuum plasma by r- f electric field gradient forces is examined. The conditions imposed on the electric field in order to effect sonic transitions are given. The case of isentropic flow is treated, and departure from the particle model is indicated for pressure> 100 mu and temperatures approx equal = 0.1 ev.

Fast-wave Propagation in Plasma-filled Waveguides†

ABSTRACT In the analysis of the waves which propagate in a waveguide completely or partially filled with a stationary plasma column or an electron beam drifting with a uniform velocity and collimated with a magnetic field of finite magnitude, it is necessary to include the effects of the rotation of the electric field in order to treat the waves which have phase velocities comparable to or greater than the velocity of light. Certain interesting features of perturbed waveguide modes and cyclotron modes become apparent only when the dynamic nature of the electric field is recognized in the analysis. These features are the influence of cyclotron frequency and the polarization of waves on the propagation characteristics. The cyclotron modes are found to be purely transverse electric at cut-off, gradually changing in character along their propagation curves through hybrid to purely transverse magnetic modes at cyclotron resonance. A classification of modes of propagation is proposed, and formulae for cut-off frequencies as functions of the cyclotron frequency as well as some representative Brillouin diagrams are given. Equations for neutralized electron beams are obtained when the Lorentz transformation is applied to the equations for stationary plasmas. The solutions for stationary plasmas can be transformed directly into the solutions for neutralized electron beams.

Secondary emission from copper due to slow positive ions of argon

The dependence of the secondary emission from the cathode on the energies of the impinging positive ions is of importance in the mechanism of the corona, or asymmetric, discharge through gases. In particular, it is important to find whether positive ions are able to liberate electrons from an electrode on to which they drift when their mean energy of agitation is no greater than that of the atoms of the gas through which they move. This paper describes experiments performed with positive ions of argon moving in that gas under weak electric fields in order to measure the secondary emission produced at a copper cathode. No secondary emission was detected when the electric intensity Z was less than 20 V./cm. and Z/p was less than 40, but the secondary emission attained a value of about 1.1 per cent when the electric field was increased above 60 V./cm. corresponding to values of Z/p greater than 100. This result is compared with those previously obtained with hydrogen and helium ions under similar conditions.