Nonzero Electric Field Research Articles

Efficiency of lower hybrid current drive at 2.45 GHz in ASDEX

The experimental lower hybrid current drive efficiency at zero electric field is found to agree with theory if the spectrum of the lower hybrid waves is fully accessible to the central region of the plasma. For broad or very low parallel wave number spectra, N∥, for which the plane wave accessibility condition is not fully satisfied, the observed efficiency is higher than predicted. At non-zero electric field, the measurements agree with theories for an electric field dependent efficiency. The model can also be applied to experiments in which a symmetric spectrum of waves is launched, or to cases where the current is driven opposite to the DC electric field if runaway electrons can be neglected. A shift of the deposition zone by superposition of a low N∥ current drive spectrum and a high N∥ symmetric spectrum does not change the current drive efficiency.

Growing hair on black holes.

A black hole can carry quantum numbers that are not associated with massless gauge fields, contrary to the spirit of the no-hair theorems. In the Higgs phase of a gauge theory, electric charge on a black hole generates a nonzero electric field outside the event horizon. This field is nonperturbative in ħ and is exponentially screened far from the hole. It arises from the cloud of virtual cosmic strings that surround the black hole. In the confinement phase, a magnetic charge on a black hole generates a classical field that is screened at long range by nonperturbative effects. Despite the sharp difference in their formal descriptions, the electric and magnetic cases are closely similar physically.

Deviation from the mean field theory in the critical behavior of ferroelectric TGSe

Abstract The deviation from the mean field theory in values of the ratios X(O, TM)/X(E, TM ) for TGSe was found, where X(E, TM ) is the value of the maximum in the temperature dependence of electric susceptibility in a non-zero electric field parallel to the measuring field, X(O, TM ) is the initial susceptibility of the same temperature TM . It is stressed that not only the quality of the sample (imperfections and non-homogenities) but also other mechanisms are responsible for this deviation.

Relaxation of spin [formula omitted] in a nonzero average electric field gradient

The time dependence of the density operator under relaxation and precession for spin S= 3 2 in a nonzero average electric field gradient has been evaluated. Transverse relaxation, nonselective inversion recovery and multiple-quantum relaxation experiments are discussed.

Electric-field-dependent photoconductivity in GaInAs-InP quantum wells

We report a study of the effects of an electric field on the excitonic, band-to-band photoconductivity spectra of a Ga0.47In0.53As-InP quantum well structure. In a sample with five wells of widths 10–110 Å we show that for carrier motion normal to the quantum well layers a photoconductivity signal is only observed from those wells which are in a region of nonzero electric field. The spectral line shapes of the transitions in the narrowest two wells show strong variations with field, which we attribute to exciton dissociation at high field, and possibly exciton screening by free carriers trapped in the wells at low fields. The results are compared with photoconductivity spectra of the same structure, but for carrier motion in the plane of the layers. The latter geometry is found to reflect more accurately the sample absorption.

Is a circular Rydberg atom stable in a vanishing electric field?

It is shown both experimentally and theoretically that a Rydberg circular atom keeps its maximum momentum only if it is submitted to a nonzero static electric field whose direction is constant or slowly varying. The minimum characteristic time of the field-allowed rotation is determined. The implications of this effect for a future high-resolution microwave spectroscopy experiment on circular Rydberg atoms is discussed.

Kinetic description of betatron oscillation instability for nonrelativistic nonneutral electron flow

The linearized Vlasov–Poisson equations are used to investigate the electrostatic stability properties of nonrelativistic nonneutral electron flow in a planar diode with cathode located at x=0 and anode at x=d. The electron layer is immersed in a uniform applied magnetic field B0êz, and the equilibrium flow velocity V0yb(x) is in the y direction. Stability properties are calculated for perturbations about the choice of self-consistent Vlasov equilibrium f0b (H,Py) =(n̂b/2πm) δ(H) δ(Py), which gives an equilibrium with uniform electron density (n̂b =const) extending from the cathode (x=0) to the outer edge of the electron layer (x=xb). Assuming flute perturbations, the eigenvalue equation is simplified and solved analytically for long-wavelength, moderately high-frequency perturbations satisfying k2x2b ≪1 and (ω−kVd)2 ≊ω2v =ω2c −ω̂2pb. The present analysis is restricted to electron densities below the Brillouin flow (ω̂2pb <ω2c) and the nonzero electric field at the cathode. The eigenvalue equation leads to a fourth-order algebraic dispersion relation for the complex eigenfrequency. The dispersion relation is solved numerically, and detailed stability properties are investigated as a function of system parameters for both the upshifted branch (ω−kVd ≊+ωv) and the downshifted branch (ω−kVd≊−ωv). For a sufficiently thin electron layer, it is found that only the upshifted branch exhibits instability. Typically, instability exists for a range of ω̂2pb/ω2c.

Beam coupling and decoupling in degenerate two-wave mixing in a reflection geometry with photorefractive Bi2GeO20 crystals

The dependence of the effective gain on the incident angle of the beams and on the initial beam intensity ratio is studied in the experiment of degenerate two-wave mixing (TWM) in a reflection geometry with photorefractive Bi12GeO20 (BGO) crystals. A saturation value of the effective coupling constant ¦g¦-0.4 cm−1 is obtained. In contrast to TWM operated in the drift mode (i.e. with a nonzero electric field applied to the crystal), for TWM operated in the diffusion mode (zero external electric field) as is our case, beam coupling is reduced by moving the crystal or the interference fringes at a constant speed. At high moving speeds, complete beam decoupling can be reached. A comparison between the theoretical and measured dependence of the effective gain on the moving speed is also made. Using this technique, complete isolation of two intersecting coherent beams inside a nonlinear medium can be achieved.

Spontaneous polarisation and transition temperature of a ferroelectric measured by a simple and accurate method

A method for measuring the spontaneous polarisation and the dielectric constant as a function of the temperature of ferroelectric materials is presented. It is based on the extrapolation of the saturated (linear) part of the hysteresis loop. This extrapolation can be easily and accurately carried out by means of simple electronic circuits. Results of measurements at triglycine sulphate are given, as well as a detailed description of the electronic circuits. The systematic error in the spontaneous polarisation caused by the measurement at nonzero electric field is discussed and a suitable correction procedure is given.

On the effect of surface recombination on some properties of semiconductor devices

A theoretical analysis of the influence of the surface recombination velocity of charge carriers on some static and dynamic parameters of planar semiconductor structures with a shallow p +-n junction is presented. The discussion is based on the solution of the continuity equation of charge carriers in a cylindrical base region for the case of a non-zero built-in electric field. Appropriate boundary conditions allow the use of the results presented here in the analysis of various devices of planar technology. The following were calculated as functions of the surface recombination velocity and the physical dimensions of the structure: the I( V) characteristic, the conductance (resistance) and the diffusion capacitance, the charge stored in the base region and the carrier transport coefficient. Typical plots are presented which can be helpful in evaluating the surface recombination effect on these parameters of semiconductor devices. It was found that the effect is strongest for structures with an n-n + interface. Using the formulae derived here the scatter of the electrical parameters of a device with the expected variation of surface recombination velocity can be evaluated for a given production line.

The AC motion of the sheath edge in a flowing high-pressure plasma

Measurements have been made of the sheath edge motion in response to a small AC perturbation of the negative potential of a planar probe immersed in an unseeded atmospheric pressure flame plasma. These measurements, which were conducted over a frequency range 100-2500 Hz, compare favourably with an analytical solution for the sheath motion. The analysis predicts a low-frequency sheath motion which is dependent on the initial ion velocity at the sheath edge; the effect of a non-zero electric field at the sheath edge on the sheath motion is also discussed.

Subshock criterion for switch-on shock waves

Conditions are established for determining whether the downstream state of a switch-on hydromagnetic shock wave will be subsonic, sonic, or supersonic, thus giving an explicit criterion for the existence of a gasdynamic subshock within the overall structure. The analysis is made for the general case of an ionizing shock wave with nonzero electric field. The results include the well-known magnetohydrodynamic case in the limit of zero-electric field. Post-shock temperatures are investigated, and it is found that it may be possible for an ionizing switch-on shock to exist with a nonzero electric field without involving a gas-dynamic subshock. In this case, the structure consists of an initial photoionization layer followed by magnetohydrodynamic compression giving a high-temperature post-shock region, the radiation from which drives the upstream photoionization.

Structural determination of electric field conditions in ionizing shock waves

Ionizing shock waves may involve a non-zero transverse electric field in the non-conducting upstream region. The possibility of this field means that two extra parameters are involved in ionizing shock transitions, in addition to the usual three MHD shock parameters. The present paper investigates the question of existence or absence of structurally determined relationships between the five parameters. The results show that ionizing shock waves can be categorized into three types, depending on the nature of their downstream singular points in a three-dimensional phase space. Thus, if shock Mach number, Alfven number, and upstream magnetic field angle are given, type 2 shooks are characterized by a unique transverse electric field in both magnitude and orientation, type 3 shocks allow one degree of freedom to the electric field but specify a relationship between the magnitude and orientation, and in type 4 shocks both electric field parameters are free to range between certain limits. These structural conditions correspond exactly to previously investigated evolutionarity conditions based on wave perturbations, the interpretation of which has been incomplete due to a lack of knowledge of the possibility of skew shocks. In light of the author's subsequent work on skew shocks and the new general three-dimensional results, a complete map of the behaviour of ionizing shock waves is now possible, and the present article attempts to construct its essential features. Earlier works are reviewed and clarified in reference to this general framework.

Comparison of theoretical and experimental values of the capacitance of diffused junctions

Using computer calculations, the exact values of the C−3-vs-U intercept with the U axis are calculated for diffused junctions. These theoretical intercepts are 0.125 V lower than the classical values obtained from Shockley's theory. This difference is due to three factors: the variation of the diffusion voltage, the nonzero electric field at the boundaries of the depletion region, and the contribution of electrons and holes. The exact values also disagree with the experimental results. It is shown that this discrepancy depends on the diffusion parameters and that it may be caused by two effects: irregularities in the diffusion profile around the metallurgical junction and incomplete ionization of impurities.

A study of dipole-broadened NMR powder lineshapes in the presence of second-order quadrupole effects with applications to polycrystalline borates

Computer simulations of NMR powder patterns for the m = 1 2 ⇋ m = − 1 2 transition broadened in second order by a quadrupole interaction having a nonzero electric field gradient asymmetry parameter are discussed. The dependence of measurable features of the derivative spectrum on the theoretical parameters is considered. It is found that the frequency dependence of the derivative linewidth is insensitive to relatively large changes in the asymmetry parameter. Thus, methods for determining the quadrupole coupling parameters from the frequency dependence of the NMR linewidth are subject to large uncertainties. In the special case when the dipole broadening is small relative to the quadrupole broadening, it is shown that reference to a library of standard, computer-simulated spectra allows precise determination of the coupling constant and asymmetry parameter without the need for computation of the specific experimental spectrum. This is illustrated with examples of 11B NMR spectra from several polycrystalline borates.

Variational method for the analysis of waveguide coupling

A variational method is described for the analysis of electromagnetic-energy transfer through the common broad wall between two rectangular waveguides. The method is of general application, providing quantitative theoretical results for the coupling and directivity of a large variety of waveguide coupling mechanisms.The problem considered is that of two adjacent rectangular waveguides having an arbitrary nonzero tangential electric field in the plane of the infinitesimally thin common wall. In terms of this arbitrary electric field, variational expressions for the forward and backward coupling coefficients to the dominant wave in the auxiliary guide are deduced. It is subsequently shown that these expressions may be used to evaluate the coupling produced by any desired aperture distribution in the common wall between the guides, if a reasonable approximation to the functional form of the tangential electric field in the plane of the apertures can be made.Numerical results have been calculated for the simple problem of two waveguides coupled by means of a single transverse rectangular aperture of any length. Where possible, comparison is made with the results of other theories. Experimental results are also presented, and using these, the relative merits of the alternative theoretical treatments are discussed.With some reservations, waveguide coupling by means of a multiple array of slots and also by means of a long axial slot may be analysed using this variational method, and some consideration is given to the additional requirement of such an analysis.