Showing papers on "Magnetic field published in 1977"

First Operation of a Free-Electron Laser

Abstract: A free-electron laser oscillator has been operated above threshold at a wavelength of 3.4 \ensuremath{\mu}m.

Low field electron-nuclear spin coupling in gallium arsenide under optical pumping conditions

Abstract: In a semiconductor, absorption of circularly polarized light (optical pumping) leads to spin-oriented photoelectrons. In this situation, the nuclei of the crystal are dynamically polarized through their hyperfine interaction with the electronic spins. Consequently the electrons experience a hyperfine magnetic field due to the oriented nuclei which may reach several kilogauss. When this large nuclear field is driven obliquely with respect to the direction of the exciting light, the precession of the electronic spins around the nuclear field leads to a decrease of the electronic polarization along the light excitation: it is a nuclear Hanle effect. This work is an experimental and theoretical study of these effects in weak external magnetic fields, of the order of the local field which characterizes the nuclear spin-spin interactions (a few gauss). Large nuclear fields are obtained at 77\ifmmode^\circ\else\textdegree\fi{}K in strongly doped and compensated $p$-type GaAs samples. We present a model which includes the different effects of the hyperfine coupling when there is a nuclear spin temperature among all the nuclei of the sample: Dynamic polarization, nuclear field, but also, existence of an electronic field acting on the nuclei. We show that a small external field is able to drive the large nuclear field acting on the electrons; consequently the electronic polarization is very sensitive to external fields which are too small to have a direct effect on the electronic spin motion. We study experimentally the variation of the electronic polarization with the direction and magnitude of a small external magnetic field, by measuring the circular polarization of the luminescence light. The experimental results are in quantitative agreement with the theoretical predictions. The usual Hanle Lorentzian depolarization curve is strongly modified in low fields and $W$-like singularities appear around zero field. The experimental values of the average electronic and nuclear fields are in reasonable agreement with theoretical evaluations. These nuclear effects may strongly alter the measurement of the Hanle linewidth in standard optical pumping experiments.

Saturation of the tearing mode

Abstract: A quasi‐linear analytical model is used to describe the nonlinear growth and saturation of tearing modes with mode number m⩾2. The saturation of the magnetic island growth is the quasi‐linear development of a single mode rather than a mode coupling process. The saturation amplitude, which is dependent on the form of the resistivity, is in good agreement with results obtained previously by numerically advancing the full set of nonlinear equations.

Atoms in high magnetic fields (white dwarfs)

Abstract: High magnetic fields have been of interest for some time in solid-state physics. Recent spectroscopic experiments on highly excited barium atoms and the discovery of very large magnetic fields in white dwarf stars and neutron stars have stimulated renewed interest in the spectroscopy of free atoms in high magnetic fields. The various spectroscopic regimes ranging from the low-field Zeeman effect to the high-field Landau regime are reviewed. Calculations of the energy levels of hydrogen, helium and their isoelectronic ions in fields of any strength are surveyed. Observations of the quadratic Zeeman effect in the alkalis and high-field effects in the neutral barium spectrum are described. Finally, recent discoveries of continuum polarization in white dwarf stars, its interpretation in terms of the presence of high magnetic fields, the confirmation of such fields spectroscopically, and some remaining mysteries in these remarkable stellar spectra are reviewed.

Solitons in a one-dimensional magnet with an easy plane

Abstract: An easy plane magnetic chain in a magnetic field is equivalent to a Sine-Gordon system. At low temperatures and moderate fields the dynamical behaviour of this system can be discussed in terms of magnons and solitons. Solitons are responsible for a zero frequency peak in the longitudinal part of the dynamic structure factor. They are thus observable in neutron scattering experiments.

Magnetic field-line reconnexion by localized enhancement of resistivity: Part 1. Evolution in a compressible MHD fluid

Abstract: The temporal dynamics of field-line reconnexion is studied by numerical experiment in a compressible conducting fluid. It is shown that because of localized enhancement of resistivity the reconnexion takes place in an initially antiparallel magnetic field and that an X-type field configuration develops, occupying an extended region. There is also a remarkable release of magnetic energy into kinetic and thermal energies. The global flow pattern can spontaneously be set up through the field-line reconnexion under no specially imposed boundary conditions.

Electric currents in cosmic plasmas

Abstract: It is suggested that dualism is essential for the physics of cosmic plasmas, that is, that some phenomena should be described by a magnetic field formalism, and others by an electric current formalism. While in earlier work the magnetic field aspect has dominated, at present there is a systematic exploration of the particle (or current) aspect. A number of phenomena which can be understood only from the particle aspect are surveyed. Topics include the formation of electric double layers, the origin of 'explosive' events like magnetic substorms and solar flares, and the transfer of energy from one region to another. A method for exploring many of these phenomena is to draw the electric circuit in which the current flows and then study its properties. A number of simple circuits are analyzed in this way.

Evolution of pulsar magnetic fields

Abstract: Theoretical considerations of neutron star matter and magnetic fields suggest a picture of the evolution of pulsar dipole moments. At birth the spin axis and magnetic dipole are argued to be roughly aligned. Subsequently the magnetic dipole greatly diminishes in strength and changes its direction until it ultimately makes a large angle with the spin axis. This view is supported by observations of pulsar subpulse drifting and the relation of pulsar periods to spin-down rates.

Magnetic Fields of a Dipole in Special Volume Conductor Shapes

Abstract: Expressions are presented for the magnetic fields produced by current dipoles in four basic volume conductor shapes. These shapes are the semi-infinite volume, the sphere, the prolate spheroid (egg-shape), and the oblate spheroid (discus-shape). The latter three shapes approximate the shape of the human head and can serve as a basis for understanding the measurements of the brain's magnetic fields. The semi-infinite volume is included in order to investigate the effect of the simplest boundary between a conductor and nonconductor. The expressions for the fields are presented in a form which separates the total field into two parts. One part is due to the dipole alone (the dipole field); the other is due to the current generated in the volume conductor by the dipole (the volume current field). Representative plots of the total field and the volume current field are presented for each shape. The results show that for these shapes the component of the total field normal to the surface of the volume conductor is produced completely or in large part by the dipole alone. Therefore, measurement and use of this component will greatly reduce the complexity of determining the sources of electrical activity inside a body from measurements outside the body by removing the necessity of dealing with the volume current field.

Magnetic excitations in the weak itinerant ferromagnet MnSi

Abstract: Magnetic excitations in an intermetallic compound MnSi have been studied by neutron scattering. At 5 K in a magnetic field of 10 kOe, well-defined spin-wave excitations have been observed below 2.5 meV. The dispersion relation is almost isotropic and is $h{\ensuremath{\omega}}_{q}(\mathrm{meV})=0.13+52{q}^{2}$ (${\mathrm{\AA{}}}^{\ensuremath{-}2}$) in the [100] direction. Above 3 meV, the excitation linewidth increases substantially, suggesting that the dispersion relation merges into the Stoner continuum. The Stoner excitations, which extend over almost all of the Brillouin zone, show a broad peak on the extension of the spin-wave dispersion relation. The spin-wave excitation renormalizes with increasing temperature and collapses into critical scattering above 40 K. On the other hand, the excitation in the Stoner continuum is affected little by temperature; the excitations are qualitatively the same at $\frac{T}{{T}_{N}}=10$ as at 5 K. The Stoner boundary energy decreases with increasing temperature. The results provide us with the first example of magnetic excitations in a weak itinerant ferromagnet.

Comparative study of the axial and azimuthal bunching mechanisms in electromagnetic cyclotron instabilities

Abstract: A comparative study is presented of the axial and azimuthal bunching mechanisms of the electromagnetic electron cyclotron instability. Axial bunching can be described with a nonrelativistic treatment, but azimuthal bunching is relativistic in origin. As is well known, the axial bunching mechanism drives the Weibel‐type instability while the azimuthal bunching mechanism drives the electron cyclotron maser instability. For an electron ensemble of cold helical trajectories, a unified physical interpretation of both instabilities is presented. It is shown that the two bunching mechanisms are actually simultaneously present in either instability and compete with one another. As a result, the dominant mechanism determines the type of instability. A criterion for distinguishing the two types of instabilities is derived. It is shown that the energy of the electrons plays an insignificant role in the criterion and, hence, should not be a factor in the justification of a nonrelativistic treatment. Regimes of validi...

Transport coefficients in strong magnetic fields

Abstract: Formulae describing the electron transport coefficients as functions of the conductivity tensor are derived on the basis of the linear response theory. It is shown that the Wiedemann-Franz law and the Mott rule are obeyed even in the presence of a quantizing magnetic field omega tau >1 if the scattering of electrons is elastic and if h(cross) omega >>kT. As no assumptions about the form of the one-electron potential are made, the conclusions are valid not only in the case of the Shubnikov-de Haas oscillations but also if effects such as magnetic breakdown and the size-effect appear.

Magnetospheric Magnetic Field Modeling.

Abstract: : This report describes work that has been performed within the last year on the modeling of the disturbed magnetospheric magnetic field. The reasons for developing quantitative magnetospheric models are twofold: first, there are many model users who have practical needs. Their requirements are to specify and to predict such things as: radiation dose in the earth orbital environment, fluctuations in ionospheric electron density, variations in the earth's upper atmosphere density, and the impact of magnetic storms on loading changes in electrical power grids. The second set of model users have technical and scientific needs. They use the magnetic field models in studies of other magnetospheric, ionospheric, and upper atmospheric phenomena, or to understand the interactions of magnetospheric magnetic and electric fields with particle populations. This report should be the last of a series quantitatively describing the magnetospheric magnetic field. Some of the history of the magnetic field work that leads to the efforts pursued this last year is described. (Author)

The mean magnetic field of the Sun: observations at Stanford.

Abstract: A solar telescope has been built at Stanford University to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph which is connected to a 22.9 m vertical Littrow spectrograph. Sun-as-a-star integrated light measurements of the mean solar magnetic field have been made daily since May 1975. The typical mean field magnitude has been about 0.15 G with typical measurement error less than 0.05 G. The mean field polarity pattern is essentially identical to the interplanetary magnetic field sector structure (see near the Earth with a 4 day lag). The differences in the observed structures can be understood in terms of a ‘warped current sheet’ model.

Single-Pulse Superfluorescence in Cesium

Abstract: Superfluorescence has been observed at 2931 nm from the 7P3/2, to 7S1/2, transition in cesium. It is shown that a completely inverted two-level system can be prepared in a magnetic field. Experimental results on the superfluorescent output are reported for a range of sample lengths (1.0 cm to 5.0cm)and densities (8 × 109 cm−3 to 2 × 1011 cm−3 ) and for several values of the inhomogeneous dephasing time (5 ns, 18 ns, and 32 ns). For sufficiently long delay times the emission from the pencil-shaped volume of Fresnel number one consists of a single pulse for all sample lengths, both in an atomic beam and in a cell. Neither homogeneous relaxation nor inhomogeneous dephasing can explain the absence of ringing. For increased densities and reduced delay times, depending on the length of the sample,multiple-pulse output occurs. The occurrence of single and multiple pulses, the delay times, and the pulsewidths are discussed.

High resolution mapping of the magnetic field of the solar corona

Abstract: The mapping of the current-free magnetic field of the solar corona and the reliability of the spherical harmonic analysis of the photospheric magnetic field pattern are improved by data with much greater dynamic range and spatial resolution than previously available and a new algorithm which permits spherical harmonic expansion to a much higher value of the principal index. Coronal field maps can be drawn for local regions, for just the open field lines, and for various spatial resolutions on a global scale.

Theory of the ubiquitous mode.

Abstract: Standing modes along the magnetic field lines and with frequencies between the mean ion transit and electron bounce frequencies are driven unstable by a combination of effects including inverse Landau damping, collisional de-trapping of electrons and magnetic curvature drifts in a magnetically confined plasma. The mode phase velocity, depending on the plasma parameters and on the mode perpendicular wavelength, is either in the direction of the electron diamagnetic velocity (electron drift mode) or in the direction of the ion diamagnetic velocity (ubiquitous mode). An intermediate region exists where the mode becomes fluid-like and is driven by the magnetic curvature drift. This fluid-like instability is shown to appear for wavelengths longer than the ion gyroradius and is expected to affect significantly the rate of particle and thermal energy transport in high-temperature toroidal-confinement devices.

Interface free energy and transition temperature of the square-lattice Ising antiferromagnet at finite magnetic field

Abstract: We use a simple method to calculate interface properties of the square-lattice Ising antiferromagnet with nearest neighbour interaction. The method bypasses the more complicated bulk problem by taking into account only interface configurations of spins and allows the inclusion of a finite magnetic field. From this we derive two new results: 1) the interface free energy associated with the coexistence of the two antiferromagnetic phases at finite magnetic field, and 2) the transition temperature as a function of the magnetic field which determines the phase boundary.

The determination of vector magnetic fields from stokes profiles

Abstract: The application of Unno's (1956) solution of the transfer equation for polarized radiation to the determination of thevector magnetic field is investigated. An analysis procedure utilizing non-linear least squares techniques is developed that allows one to automate the reduction of measured spectral profiles of the Stokes parameters to determine the field angles, strength as well as other parameters. The method is applied to synthetic spectra generated using a model solar atmosphere and yields results of remarkably high accuracy. The influence of additional factors upon determination of the vector field are also considered. These factors include effects of asymmetric profiles, magneto-optical effects, magnetic field gradients, unresolved field elements, scattered light, and instrumental noise.

Formation of the hexagonal pattern on the surface of a ferromagnetic fluid in an applied magnetic field

Abstract: When a ferromagnetic fluid with a horizontal free surface is subjected to a uniform vertical applied magnetic field B0, it is known (Cowley & Rosensweig 1967) that the surface may be unstable when the field strength exceeds a certain critical value Bc. In this paper we consider, by means of an energy minimization principle, the possible forms that the surface may then take. Under the assumption that |μ − 1| [Lt ] 1 (where μ is the magnetic permeability of the fluid), it is shown that when B0 is near to Bc there are three equilibrium configurations for the surface: (i) flat surface, (ii) stationary hexagonal pattern, (iii) stationary square pattern. Configuration (i) is stable for B0 Bc and B0−Bc sufficiently small, and (iii) is stable for some higher values of B0. In each configuration the fluid is static, and the surface is in equilibrium under the joint action of gravity, surface tension, and magnetic forces. The amplitude of the surface perturbation in cases (ii) and (iii) is calculated, and hysteresis effects associated with increase and decrease of B0 are discussed.

Nuclear magnetic resonance apparatus and methods

Abstract: A nuclear magnetic resonance spin density distribution in a sample is obtained for a selected plane or planes by placing the sample in a static magnetic field, applying a gradient to the field and simultaneously applying selective rf pulses to select a plane or planes in the sample, switching to an orthogonal gradient and simultaneously applying selective rf pulses to select strips in the selected plane or planes and then applying orthogonal field gradients to the sample of such relative magnitudes that each point of the selected strips is subjected to a resultant magnetic field of amplitude unique to that point. The free induction decay signal is then read out from the strips.

Slotted tube resonator: A new NMR probe head at high observing frequencies

Abstract: Superconducting magnetic systems permit increasing observing frequencies for NMR experiments. The sensitivity and range of application of the single‐layer detection system is evaluated. High measuring frequencies demand new detection systems because the conventional detection systems lose their good features above about 100 MHz. In this paper a new detection system is presented. The slotted tube resonator (STR) is suitable for all intensities of magnetic field which will be produced in the near future. The characteristic parameters of the STR are calculated and the construction is described in detail. Theoretical sensitivity considerations are compared with practical measurements.

Quadrupole Influence on the Dipolar-Field Width for a Single Interstitial in a Metal Crystal

Abstract: The dipolar broadening of the magnetic field sensed by an interstitial impurity in a rigid lattice is calculated with the electric-field gradient set up by the impurity taken into account. This is shown to give a strong dependence of the dipolar width on the applied magnetic field. The theory is especially applicable to the linewidth of precessing muons in metals.

Magnetic fields of finite track width heads

Abstract: Closed form field solutions are given for finite gap, infinite length heads of either semi-infinite or zero track width. A three dimensional head of finite track width is approximated by superposition. By viewing the finite width head at a different angle, the results also apply to a semi-infinite width head with finite gap depth. At distances greater than one-half gap length removed, the maximum field error of the superposition does not exceed 8% and decreases with increasing head width.

Nonrelativistic electron stream propagation in the solar atmosphere and type III radio bursts.

Abstract: Electron streams with type III burst characteristics are numerically modeled. The electronplasma wave quasilinear interaction is assumed to be the dominant velocity diffusion process. The quasilinear equations with the addition of spontaneous emission, magnetic and collisional effects are numerically solved as an initial value and a half-space boundary value problem with time, distance and velocity as the independent variables for a solar-type background plasma and a type-III-like stream. Background density and temperature coordinate structure, spontaneous emission, magnetic fields, electron-ion collisions, stream reabsorption and wave pileup are shown to affect propagation and are incorporated into a physical description of the stream motion. The calculated electron flux-time profiles at the Earth suggest scatter-free propagation and compare well with type III stream observations.