Vacuum Propagation Research Articles

On the dispersion of gravitational waves

The propagation of gravitational waves in cold matter has been analyzed in the geometrical-optics approximation showing no difference from the propagationin vacuum. In a cosmological background a plasmalike dispersion relation has been shown to appear in the postgeometrical optics approximation only. A qualitative discussion of the physical meaning of the present result is given and compared with previous works.

Vacuum propagation of solid relativistic electron beams

An investigation of equilibrium, stability, and space-charge- limiting current of a solid relativistic beam propagating along a finite external magnetic field has been carried out and compared with experiments. The concept of a rigid-rotor equilibrium is only approximately valid when the beam current is much less than the Alfvèn critical current. Typically, low-voltage beams rotate fastest at the beam edge whereas high-voltage beams rotate fastest near the beam axis. A limited investigation of the rigid rotor stability condition indicates, at worst, a weak instability may be present if the rigid-rotor equilibrium is artificially imposed, and no instability at all for a self-consistent equilibrium. Numerical solutions for the space-charge limiting current and relativistic factor on axis are presented and compared with two-dimensional, cylindrical, space- and time-dependent simulations. Analytical expressions for the limiting current are valid for ωc/ωb≳5, where ωc is the cyclotron frequency and ωb is the beam frequency. For external magnetic field strengths below this level, the limiting current increases monotonically with decreasing field strength until the beam collides with the drift tube. Expressions for foil scattering are also presented. The results are in good agreement with experiments.

A contribution to the study of fatigue crack retardation in vacuum

Results of an experimental work on a comparative study of the delay mechanism following a spike type overload during fatigue crack propagation in vacuum and air, in an aluminium alloy 2024-T351 are presented. The analysis that follows suggests that phenomenologically, the mechanism of delay in the two environments is essentially the same, the quantitative differences being due to the inherent differences in the fatigue crack propagation behaviour in the two environments considered.

On the propagation of energetic ion beams through a fusion target chamber

This article treats some aspects of the propagation of a highly energetic heavy ion beam through a fusion target chamber. The stability of this beam to various perturbations is especially emphasized, first for vacuum propagation, and then for propagation through a background plasma. The results are illustrated by detailed calculations for a beam carrying 3 kA of current and consisting of 70GeV, singly ionized uranium ions. We find that such a beam will propagate stably through the target chamber. As a second example we have applied some of our results to the recently proposed I53+ concept in which up to a hundred beams of 40GeV iodine ions each carrying 4.2 kA of current are directed to the target. Our conclusions for this case are less optimistic, as such beams are likely to be subject to space-charge problems due to the increased value of v/γ and are prone to instabilities of the two-stream type.

Dielectric screening and susceptibility fluctuations in optical scattering

An internally consistent theory of optical scattering in molecular fluids is developed in terms of a ‘screened dipole radiator’ and a screened dipole photon propagator, related by a Bohr-Peierls-Placzek-type relation. The screened dipole radiator describes the propagation of scattered radiation in the dielectric medium, the transition at the surface of the scattering sample, and the ultimate propagation in vacuum outside the material system. The scattering is obtained as a series in terms of Ursell functions, explicit to all orders. In an approximation describing light scattering by matter in bulk, the screened dipole radiator is obtained in closed form involving an intrinsic directional average. The directional average has important consequences for depolarization, for critical scattering, and for questions of convergence. A microscopic representation of local susceptibility fluctuations emerges, and it is shown that polarization fluctuations can naturally and rigorously be decomposed into one contribution from local susceptibility fluctuations and one from field fluctuations. An approximation in closed form to the scattering cross section can be interpreted in macroscopic terms on the basis of this decomposition. In this approximation, critical scattering deviates from Ornstein-Zernike behaviour, but depolarized scattering is incompletely included.

Propagation of an unneutralized intense relativistic electron beam in a magnetic field

In this paper the results of a series of electron-beam vacuum propagation experiments are presented and compared with theoretical predictions. Important conclusions pertaining to time-dependent diode behavior, virtual cathode formation, and various equilibrium and stability criteria are reported.

Remarks on the Poukey‐Rostoker‐model of relativistic electron beam propagation

AbstractSome properties of an analytic electrostatic model of relativistic electron beam propagation in vacuum, developed by Poukey and Rostoker [1], are evaluated. It is shown that the limits of validity of this model result from the mixing of electrons, from space charge singularities and the backflow of electrons to the cathode.

Nonlinear Inverse Comption Radiation and the Circular Polarization of Diffuse Radiation from the Crab Nebula

view Abstract Citations (37) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonlinear Inverse Comption Radiation and the Circular Polarization of Diffuse Radiation from the Crab Nebula Arons, Jonathan Abstract A detailed calculation is given of the high-frequency radiation from very relativistic particles moving in a strong, circularly polarized electromagnetic wave. Tbe wave has amplitude E0 such that v0 = eE0/mc 1, where Q is the circular frequency of the wave and m and e are respectively the mass and the absolute value of the charge, and the particle density is assumed to be low enough for vacuum propagation conditions to apply. The particles are assumed to have energies yfllc2 such that y v02. The resulting radiation is shown to have a degree of circular polarization oCv0 -` independent of the radiated frequency, but the total intensity emitted by a stationary system of electrons, all of which drift at angles greater than voly to the propagation direction of the wave, is shown to be identical to synchrotron radiation from relativistic electrons gyrating at large pitch angles in a magnetic field of strength E0/ 2. The results are used to show that the circular polarization of this "nonlinear inverse Compton" mechanism, when combined with the vacuum oblique-rotator approximation for pulsar environments, may be in disagreement with observational upper limits on the fractional circular polarization of the diffuse emission from the Crab nebula. Publication: The Astrophysical Journal Pub Date: November 1972 DOI: 10.1086/151717 Bibcode: 1972ApJ...177..395A full text sources ADS |

Microwave Frequency Acoustic Surface-Wave Loss Mechanisms on LiNbO3

The temperature and frequency dependence of the attenuation of 0.5–5-GHz acoustic surface waves on LiNbO3 has been measured. For propagation in vacuum a frequency-squared dependence of the total attenuation is obtained with a value at 1 GHz of 0.9 dB/μsec. Temperature-dependence measurements using a novel three-transducer technique show the dominant loss (0.7 dB/μsec at 1 GHz) mechanism to be the interaction with thermally excited elastic waves. Propagation in air results in an additional loss linearly proportional to frequency with a value of 0.2 dB/μsec at 1 GHz. The effects of beam steering and diffraction losses are also investigated both theoretically and experimentally. Both misalignment of transducers with respect to pure mode propagation axes and misalignment of the propagation-plane perpendicular can add significantly to delay line insertion loss. This beam steering loss on Y-cut Z-propagating LiNbO3 is considerably higher than on the 41.5° rotated-cut X-propagating orientation. The loss mechanisms measured in this paper are sufficient to completely account for the insertion loss of surface-wave delay lines.

Ionospheric Wave Theory Using Coupled Vacuum Modes

In coupled‐mode theory the wave field in a stratified anisotropic medium appears decomposed into modes, which are coupled in accordance with a set of coupled wave equations. The modes can be defined in various ways. In the conventional formulation they correspond to waves in a homogeneous magnetoionic medium; the present theory uses modes of vacuum propagation characteristics. The coupled wave equations for these vacuum modes have a fairly simple form. The effect of the medium appears in the coupling matrix, whose elements contain a number of terms depending on the various parameters of the medium.The coupled wave equations, originally obtained from a study of partial reflection at differential slabs of the medium, are now derived in two different ways, both rather direct. In one derivation the vector potential is split into ascending and descending parts. The other derivation adheres to the matrix formalism of the conventional coupling theory as far as possible, thus indicating the essential deviations from the conventional theory. The vertical energy flux densities of the modes are expressed in terms of the field quantities used.

Electromagnetic Scattering by Finite Dense Plasmas

The often used hypothesis that a large overdense plasma medium may be treated as a perfect reflector is critically examined using a large homogeneous plasma sphere for particular emphasis as a simple scattering volume. This hypothesis is found to be in substantial error for densities between about four and κa/2 times the critical density where κ is the vacuum propagation constant of the incident field and a is the radius of the scatterer. The ``anomalous'' scattering in this restricted density range arises because of interference effects between the backscattered wave and tightly bound surface waves. These surface waves are of considerable importance for even very large (κa∼700) scatterers. The effects of absorption and inhomogeneity are also discussed. In general, it appears that deductions based on radar and other backscatter measurements are of questionable validity for a rather broad range of ionized scattering media.