Volume 15, Issue 2
August 1980, pages 117-223
pp 117-136 August 1980 Plasma Physics
The paper emphasises the importance of plasma-neutral boundary layer in a wide variety of physical phenomena occurring in laboratory and cosmic plasmas. The interaction of a magnetised plasma stream penetrating a neutral gas cloud is discussed in the light of Alfvén’s critical velocity and Varma’s threshold velocity on the ionising interaction.
Interaction of a moving magnetised plasma with a stationary neutral gas has been studied and described. The device comprises of a plasma gun and an interaction region where neutral gas cloud is injected. The interaction region is provided with a transverse magnetic field of upto 1000 G. Several diagnostics deployed at the interaction region to make measurements on the macroscopic parameters of plasma and neutral gas are described. The parameters of discharge circuits are measured with high current and voltage probes.
An interaction between a magnetised plasma stream and a neutral gas cloud is demonstrated. It is shown that this interaction does not have Varma’s threshold on their relative velocity. The Alfvén’s critical velocity phenomenon is shown to depend on the integrated column neutral gas density that a plasma stream encounters while penetrating through it and not on the neutral gas density in the range of 1017–1021 m−2.
pp 137-143 August 1980 Plasma Physics
The application of high power microwaves to a gas-filled resonant cavity may be an ideal way to start up Tokamak, mirror or other thermonuclear reactors. RF coupling problems are avoided by input of microwave power prior to wave cut-off.
pp 145-151 August 1980 Astrophysics
The transmission probability of charged particle reactions in a stellar plasma is examined by considering the effective potential between the interacting charges as having an exponential factor in the Coulomb term arising out of the spherical distribution of electrons with its uniform background of positive charges (Debye-Hückel electrolyte). The expression for the ratio of this transmission probability to that due to pure Coulomb field, called the enhancement factor, is then obtained. Thermonuclear reactions of astrophysical interest such as12C(α, γ)16O and16O(α, γ)20 Ne are then considered. Our enhancement factor in the12C(α, γ)16O reaction rate is found to agree reasonably well with those calculated from the expression given by Alastuey and Jancovici.
pp 153-161 August 1980 Mathematical Physics
Reduced expansions for electromagnetic fields associated with spin-1 particles of imaginary mass (tachyons) have been derived in terms of standard helicity representations of inhomogeneous Lorentz group. The effects of wave equation and reality condition on these reduced expansions to satisfy Maxwell’s field equations have been derived. The reduced expansions of charge and current source densities associated with these fields have also been derived and it has been shown that these fields cannot satisfy the Maxwell’s equations in the absence of both the current and charge source densities.
pp 163-173 August 1980 Atomic Physics
Energy levels: 1s1/2, 2p1/2, 2p3/2, 3d3/2, and 3d5/2 of the muon in the spherical nuclei:120Sn,197Au and208Pb have been calculated under the assumption of harmonic oscillator potential. The levels are corrected for vacuum polarisation. The agreement with experimental values is better than 0·5%. An accurate method of solving the Dirac equation to obtain the energy eigenvalues is outlined. The importance of choosing the ‘classical turning point’ as the radius for matching the interior and exterior solutions is discussed.
pp 175-179 August 1980 Solid State Physics
Poor beam collimation leads to overcompensation of suppression of coherent bremsstrahlung in the axial channelling case. This fact has been used to study the effect of beam divergence on the radiation emitted by relativistic (axially) channelled positrons. It has been found that due to beam divergence in the experiment of Alguard and co-workers in which radiation of 56 MeV positrons channelling along 〈110〉 rows of a silicon crystal is observed, coherent bremsstrahlung becomes an important contributing factor to the high frequency part of the observed spectrum.
pp 181-188 August 1980 Solid State Physics
Photo and electroluminescent properties of ZnO phosphors prepared by burning zinc in an atmosphere of pure oxygen have been reported. The peak positions obtained by resolving the experimental curves into Gaussian profiles are explained by assuming that the rare earths form the donor levels while the acceptor levels cause the emission of undoped phosphors. Voltage dependence of electroluminescent brightness shows that the mechanism of excitation consists of acceleration-collision. Brightness waves at different concentrations of rare earths and frequency of excitation are also studied and discussed.
pp 189-205 August 1980 Solid State Physics
Computer simulation studies have been made to investigate the static properties of mono-, di- and tri-vacancy clusters and of self-interstitials in hcp magnesium in different configurations. Three interatomic potentials have been chosen for which the results have been compared. A crystallite containing about 1500 atoms and a model with the interatomic interaction extending upto the fourth neighbour distance have been used. Relaxation field, defect relaxation and formation energies, strength dipole tensors and relative changes in volume in the above defects have been computed and our final results compared with those of earlier workers. The formation energies of the defects are highly sensitive to the choice of the potential whose detailed structure guides the nature of relaxation and the dipole tensors. Calculations have been done for octahedral, tetrahedral and dumb-bell interstitials of which the last is found to be the most stable.
pp 207-223 August 1980 Solid State Physics
Using polarised neutrons, the full three-dimensional magnetic structure amplitudes in the Ni1−c Ruc single crystals forc = 0·027, 0·033 and 0·046 were measured. Moment density maps in various portions of the Wigner-Seitz cell were obtained. It is seen from these maps that unlike Ni-based alloys with 3d impurities, the introduction of Ru to the Ni matrix produces extensive perturbations in the diffuse moment density, giving rise to a netpositive diffuse moment which tends to increase with Ru concentration. The asphericity of the host moment at first increases and then decreases with increasing Ru content. Another significant outcome of the present study is the evidence for the reversal of the sign of the Ru moment, from negative to positive, obtained by comparing the shape of the spherical site form factors of the three-alloy concentrations with the Ni spherical form factor itself. The sign reversal of the impurity moment is confirmed by the form factor analyses. Strong local environmental effects seem to play a major role in this alloy system.
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