• Volume 61, Issue 6

December 2003,   pages  1055-1234

• Cosmological models in general relativity

LRS Bianchi type-I space-time filled with perfect fluid is considered here with deceleration parameter as variable. The metric potentialsA andB are functions of x as well as t. Assuming B′/B = f (x), where prime denotes differentiation with respect to x, it was found thatA = (l′/l)B andB = lS(t), wherel = f (x) andS is the scale factor which is a function of t only. The value of Hubble’s constantH0 was found to be less than half for non-flat model and is equal to 1.3 for a flat model

• Types of two-dimensionalN = 4 superconformal field theories

Various types ofN = 4 superconformal symmetries in two dimensions are considered. It is proposed that apart from the well-known cases ofSU (2)and SU(2) × SU(2) ×U (1), their Kac-Moody symmetry can also be SU(2) × (U (1))4. Operator product expansions for the last case are derived. A complete free field realization for the same is obtained

• The continuum shell-model neutron states of209Pb

The neutron strength distributions of the three high-spin 1k17/2, 2h11/2 and 1j13/2 states of209Pb have been obtained within the formalism of the core-polarisation effect where the effect of interaction of the neutron shell-model states of209Pb with the collective vibrational states (originating also from the giant resonances) have been taken into consideration. The theoretical results have been discussed in the light of works on 1k17/2, 2h111/2 and 1j13/2 neutron orbitals of209Pb. The shell-model energies of the neutron states have been obtained by Skyrme-Hartree-Fock method

• Dynamics of cascade three-level system interacting with the classical and quantized field

We study the exact solutions of the cascade three-level atom interacting with a single mode classical and quantized field with different initial conditions of the atom. For the semiclassical model, it is found that if the atom is initially in the middle level, the time-dependent populations of the upper and lower levels are always equal. This dynamical symmetry exhibited by the classical field is spoiled on quantization of the field mode. To reveal this non-classical effect, a Euler matrix formalism is developed to solve the dressed states of the cascade Jaynes-Cummings model (JCM). Possible modification of such an effect on the collapse and revival phenomenon is also discussed by taking the quantized field in a coherent state

• A DC excited waveguide multibeam CO2 laser using high frequency pre-ionization technique

High power industrial multibeam CO2 lasers consist of a large number of closely packed parallel glass discharge tubes sharing a common plane parallel resonator. Every discharge tube forms an independent resonator. When discharge tubes of smaller diameter are used and the Fresnel numberN ≪ 1 for all resonators, they operate in waveguide mode. Waveguide modes have excellent discrimination of higher order modes. A DC excited waveguide multibeam CO2 laser is reported having six glass discharge tubes. Simultaneous excitation of DC discharge in all sections is achieved by producing pre-ionization using an auxiliary high frequency pulsed discharge along with its other advantages. Maximum 170 W output power is obtained with all beams operating in EH11 waveguide mode. The specific power of 28 W/m is much higher as compared to similar AC excited waveguide multibeam CO2 lasers. Theoretical analysis shows that all resonators of this laser will support only EH11 mode. This laser is successfully used for woodcutting

• Electrothermal efficiency, temperature and thermal conductivity of plasma jet in a DC plasma spray torch

A study was made to evaluate the electrothermal efficiency of a DC arc plasma torch and temperature and thermal conductivity of plasma jet in the torch. The torch was operated at power levels from 4 to 20 kW in non-transferred arc mode. The effect of nitrogen in combination with argon as plasma gas on the above properties was investigated. Calculations were made from experimental data. The electrothermal efficiency increased significantly with increase in nitrogen content. The plasma jet temperature and thermal conductivity exhibited a decrease with increase in nitrogen content. The experiment was done at different total gas flow rates. The results are explained on the basis of dissociation energy of nitrogen molecules and plasma jet energy loss to the cathode, anode and the walls of the torch

• Optimization of soft x-ray line emission from laser-produced carbon plasma with laser intensity

Absolute measurement for He-α resonance (1s21S0−1s2p1P1, at 40.2 Å) line emission from a laser-produced carbon plasma has been studied as a function of laser intensity. The optimum laser intensity is found to be ≈1.3×1012 W/cm2 for the maximum emission of 3.2 × 1013 photons sr−1 pulse−1. Since this line lies in the water window spectral region, it has potential application in x-ray microscopic imaging of biological sample in wet condition. Theoretical calculation using corona model for the emission of this line is also carried out with appropriate ionization and radiative recombination rate coefficients

• Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

Isothermal remanent magnetization decay,Mr(t), and ‘in-field’ growth of zero-field-cooled magnetization,MZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25Tc to 1.25Tc (whereTc is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni3Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofMr(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions$$M_r (t) = M_0 [M_1 \exp ( - t/\tau _1 ) + (t/\tau _2 )^{ - \alpha } ]$$ and$$M_{ZFC} (t) = M'_0 [1 - \{ M'_1 \exp ( - t/\tau '_1 ) + (t/\tau '_2 )^{ - \alpha '} \} ]$$ closely reproduce such data in the present case. The most striking features of magnetic relaxation in the system in question are as follows: Aging effects are absent in bothMrt andMZFC(t) at all temperatures in the temperature range covered in the present experiments. A cross-over in equilibrium dynamics from the one, characteristic of a pured = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from Tc, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT → Tc. The relaxation times τ1(T)(τ1(T)) and τ2(T)(τ2(T)) exhibit logarithmic divergence at critical temperatures$$T_C^{\tau _1 } (T_C^{\tau '_1 } (H))$$ and$$T_C^{\tau _2 } (T_C^{\tau '_2 } (H))$$;$$T_C^{\tau '_1 }$$ and$$T_C^{\tau '_2 }$$ both increase with the external magnetic field strength,H, such that at any given field value,$$T_C^{\tau '_1 } = T_C^{\tau '_2 }$$. The exponent characterizing the logarithmic divergence in τ1(T) and τ2T possesses a field-independent value of ≃16 for both relaxation times. Of all the available theoretical models, the droplet fluctuation model alone provides a qualitative explanation for some aspects of the present magnetic relaxation data

• Order parameter anisotropy of MgB2 using specific heat jump of layered superconductors

The recently obtained analytical result [1] for renormalization of the jump of the heat capacity (CSCN)/CN by anisotropy of the order parameter is applied to the layered superconductors. The graph of (CSCN)/CN vs. the anisotropy of the order parameter allows a direct determination of the gap anisotropy in MgB2 using available experimental data

• Room temperature Compton profiles of conduction electrons in α-Ga metal

Room temperature Compton profiles of momentum distribution of conduction electrons in α -Ga metal are calculated in band model. For this purpose, the conduction electron wave functions are determined in a temperature-dependent non-local model potential. The profiles calculated along the crystallographic directions, (100), (010), and (001) are found to be nearly isotropic. This conclusion is in reasonable agreement with experimental observations

• Observation of high spin levels in131Cs from131Ba decay

The γ- and conversion electron spectra following131Ba ε-decay are investigated, using HPGe detector and mini-orange electron spectrometer. Attention is particularly focussed on identifying weak transitions associated with low energy high spin levels in131Cs level scheme earlier inferred in reaction studies but not yet observed in131Ba decay. Our experiment identifies 15 new gammas and 6 new conversion lines in this decay. Internal conversion coefficients and multipolarities of several transitions are determined. Five new levels (3 with Iπ = 7/2+ and one each with Iπ 9/2+ and 11/2) are introduced in the131Cs level scheme based on our observations taken together with the results from reaction studies. Spin-parity assignments to a few other levels are also suggested

• Dynamical interplay between fluctuations, electric fields and transport in fusion plasmas

A view of recent experimental results and progress in the characterization of the statistical properties of electrostatic turbulence in magnetically confined devices is given. An empirical similarity in the scaling properties of the probability distribution function (PDF) of turbulent transport has been observed in the plasma edge region in fusion plasmas. The investigation of the dynamical interplay between fluctuation in gradients, turbulent transport and radial electric fields has shown that these parameters are strongly coupled both in tokamak and stellarator plasmas. The bursty behaviour of turbulent transport is linked with a departure from the most probable radial gradient. The dynamical relation between fluctuations in gradients and transport is strongly affected by the presence of sheared poloidal flows which organized themselves near marginal stability. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way to characterize transport based on the computation of effective transport coefficients

• Parametric instabilities in magnetized bi-ion and dusty plasmas

The excitation of low frequency modes of oscillations in a magnetized bi-ion or dusty plasma with parametric pumping of the magnetic field is analysed. The equation of motion governing the perturbed plasma is derived and parametrically excited transverse modes propagating along the magnetic field are found. With multiple ion species or charged dust present, a number of different circularly polarized modes can be excited. The stability of these modes is investigated as a function of the plasma parameters. The modulational instabilities of large amplitude normal modes, modified by the extra ion species or dust and propagating along the magnetic field, are also investigated

• Application of plasma focus installations for a study of the influence of deuterium cumulative flows on materials

In this work, as an example of an application of the plasma focus (PF) device, we study the influence on alloys of vanadium of a cumulative flow producing in the PF device. The experiment was done in a 4-kJ PF device with various gas fillings and various anode shapes. It was found that the velocity of the axial cumulative flow depends on the type of gas and is about 5.107 cm/s for deuterium and 2.107 cm/s for argon fillings of plasma focus chamber; the shape of the flow is changed from a broad conical fly for deuterium to a quasi-one-directional stream for argon. The dynamics and structure of such flows are investigated by means of laser diagnostics and an image converter camera. The experiments show that cumulative flows produce various defects in tested samples. The appearance of a large number of cracks on the surface of vanadium under the impulse influence of deuterium plasma shows that pure vanadium cannot be used for the construction of thermonuclear fusion reactors

• Hard X-ray bursts and DD microfusion neutrons from complex plasmas of vacuum discharge

We create the random complex media of high-power density in low-energy nanosecond vacuum discharges. Hard X-ray emission efficiency, generation of energetic ions (∼1 MeV) and neutrons, trapping and releasing of fast ions and/or X-rays from interelectrode aerosol ensembles are the subject of our study. The neutrons from DD microfusion, as well as the modelling of some interstellar nuclear burning due to microexplosive nucleosynthesis are discussed. The value of neutron yield from DD fusion in interelectrode space varies and amounts to ∼105–107/4π per shot under ≈ 1 J of total energy deposited to create all discharge processes

• Dust acoustic shock wave generation due to dust charge variation in a dusty plasma

In a dusty plasma, the non-adiabaticity of the charge variation on a dust grain surface results in an anomalous dissipation. Analytical investigation shows that this results in a small but finite amplitude dust acoustic (DA) wave propagation which is described by the Korteweg-de Vries-Burger equation. Results of the numerical investigation of the propagation of large-amplitude dust acoustic stationary shock wave are presented here using the complete set of non-linear dust fluid equations coupled with the dust charging equation and Poisson equation. The DA waves are of compressional type showing considerable increase of dust density, which is of significant importance in astrophysical context as it leads to enhanced gravitational attraction considered as a viable process for star formation. The DA shock transition to its far downstream amplitude is oscillatory in nature due to dust charge fluctuations, the oscillation amplitude and shock width depending on the ratioωpd/Vch and other plasma parameters

• Centrifugal acceleration of plasma in pulsar magnetosphere

We present a relativistic model for the centrifugal acceleration of plasma bunches and the coherent radio emission in pulsar magnetosphere. We find that rotation broadens the width of leading component compared to the width of trailing component. We explain this difference in the component widths using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters, and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyse the single pulse polarization data of PSR B032954 at 606 MHz, and find that in its conal components, one sense of circular polarization dominates in the leading component while the other sense dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sense of circular polarization as well as the swing of polarization angle

• Non-linear high-frequency waves in the magnetosphere

Using fluid theory, a set of equations is derived for non-linear high-frequency waves propagating oblique to an external magnetic field in a three-component plasma consisting of hot electrons, cold electrons and cold ions. For parameters typical of the Earth’s magnetosphere, numerical solutions of the governing equations yield sinusoidal, sawtooth or bipolar wave-forms for the electric field

• Effect of ionized plasma medium on the radiation from a RITMA structure on ferrite substrate

This paper presents theoretical investigations on the radiation properties of a right isosceles triangular microstrip antenna (RITMA) printed on a magnetized ferrite substrate Ni0.62Co0.02Fe1.948O4 in the presence of ionized plasma medium. The theoretical study on RITMA structure in free space is carried out in TM11 mode of excitation by applying cavity model-based modal expansion technique while hydrodynamic theory is used for its analysis in plasma medium. By varying the bias magnetic field, far-field radiation patterns in free space and plasma medium are obtained which in turn are applied in computing radiated power, directivity, quality factor and bandwidth of antenna. It is found that the presence of plasma medium affects the performance of RITMA structure significantly

• Subject Index of Volume 61

• Author Index of Volume 61

• # Pramana – Journal of Physics

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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019