سال:1393 | دوره: | شماره: |تعداد مقالات:10

An inhomogeneous plasma comprising ions, two temperature electrons and dust grains with variable charge is explored for its unperturbed state for ions’ drift due to density gradient and perturbed state for the evolution of ion acoustic solitary waves and their reflection under the effect of an external magnetic field. The ion drift velocity is found to depend on the plasma parameters and magnetic field. The perturbed state of plasma supports two types of ion acoustic waves, which evolve into fast and slow compressive solitary structures under certain conditions.However, only the fast solitary wave is observed to be reflected and acquired opposite polarity to that of the incident solitary wave. The solitary waves are found to be downshifted after their reflection. The reflection coefficient acquires higher values in the case of dust grains of fixed charge in comparison with the case of fluctuating charge on the dust grains. It means the reflection becomes stronger when the charge on the dust grains does not fluctuate and remains fixed. The effect of dust grain density is to enhance the amplitude of solitary waves but to weaken their reflection. The amplitudes of both the incident and reflected solitons remain higher for the case of fluctuating charge on the dust grains in comparison with the case of fixed charge. The effective temperature of the plasma is also found to alter the solitary structures significantly in the case of dust grains having fluctuating charge.

The reflection of nonlinear solitary waves is studied in a plasma under the effect of an external magnetic field and constant ionization along with finite ion temperature.To investigate the reflection of solitary waves, relevant modified Korteweg–deVries equations for the right and left going waves are derived, and coupled at the point of reflection for obtaining the expression of reflection coefficient. The solitary waves are found to shift after their reflection. Variation of reflection coefficient and shift are studied for different plasma parameters like ion temperature, ionization rate and wave propagation angle or the obliqueness of magnetic field.

The full-energy peak efficiency (FEPE) curves of the (2” x 2” and 3” x 3”) NaI (Tl) detectors were measured at seven different axial positions from their surfaces. The calibration process was done using radioactive point sources, which produce a wide energy range from 59.53 up to 1, 408.01 keV. This work has been undertaken to explain the effects of source energy and sourcE-to-detector distance on the detector efficiency calculations. The study provides an empirical formula to calculate FEPE based on the efficiency transfer method for different detectors using the effective solid angle ratio at very large distances and for higher energies. A remarkable agreement between the measured and calculated efficiencies for the detectors at the sourcE-to-detector distances \35 cm and above that slight difference was observed.

A linearized relativistic field theory of a plasma-loaded helix traveling-wave tube is presented for a configuration where a solid electron beam propagate through a sheath helix enclosed within a loss-free wall in which the gap between the helix and the outer wall is filled with a dielectric. Numerical study of the effect of plasma density on the phase velocity and growth rate has been done. Numerical results show that the plasma have different behaviors in different density limits.

Langmuir probe measurements are performed in cylindrical dc glow discharge plasma. Plasma was generated in an evacuated glass tube, with circular plane disk electrodes. Measurements were carried out at different points along the axis of tube for different working pressures of pure argon and mixture of argon–oxygen gasses to obtain the plasma density and temperature as well as plasma and floating potentials. Variation of discharge potential as a function of discharge pressure for both plasmas is observed. It is shown that electron temperature, plasma potential, and floating potential in constant current mode and constant pressure are increased from cathode to anode on the axial points of the discharge tube, while electron density is decreased. To sustain the discharge process after adding oxygen to plasma, higher voltage is required since electrons are more energetic at lower density.

High-resolution confocal Raman microscopy was used to investigate the effects of nitrogen plasma on unsized high strength (HS) PAN-based carbon fiber surfaces.The fibers were treated by a high frequency (40.68 MHz) capacitively coupled single RF-PECVD reactor under different processing conditions (exposure times, RF powers and gas pressures). It was found that the order/ disorder structure of the treated carbon fiber changed with different processing conditions. At low pressures, the degree of disordered structure increased with HF–RF power and process time. However, at high pressures, highorder structure (IG=IT ¼ 84: 51%) was observed and almost no observable structural effects appeared at long treatment time. Also, the first-order Raman-band peaks (D and G) of the treated carbon fibers shifted. And, FWHM (wD=wG), intensity (ID=IG) and D-band relative integrated intensity (ID=IT) ratios increased with ordering whereas they decreased with disordering.

A formalism which enables one to strictly conserve the number of particles when taking into account the isovector pairing correlations is presented in the case of odd-mass nuclei. With this aim, we had to first establish the expression of the projector for such systems. Expressions of the ground state and its energy have been exhibited. The model has been numerically tested in the framework of a schematic model.

A theory of self-fields in a one-dimensional helical wiggler free electron laser with ion-channel guiding and axial magnetic field is presented. The steady-state orbits under the influence of self-field are derived and discussed. The U function that determines the rate of change of axial velocity with energy is derived. The numerical results show the effects of self-fields and the two electron-beams guiding devices (ion-channel and axial magnetic field) on the trajectories when used separately and simultaneously. The study shows that new unstable orbits, in the first part of the group I and II orbits, are found. A detailed stability analysis of orbits is presented.

Effects of low-intensity UV treatment on the optical constants of red BS dye-doped polymethyl methacrylate (PMMA) film including refractive index, extinction coefficient, real and imaginary parts of dielectric constant, band gap energy, Urbach energy, and refractive index dispersion parameters are measured and calculated.Changes in optical constants mainly occurred in the UV–VIS range. Red BS dye, which is used as a UV absorber impurity in PMMA films, generated another energy band gap which was increased by UV treatment. Although some of the optical constants of PMMA are not changed noticeably by the treatment in this low range of UV radiation, our results confirm that polymeric changes such as chain scission and depolymerization can directly affect the optical constants of PMMA.

The effect of the frequency variation of laser pulse in uniform magnetized plasma is considered in onedimensional laser wakefield acceleration and carried out particle simulation. It is shown that wakefield amplitude is increased threefold for the negative Gaussian chirped laser pulse in the magnetized plasma. In our simulation, electrons with initial energy about 0.3 MeV with initial energy spread about 10 % were trapped, effectively compressed in longitudinal direction and accelerated to ultra-relativistic energy about 1.3 GeV with final energy spread about 6%.