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Effect of energy pooling collisions in formation of a cesium plasma by continuous wave resonance excitation

Mahmoud M. A., Gamal Y. E. E.

Optica Applicata

2010

Vol. 40, nr 1

129--141

The generation of plasma in cesium vapor excited by cw laser is investigated theoretically. We have developed a computational model which quantitatively explains the laser power dependences of electron energy distribution function (EEDF) and the level population densities which were created during the interaction. The energy spectra of the electrons emerging from the interaction show that the mechanisms by which electrons gain energy through the resonant system can significantly increase the density of the resonant plasma. The nonlinear behaviour of the ion current suggests that the plasma formation is initiated via collisional ionization and collisional excitation, such as associative ionization, Penning ionization, or photoionization as well as energy pooling collisions.The Cs - Cs atom collisions play important role in the formation of a highly Cs plasma with the use of a relatively low power laser to excite a resonance transition. A comparison between the behaviour of the calculated ion current as a function of a laser power is proved to be a reasonable agreement with that measured by Pappas et al. (Appl. Spectrosc. 54, 2000, p. 1245). The results may be useful in designing and developing cw metal vapor lasers.

Nonlinear Instability of Two Superposed Magnetic Fluids in Porous Media Under Vertical Magnetic Fields

Elhefnawy A. R. F., Mahmoud M. A., Mahmoud M. A. A., Khedr G. M.

Mechanics and Mechanical Engineering

2007

Vol. 11, nr 1

65-85

The nonlinear analysis of the Rayleigh - Taylor instability of two immiscible, viscous magnetic fluids in porous media, is performed for two layers, each has a finite depth. The system is subjected to both vertical vibrations and normal magnetic fields. The influence of both surface tension and gravity force is taken into account. Although the motions are assumed to be irrotational in each fluid for small perturbations, weak viscous effects are included in the boundary condition of the normal stress balance. The method of multiple scale expansion is used for the investigation. The evolution of the amplitude is governed by a nonlinear Ginzburg - Landau equation which gives the criterion for modulational instability. When the viscosity and Darcy's coefficients are neglected, the cubic nonlinear Schrodinger equation is obtained. Further, it is shown that, near the marginal state, a nonlinear diffusion equation is obtained in the presence of both viscosity and Darcy's coefficients. Stability analysis and numerical simulations are used to describe linear and nonlinear stages of the interface evolution and then the stability diagrams are obtained. Regions of stability and instability are identified.