Effect of Radial Density Profile on Resonance Absorption in Laser‐Cluster Interaction

The spatial non‐uniformities in the hydrodynamic parameters of an expanding plasma in laser‐cluster interaction plays an important role in determining the region of resonance absorption. It is shown that, in the case of uniform density, the surface resonance at three times the critical density is responsible for laser absorption. However, it is volume resonance at critical density responsible for the laser absorption, if the non‐uniform nature of plasma density inside the hydrodynamically expanding cluster is taken into account (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of electron‐absorbing boundaries on the plasma parameters of a hot‐filament discharge

In this paper we investigate theoretically and experimentally the plasma parameters in a double‐plasma device in the presence of an additional electron‐absorbing boundary. The latter is formed by an electrode of variable size immersed in the plasma. It is found that, depending on its size and bias potential, such an anode can considerably influence the plasma parameters. Good qualitative and fair quantitative agreement between theoretical predictions and laboratory measurements of the plasma parameters is found for various discharge conditions. In addition we discuss the consequences of our results with respect to the existence conditions of anode‐type double layers in double‐plasma devices.     

Characteristics of an atmospheric‐pressure radio frequency‐driven Ar/H2 plasma discharge with copper wire in tube

This paper investigates a plasma discharge driven by a 13.56 MHz radio frequency (RF) power supply at atmospheric pressure, in which a copper wire is inserted in the discharge tube for the deposition of Cu films. The results show that the jet plasma formation originates from the discharge between the copper wire and induction coil because of its electrostatic field. The axial distribution of the plasma parameters in the RF plasma jet, namely the gas temperature, excitation temperature, and electron number density, is determined by diatomic molecule OH fitting, Boltzmann slope, and H_β Stark broadening, respectively. The discharge current significantly declines when a small amount of hydrogen is added to the argon as the plasma‐forming gas, and the gas temperature of discharge plasma increases considerably.     

Collision processes of Hydride species in Hydrogen plasmas: III. The Silane family

Cross sections are provided for most important collision processes of the Silicon‐Hydrides from the “Silanefamily”: SiH_y (y = 1 ? 4) molecules and their ions SiH⁺_y, with (plasma) electrons and protons. The processes include: electron impact ionization and dissociation of SiH_y, dissociative excitation, ionization and recombination of SiH⁺_y ions with electrons, and charge ‐ and atom ‐ exchange in proton collisions with SiH_y. All important channels of dissociative processes are considered. Information is also provided on the energetics (reactants/products energy loss / gain) of each individual reaction channel. Total and partial cross sections are presented in compact analytic forms. The critical assessment of data, derivation of new data and presentation of results follow closely the concepts of the recently published related databases for Carbon‐Hydrides, namely for the Methane family [1, 2], and for the Ethane‐ and the Propane families [3], respectively. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron Temperatures and Electron Densities in Microwave Helium Discharges with Pressures Higher than 0.1 MPa

We adopted laser Thomson scattering for measuring the electron density and the electron temperature of microwave plasmas produced in helium at the pressures higher than the atmospheric pressure. The electron density decreased while we observed the increase in the electron temperature with the pressure. These are reasonable results by considering the decrease in the reduced electric field, the dominant loss of electrons via three‐body recombination with helium as the third body, and the production of electrons with medium energy via heavy particle collisions at the high gas pressure. The temporal variation of the electron temperature had the rise and the fall time constants of approximately 10 ns. The rapid heating and cooling of the electron temperature are due to the fast energy transfer from electrons to helium because of the high collision frequency in the high‐pressure discharge. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatiotemporal Evolution of Intense Short Gaussian Laser Pulses in Weakly Relativistic Magnetized Plasma

The weakly relativistic regime of propagation of a short and intense laser pulse in the magnetized plasma is investigated. By considering relativistic nonlinearity and using non‐linear Schrödinger equation with paraxial approximation, two second‐order coupled differential equations are obtained for the longitudinal pulse width parameter (in time) and for the transverse pulse width parameter (in space). The simultaneous evolution of spot size and length of a relativistic Gaussian laser pulse in a magnetized plasma can be calculated by the numerical solution of the equations. The effect of magnetic field is investigated. It is observed that in the presence of magnetic field both the self‐compression and the self‐focusing can be enhanced. Furthermore, the interplay between the longitudinal self‐compression and the transverse self‐focusing in a magnetized plasma is investigated. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear ion‐acoustic cnoidal wave in electron‐positron‐ion plasma with nonextensive electrons

Effects of plasma nonextensivity on the nonlinear cnoidal ion‐acoustic wave in unmagnetized electron‐positron‐ion plasma have been investigated theoretically. Plasma positrons are taken to be Maxwellian, while the nonextensivity distribution function was used to describe the plasma electrons. The known reductive perturbation method was employed to extract the KdV equation from the basic equations of the model. Sagdeev potential, as well as the cnoidal wave solution of the KdV equation, has been discussed in detail. We have shown that the ion‐acoustic periodic (cnoidal) wave is formed only for values of the strength of nonextensivity (q). The q allowable range is shifted by changing the positron concentration (p) and the temperature ratio of electron to positron (σ). For all of the acceptable values of q, the cnoidal ion‐acoustic wave is compressive. Results show that ion‐acoustic wave is strongly influenced by the electron nonextensivity, the positron concentration, and the temperature ratio of electron to positron. In this work, we have investigated the effects of q, p, and σ on the characteristics of the ion‐acoustic periodic (cnoidal) wave, such as the amplitude, wavelength, and frequency.     

Dust in Plasma I. Particle Size and Ion‐Neutral Collision Effects

A fully kinetic self‐consistent model of an absorbing particle immersed in stationary isotropic weakly collisional plasma has been developed. The combined effects of particle size and ion‐neutral charge exchange collisions have been investigated for intermediate regimes, where no analytic theories are available. It is shown that collisional effects related to the ion orbital destruction (presence of extrema in ion flux collected on the particle surface and in particle potential and charge) are important for small particles, while they are totally absent for large particles. The potential distribution around the particle is quite well represented by a Yukawa form, but with an effective screening length that shows different dependences from the gas pressure for small and large particle size. Analytical fitting formulas of particle charge and potential and screening length depending on the particle radius parameter and on the Knudsen number have been obtained (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self‐focusing of a cosh‐Gaussian laser beam in magnetized plasma under relativistic‐ponderomotive regime

The combined effect of relativistic and ponderomotive nonlinearities on the self‐focusing of an intense cosh‐Gaussian laser beam (CGLB) in magnetized plasma have been investigated. Higher‐order paraxial‐ray approximation has been used to set up the self‐focusing equations, where higher‐order terms in the expansion of the dielectric function and the eikonal are taken into account. The effects of various lasers and plasma parameters viz. laser intensity (a₀), decentred parameter (b), and magnetic field (ω_c) on the self‐focusing of CGLB have been explored. The results are compared with the Gaussian profile of laser beams and relativistic nonlinearity. Self‐focusing can be enhanced by optimizing and selecting the appropriate laser‐plasma parameters. It is observed that the focusing of CGLB is fast in a nonparaxial region in comparison with that of a Gaussian laser beam and in a paraxial region in magnetized plasma. In addition, strong self‐focusing of CGLB is observed at higher values of a₀, b, and ω_c. Numerical results show that CGLB can produce ultrahigh laser irradiance over distances much greater than the Rayleigh length, which can be used for various applications.     

Raman spectroscopy of melamine‐formaldehyde resin microparticles exposed to processing in complex plasma

We explore the characterization of melamine formaldehyde resin (MF‐R) micron‐sized particles, immersed in argon, neon and argon–oxygen plasmas, using Raman spectroscopy. It is shown that plasma treatment of MF‐R results in modification of its chemical composition. Particularly, a decrease in the intensities of the Raman scattering bands, corresponding to both formaldehyde C―H and melamine C―N and N―H bonds, is observed. The band at 980–990 cm⁻¹, associated with breathing vibrations of the triazine rings, undergoes the most significant changes, and the greatest modifications of the spectra are observed after exposure to Ar and Ar–O₂ plasma, whilst for the MF‐R particles exposed to Ne plasma these modifications are less pronounced. Copyright © 2016 John Wiley & Sons, Ltd.     

Ion Heating in Dusty Plasma of Noble Gas Mixtures

Ion heating in dusty plasma of noble gas mixtures is studied by the observation of dust particles in stratified glow discharge. The particles and their formations can be used as a “contact‐free” probe of the ion flows. It is shown that under condition of experiments transition of dust particles into crystalline state in pure gases occur at much lower pressures in comparison to the case of gas mixtures. This observation is also supported by the evaluation of “effective” kinetic temperature of dust particles as defined from the velocity distribution function at the same set of discharge parameters. Absolute value of temperature of dust component in the mixture of helium and argon indicates important role of argon ionization process (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Longitudinal electron waves in plasma formed at multi‐photon ionization of atoms by a short laser pulse

The dispersion law and the collisionless damping decrement of high‐frequency waves in a plasma formed as a result of multi‐photon ionization of atoms by a laser pulse are found. Estimates indicating that such waves may exist in a weakly ionized xenon plasma are given.     

Dynamics of Filament Formation in Laser Produced Collisional Magnetoplasma

The paper presents an investigation of growth dynamics of spike in a collisional magneto plasma in the presence of externally applied magnetic field. We have set up and solved the non‐linear differential equations for growth and width of the laser spikes. We have considered the mechanism of nonuniform heating of carriers along the wavefront which is important in collisional plasmas. The effect of intensity of main beam and magnetic field on growth dynamics of spikes is studied (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of the evolution of two cross‐focused Gaussian laser beams on the terahertz generation in thermal collisional plasma

Generation of the terahertz (THz) radiation based on the beating of two cross‐focused high intensity Gaussian laser beams in a warm rippled density plasma is numerically investigated, taking into account the ponderomotive force, Ohmic heating, and collisional nonlinearities. The beat ponderomotive force as a result of cross‐focusing of beams induces a vertical velocity component that by coupling with the rippled density gives rise to a nonlinear current deriving THz radiation. The effect of laser beams spot size evolution and plasma parameters on the THz generation is studied. It was found that there exist special electron temperature and laser intensity ranges with “turning points” where the generation of THz radiation reaches its maximum value and outside of these ranges, it disappears. The results also indicated that increasing the background electron density as well as taking into account the collision frequency help THz generation. Moreover, the maximum yield of THz radiation occurs when the beat wave frequency approaches the plasma frequency.     

Dust in Plasma II. Effects of Secondary Electrons: Ionization and Surface Emission

In this second paper, the effect of secondary electrons on the charge and potential of a dust particle immersed in plasma has been studied. The processes of electron‐induced ionization and those of photo‐electron and secondary electron emission from the particle surface as a function of primary electron temperature have been taken into account. Starting from temperatures as low as 6 eV in an Ar plasma, ionization produces an extra ion flux to the dust surface comparable to that of the ion charge exchange effect. For what concerns the surface emission, results show that a transition from negative to positive dust charge/potential takes place, and that the transition regime is characterized by a non‐monotonic behavior of the electric potential around the particle. In the case of photoelectric emission, the dust charge and potential are monotonic decreasing functions of the electron temperature, while in the case of emission induced by primary electrons a minimum charge/potential is reached before they grow towards positive values. In no case multiple dust charge states have been observed due to the presence of the potential well attached to the particle surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Proton Crystallization in a Dense Hydrogen Plasma

We perform a first‐principle analysis of Coulomb crystallization in neutral two‐component mass asymmetric plasmas by large‐scale path integral Monte Carlo simulations for a dense hydrogen plasma in a broad density range. We observe two large jumps in the relative distance fluctuations of protons that are connected with qualitative changes in the behavior of the proton pair distribution function and are attributed to the formation of a spatially ordered state. A third smaller jump inside the ordered phase indicates a structural transition (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ion‐Acoustic Waves Instability in a Three Components Magneto‐Plasma with Nonthermal Electrons

A theoretical investigation has been made on obliquely propagating ion‐acoustic (IA) solitary structures in a three components magneto‐plasma containing cold inertial ions, Boltzmann distributed positrons, and hot non‐thermal electrons. The Zakharov‐Kuznetsov equation has been derived by the reductive perturbation method, and its solitary wave solution has been analyzed. Multi‐dimensional instability has also studied by the small‐k (long wave‐length plane wave) perturbation expansion technique, which is found to exist in such a plasma. The effects of the external magnetic field, nonthermal electrons, obliqueness and temperature ratio have significantly modified the basic properties of small but finite‐amplitude IA solitary waves, such as amplitude, width, instability criterion and the growth rate. The present investigation contributes to the physics of the nonlinear IA waves in space and laboratory electron‐positron‐ion magneto‐plasmas in which wave damping produces an electron tail. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)