Suresh C. Sharma

Ion beam driven ion-acoustic waves in a plasma cylinder with negative ions

An ion beam propagating through a magnetized plasma cylinder containing K+ positive ions, electrons, and SF6− negative ions drives electrostatic ion-acoustic (IA) waves to instability via Cerenkov interaction. Two electrostatic IA wave modes in presence of K+ and SF6− ions are studied. The phase velocity of the sound wave in presence of positive and negative ions increase with the relative density of negative ions. The unstable wave frequencies and the growth rate of both the modes in presence of positive and negative ions increase with the relative density of negative ions. The growth rate of both the unstable modes in presence of SF6− and K+ ions scales as the one-third power of the beam density. Numerical calculations of the phase velocity, growth rate, and mode frequencies have been carried out for the parameters of the experiment of Song et al. [Phys. Fluids B 3, 284 (1991)].

Excitation of lower hybrid waves by a spiraling ion beam in a magnetized dusty plasma cylinder

A spiraling ion beam propagating through a magnetized dusty plasma cylinder drives electrostatic lower hybrid waves to instability via cyclotron interaction. Numerical calculations of the growth rate and unstable mode frequencies have been carried out for the Princeton Q-1 device using the experimental dusty plasma parameters [e.g., Barkan et al., Planet. Space Sci. 43, 905 (1995)]. It is found that as the density ratio δ(=nio∕neo, where ni0 is the ion plasma density and ne0 is the electron density) of negatively charged dust grains to electrons increases, the unstable mode frequency of the lower hybrid waves increases. In addition, the growth rate of the instability also increases with the density ratio δ. In other words, the presence of negatively charged dust grains can further destabilize the lower hybrid wave instability. The growth rate has the largest value for the modes where Jl(pnro) is maximum [here pn=xn∕r0, where pn is the perpendicular wave number in cm−1, r0 is the plasma radius, and xn are ...

Gain and efficiency enhancement of a Cerenkov free electron laser by a prebunched electron beam in a dielectric loaded waveguide

A density prebunched electron beam on a Cerenkov free electron laser (CFEL) offers considerable enhancement in gain and efficiency in a dielectric loaded waveguide. The growth rate, efficiency, and gain were evaluated based on experimentally known parameters relevant to the CFEL. It was found that the growth rate, efficiency, and gain of the CFEL increase with the modulation index and has the largest value when the modulation index approaches unity in addition to the frequency and wave number of the prebunched beam are comparable to that of the radiation wave, i.e., when the prebunched beam velocity is comparable to the phase velocity of the radiation wave. The growth rate of the CFEL instability scales as one third power of the beam current. Moreover, the phase velocity of the radiation wave scales as minus one half power of the effective permittivity.

Excitation of upper-hybrid waves by a gyrating relativistic electron beam in a magnetized dusty plasma cylinder

A gyrating relativistic electron beam propagating through a magnetized dusty plasma cylinder drives an upper-hybrid wave to instability via cyclotron interaction. Numerical calculations of the growth rate and unstable mode frequencies have been carried out for the typical parameters of dusty plasma experiments. It is found that as the density ratio of negatively charged dust grains to electrons increases, the unstable mode frequency of the upper-hybrid waves increases. Moreover, the growth rate of the instability also increases with the density ratio. The growth rate of the unstable mode has the largest value for the modes whose eigenfunctions peak at the location of the beam. The growth rate of the unstable mode scales as the one-third power of the beam density.

Effect of beam prebunching on gain and efficiency in a free electron laser: Nonlocal theory

Beam prebunching on the free electron laser (FEL) offers considerable enhancement in growth rate, efficiency, and gain when the prebunched beam electrons are in the retarding zone and the electron beam is highly modulated implying the largest beam oscillatory velocity due to wiggler. A nonlocal theory of this process has been developed. The growth rate, efficiency, and gain were evaluated based on existing experimental parameters and compared with available theoretical/experimental results. Nonlocal effects reduce the linear growth rate of the FEL instability. The growth rate, efficiency, and gain of the FEL instability increase with the modulation index and has the largest value when the modulation index approaching unity in addition to the frequency and wave number of the prebunched beam are comparable to that of the radiation wave. The growth rate of the FEL instability scales as one-third power of the beam current in the Compton regime and one-fourth power in the Raman regime.

Modeling carbon nanotube growth on the catalyst-substrate surface subjected to reactive plasma

The paper presents a theoretical model to study the growth of the carbon nanotube (CNT) on the catalyst substrate surface subjected to reactive plasma. The charging rate of the CNT, kinetics of electron, ions and neutral atoms, the growth rate of the CNT because of diffusion and accretion of ions on the catalyst nanoparticle inclusion of the issue of the plasma sheath is undertaken in the present model. Numerical calculations on the effect of ion density and temperature and the substrate bias on the growth of the CNT have been carried out for typical glow discharge plasma parameters. It is found that the height of CNT increases with the ion density of carbon ions and radius of CNT decreases with hydrogen ion density. The substrate bias also affects the growth rate of the CNT. The field emission characteristics from the CNTs can be analyzed from the results obtained.

Theoretical modeling of the plasma-assisted catalytic growth and field emission properties of graphene sheet

A theoretical modeling for the catalyst-assisted growth of graphene sheet in the presence of plasma has been investigated. It is observed that the plasma parameters can strongly affect the growth and field emission properties of graphene sheet. The model developed accounts for the charging rate of the graphene sheet; number density of electrons, ions, and neutral atoms; various elementary processes on the surface of the catalyst nanoparticle; surface diffusion and accretion of ions; and formation of carbon-clusters and large graphene islands. In our investigation, it is found that the thickness of the graphene sheet decreases with the plasma parameters, number density of hydrogen ions and RF power, and consequently, the field emission of electrons from the graphene sheet surface increases. The time evolution of the height of graphene sheet with ion density and sticking coefficient of carbon species has also been examined. Some of our theoretical results are in compliance with the experimental observations.

Effect of dust on an amplitude modulated electromagnetic beam in a plasma

A large amplitude modulated Gaussian electromagnetic beam propagating in a dusty plasma has been studied. The electrons are heated nonuniformly by the beam. For the nonsteady state, we obtain nonlinear current density in the presence of dust grains. This expression has been used to study the nonstationary self-focusing and resulting self-distortion of the amplitude modulated electromagnetic beam. It has been observed that the effect of dust is drastic on the modulation index in comparison to dust free plasma. A simple scaling showing the dependence of modulation index on dust grain size and charge has been presented.

The effect of dust charge fluctuations on lower-hybrid suppression of drift waves in a magnetized plasma cylinder

The nonlinear coupling between drift waves and a lower-hybrid pump wave is studied in a magnetized dusty plasma cylinder. The growth rate and mode frequencies were evaluated based on typical dusty plasma parameters. It is found that the unstable drift mode frequency increases and the growth rate decreases sharply with the relative density of negatively charged dust grains. In addition, the growth rate and unstable mode frequency depend on pump wave amplitudes.

The effect of beam pre-bunching on the excitation of terahertz plasmons in a parallel plane guiding system

The excitation of terahertz (THz) plasmons by a pre-bunched relativistic electron beam propagating in a parallel plane semiconducting guiding system is studied. It is found that the n-InSb semiconductor strongly supports the confined surface plasmons in the terahertz frequency range. The growth rate and efficiency of the THz surface plasmons increase linearly with modulation index and show the largest value as modulation index approaches unity. Moreover, the growth rate of the instability scales as one-third power of the beam density and inverse one-third power of the THz radiation frequency.