Pawan Kumar

A surface plasmon laser

We examine the possibility of stimulated emission of a surface plasma wave (SPW) on a metal-vacuum interface by electron-hole recombination in a forward biased p-n junction located near the interface. We consider a thin layer of n-type semiconductor sandwiched between a metal and a p-type semiconductor, and the p-n junction is forward biased. The mode structure of the SPW, propagating along the metal surface, extends up to the p-n junction, where it induces electron-hole recombination and gets amplified. The optical gain of the SPW laser can be made comparable to that of a diode laser by reducing the thickness of the sandwich layer.

A two-layer model for studying the effect of plasma layer on the delivery of oxygen to tissue using a finite element method

A mathematical model is described for the transport of oxygen in the systemic capillaries and the surrounding tissue. The model takes into account the molecular diffusion, the convective effect of the blood, the nonlinear effects of oxyhaemoglobin, and the consumption of oxygen in the metabolic process. A two-layer model for the blood consisting of a core of erythrocytes surrounded by a cell-free plasma layer has been considered. A finite element formulation has been given to solve the resulting nonlinear convective-diffusion equations with the physiologically relevant boundary conditions. A fixed point iterative technique is used for the nonlinear terms. It is found that PO2 (partial pressure of oxygen) in the tissue increases as the core-to-capillary diameter ratio increases. The tissue PO2 is found to be lower with a heterogeneous model in comparison with a homogeneous model, and thus the analysis shows that the plasma layer obstructs the transport of oxygen from the blood to the tissue. The effect of capillary diameters and core radii on the delivery of oxygen to tissue has also been examined.

Parametric conversion of a lower hybrid wave into a whistler in a plasma

A large amplitude lower hybrid wave parametrically decays into a whistler wave and a low frequency lower hybrid wave in a plasma. The density perturbation due to the low frequency wave couples with the oscillatory velocity due to the pump wave to produce a nonlinear current, driving the whistler. The pump and whistler exert a low frequency ponderomotive force on electrons, driving the lower hybrid decay wave. The growth rate of the parametric instability scales linearly with the amplitude of the pump wave. It decreases with the electron cyclotron frequency. The process is relevant to beam plasma systems where lower hybrid waves are excited with greater ease and the whistler sideband wave can be seen outside the plasma as electromagnetic emission.

Excitation of terahertz plasmons eigenmode of a parallel plane guiding system by an electron beam

Two parallel semiconductor plates, separated by a short distance, support surface plasmon eigenmode with amplitude maxima at the inner surfaces of the plates and minimum at the center. A relativistic sheet electron beam propagating through the space between the planes resonantly excites the surface plasma wave (SPW). The frequency of the driven SPW decreases with the energy of the beam while the growth rate increases. At the beam current ≈168 A the growth rate of 5.93×108u2002rad/s is achieved at the frequency ≈0.51 THz of SPW for the 5 mm y width and spacing between the two plates of ≈2.83 mm. The growth rate scales as 1/3 root of the electron beam current.

Terahertz surface plasmon excitation via nonlinear mixing of lasers in a metal-coated optical fiber.

Terahertz (THz) surface plasmon generation via nonlinear mixing of laser eigenmodes in an optical fiber coated with ultrathin metal foil and possessing a dielectric constant ripple of wave number q is investigated. The fiber supports THz surface plasma wave (SPW) with plasmon resonance in the THz domain and is controllable by film thickness. The lasers exert a difference frequency ponderomotive force on the electrons of the metal film. The ponderomotive force induces a nonlinear current driving the difference frequency THz SPW. THz amplitude to laser amplitude ratio of the order ∼10(-2) can be achieved by CO2 laser in megawatt range.

Lower hybrid instability driven by mono-energy α-particles with finite pitch angle spread in a plasma

A kinetic formalism of lower hybrid wave instability, driven by mono-energy α-particles with finite pitch angle spread, is developed. The instability arises through cyclotron resonance interaction with high cyclotron harmonics of α-particles. The α-particles produced in D-T fusion reactions have huge Larmor radii (∼10u2009cm) as compared to the wavelength of the lower hybrid wave, whereas their speed is an order of magnitude smaller than the speed of light in vacuum. As a result, large parallel phase velocity lower hybrid waves, suitable for current drive in tokamak, are driven unstable via coupling to high cyclotron harmonics. The growth rate decreases with increase in pitch angle spread of the beam. At typical electron density of ∼1019u2009m−3, magnetic field ∼4 Tesla and α-particle concentration ∼0.1%, the large parallel phase velocity lower hybrid wave grows on the time scale of 20 ion cyclotron periods. The growth rate decreases with plasma density.

Step density model of laser sustained ion channel and Coulomb explosion

An analytical model of laser sustained ion channel in plasma is developed, assuming electron density to be zero in the inner region and constant outside. The radius of the channel is such that the ponderomotive force on electrons at the channel boundary is balanced by the channel space charge force. The laser is TM eigen mode of the system with Bessel function profile in the interior and modified Bessel function outside. The channel radius increases with laser intensity and the ratio of laser frequency to plasma frequency. Ion Coulomb explosion of the channel, on longer time scale, produces ion energy distribution, an increasing function of energy with a sharp cutoff equal to electron ponderomotive energy at the channel boundary. At peak laser intensity ≈2×1019W/cm2 at 1u2009μm wavelength and spot size of 8u2009μm, the cutoff ion energy in a plasma of density ∼1019cm−3 is ∼0.73u2009MeV.

Electron energy spectrum in circularly polarized laser irradiated overdense plasma

A circularly polarized laser normally impinged on an overdense plasma thin foil target is shown to accelerate the electrons in the skin layer towards the rear, converting the quiver energy into streaming energy exactly if one ignores the space charge field. The energy distribution of electrons is close to Maxwellian with an upper cutoff emax=mc2[(1+a02)1/2−1], where a02=(1+(2ω2/ωp2)|ain|2)2−1, |ain| is the normalized amplitude of the incident laser of frequency ω, and ωp is the plasma frequency. The energetic electrons create an electrostatic sheath at the rear and cause target normal sheath acceleration of protons. The energy gain by the accelerated ions is of the order of emax.

Fast wave stabilization/destabilization of drift waves in a plasma

Four wave-nonlinear coupling of a large amplitude whistler with low frequency drift wave and whistler wave sidebands is examined. The pump and whistler sidebands exert a low frequency ponderomotive force on electrons introducing a frequency shift in the drift wave. For whistler pump propagating along the ambient magnetic field Bsz with wave number k→0, drift waves of wave number k→=k→⊥+k||z see an upward frequency shift when k⊥2/k02>4k||/k0 and are stabilized once the whistler power exceeds a threshold value. The drift waves of low transverse wavelength tend to be destabilized by the nonlinear coupling. Oblique propagating whistler pump with transverse wave vector parallel to k→⊥ is also effective but with reduced effectiveness.

Stimulated Raman up conversion of a helicon by band-gap energy plasmons in a semiconductor

Stimulated Raman up conversion of a helicon wave in a semiconductor, where free carrier plasma frequency equals band-gap energy divided by Planck’s constant, is investigated. The stimulated electron hole recombination drives a Langmuir wave. The free carrier density oscillations associated with the Langmuir wave couple with the free carrier oscillatory velocities due to the helicon and derive a sum frequency radiation. The radiation and helicon exert a ponderomotive force on free carriers that influences the Langmuir wave.