H. D. Pandey

Second-harmonic generation of laser radiation in a plasma with a density ripple

High-power laser radiation with frequency omega /sub 1/ and wave vector k/sub 1/z propagating through a plasma at an angle to a density ripple (0,k/sub 0/) produces current and density perturbation at ( omega /sub 1/,k/sub 1/+k/sub 0/). The density perturbation couples with the oscillatory velocity at ( omega /sub 1/,k/sub 1/) to produce nonlinear current at (2 omega /sub 1/, (2k/sub 1/+k/sub 0/)) driving second-harmonic electromagnetic radiation. For a specific value of ripple wave number k/sub 0/=k/sub 0c/, the phase-matching conditions for the second-harmonic process are satisfied, leading to resonant enhancement in the efficiency of energy conversion, k/sub 0c/ decreases with the frequency omega /sub 1/ of the laser. >

Two-dimensional effects in a tunnel ionized plasma

An intense short pulse laser of finite spot size propagating through a gas produces plasma via tunnel ionization on a femtosecond time scale. The radial profile of plasma density is strongly peaked on the axis and has a defocusing property. As electron density grows with time, the trailing part of the laser pulse suffers stronger divergence than the leading front, causing severe temporal distortion of the pulse. A self-consistent paraxial ray theory of electron density evolution and defocusing of the laser reveals that a square (in time) laser pulse, after propagating one Rayleigh length, has an order of magnitude difference in the axial intensity at the front and the tail of the pulse.

Feed optimization for the slotted line antenna for high-density plasma production

Investigations on the optimization of feed structures for exciting the slotted line antenna for high-density plasma production are presented. Each feed structure used (except the direct feed) excites a preferred component of the wave electric/magnetic field. It is seen that the efficacy of plasma production using the different feeds depends directly on the relative importance of the field components (which the feeds excite) for the flow wave mode of the antenna. The optimal feed is shown to be a dipole antenna, oriented so as to excite the radial component of the electric field within the slotted line structure. The plasma characterization results as a function of the input microwave power and the magnetic field in the antenna region are also presented and discussed. The ability of the antenna to maintain high-density plasmas well away from electron-cyclotron resonance is demonstrated. >

Possibility of pulse compression of a short-pulse laser in a plasma

On account of nonlinear refraction, arising through the relativistic mass effect, a short intense laser pulse tends to accumulate its energy around the intensity maximums leading to pulse compression over a length Z c T 0 C 2 ω 2 /ω p 2 v, where τ 0 and ω are the pulse duration and frequency of the laser, v is the oscillatory electron velocity and ω p the plasma frequency. When the transverse extent of the laser is finite, nonlinear self-focusing interferes strongly with this process. The self-focusing occurs in a periodic manner on a shorter scale length. However, over long lengths of pulse propagation, pulse compression could be significant.

Relativistic cross-focusing of two coaxial Gaussian laser beams in a plasma

The paper presents a paraxial theory of the relativistic cross-focusing of two coaxial Gaussian laser beams of different frequencies in a homogeneous plasma. We discuss the self-focusing of a weaker laser beam in the plasma due to the optical inhomogeneities introduced by another stronger copropagating laser beam. In the presence of the second stronger beam (P cr21 < P 2 < P cr22 ), the plasma behaves as an oscillatory waveguide for the first, weaker, beam (P 1 < P cr11 ) as it propagates in the plasma. When both the beams are strong (P cr11,21 < P 1,2 < P cr12,22 ), the nonlinearities introduced by the relativistic effect are additive in nature, such that one beam can undergo oscillatory self-focusing and the other simultaneously defocusing, and vice versa. A comparison reveals that cross-focusing due to relativistic nonlinearity is possible for a wider range of powers of the laser pulses than is cross-focusing due to ponderomotive nonlinearity. Relativistic cross-focusing is important in plasma beat-wave excitation and collective laser particle accelerators.

Beat-wave excitation of electron plasma wave by cross-focusing of two intense laser beams

This paper presents the cross-focusing of two intense laser beams in a collisionless plasma, taking into account the relativistic non-linearity. The non-linearity is not bound to large irradiances and this non-linearity is only a perturbation. It should be noted here that while considering the self-focusing due to relativistic electron mass variation, the electron ponderomotive density depression in the channel may also be important. Therefore, these two non-linearities may simultaneously affect the self-focusing process. In the present paper we have considered the situation when only relativistic non-linearity is important. The non-linearity due to relativistic mass variation depends not only on the intensity of one laser but also on the second laser. Therefore, one laser beam affects the dynamics of the second beam and hence a cross-focusing process takes place. The electric field amplitude of the excited electron plasma wave (EPW) has been calculated and its effect on the cross-focusing process has also been discussed. It is observed that the inclusion of a resonantly excited EPW on cross-focusing is significant and the accelerating electric field of the generated EPW becomes affected. A comparison of the theory with the recent experimental observations has also been presented.

Laser excitation of surface waves over a dense plasma

An intense short laser pulse incident on a metallic target produces an overdense plasma and excites a plasma surface wave via stimulated Compton scattering. The pump and the surface wave exert a beat-frequency ponderomotive force on the electrons in the skin layer, driving a heavily damped quasimode. The density perturbations due to the quasimode couple with the oscillatory velocity due to the pump to drive the surface wave. The growth period for the process turns out to be about 0.5 ps at a Nd : glass laser intensity of 10 16 W cm -2 and a plasma density of 1.5 times the critical value.

Electromagnetic-wave-pumped free-electron laser in a plasma channel

An intense short laser pulse or a millimetre wave propagating through a plasma channel may act as a wiggler for the generation of shorter wavelengths. When a relativistic electron beam is launched into the channel from the opposite direction, the laser radiation is Compton/Raman backscattered to produce coherent radiation at shorter wavelengths. The scheme, however, requires a superior beam quality with energy spread less than 1% in the Raman regime.

Mode conversion of laser radiation in the presense of a magnetic wiggler

Laser radiation E = yA(z)e -iωt propagating through a non-uniform plasma along the direction of the density gradient suffers total reflection at the critical layer. However, when a wiggler magnetic field B ω = xB ω e ikωz exists near the critical layer, the laser drives a Langmuir wave. For suitable values of B ω and k ω , the power transfer from the laser to the Langmuir wave could be as high as 60%. The Langmuir wave deposits its energy on the electrons via Landau damping. This may be an efficient mechanism of laser absorption when large self-generated magnetic fields exist in the plasma.