Devki Nandan Gupta

COMBINED ROLE OF FREQUENCY VARIATION AND MAGNETIC FIELD ON LASER ELECTRON ACCELERATION

Laser-induced acceleration of an electron injected initially at an angle to the direction of a short laser pulse with frequency variation in the presence of an axial static magnetic field has been investigated. Due to the combined effect of frequency variation of the laser and a magnetic field, the electron escapes from the laser pulse near the pulse peak. The electron gains considerable energy and retains it even after passing of the laser pulse in the presence of magnetic field in vacuum. The frequency variation plays an important role to enhance the electron energy in the presence of a static magnetic field in vacuum.

Additional focusing of a high-intensity laser beam in a plasma with a density ramp and a magnetic field

Propagation of a high power Gaussian laser beam through a plasma with a density ramp where a magnetic field is present has been investigated. The spot size of the laser beam decreases as the beam penetrates into the plasma due to the role of a plasma density ramp. The studies show that the combined effect of a plasma density ramp and a magnetic field enhances the self-focusing property of the laser beam. Both factors not only reduce the spot size of the laser beam but also maintain it with only a mild ripple over several Rayleight lengths.

Frequency chirping for resonance-enhanced electron energy during laser acceleration

The model given by Singh-Tripathi [Phys. Plasmas 11, 743 (2004)] for laser electron acceleration in a magnetic wiggler is revisited by including the effect of laser frequency chirping. Laser frequency chirp helps to maintain the resonance condition longer, which increases the electron energy gain. A significant enhancement in electron energy gain during laser acceleration is observed.

Enhanced thermal self-focusing of a Gaussian laser beam in a collisionless plasma

Theory given by Wang-Zhou [Phys. Plasmas 18, 043101 (2011)] for the thermal self-focusing of a Gaussian laser beam in a collisionless plasma is revisited by including the effect of a localized upward plasma-density ramp. As the equilibrium electron density is an increasing function of the distance of propagation of the laser beam, the diffraction length decreases rapidly as the beam penetrates deeper into the plasma and the diffraction effect becomes reduced; thus, the laser becomes more focused. A significant enhancement in laser thermal self-focusing in a collisionless plasma is consequently observed if a localized plasma density ramp is introduced.

Energy exchange during stimulated Raman scattering of a relativistic laser in a plasma

Energy exchange between pump and daughter waves during the stimulated Raman scattering process in a plasma is investigated, including the effect of a damping coefficient of electron-ion collision at different initial three-wave phases. To obey the energy and momentum conservations, the resonance conditions are satisfied at an optimal initial phase difference between the interacting waves. The amplitudes of the interacting waves exhibit behaviors such as a parametric oscillator. The variations in initial three-wave phase difference generate a phase mismatch, which enhances the rate of the amplitude variations of the interacting waves. The relativistic mass effect modifies the dispersion relations of the interacting waves, and consequently the energy exchange during the stimulated Raman scattering is affected. The collisional damping in the plasma is shown to have an important effect on the evolution of the interacting waves.

Energetic electron beam generation by laser-plasma interaction and its application for neutron production

Acceleration of electrons in the laser and magnetic field in a plasma can lead to the generation of an energetic electron beam. Both axial and azimuthal static magnetic fields play an important role to enhance the electron energy and to collimate the accelerated electrons. If the generated energetic electrons are targeted to a high-Z solid, backed with a sample of uranium-238, a significantly large number of neutrons can be produced by photonuclear reaction initiated by the Bremsstrahlung process. The efficiency of this process is found to be considerably higher than that of the spallation neutron source. The neutron source based on this process can be used as a driver for a subcritical fission reactor.

Resonant third-harmonic generation of a short-pulse laser from electron-hole plasmas

In semiconductors, free carriers are created in pairs in inter-band transitions and consist of an electron and its corresponding hole. At very high carrier densities, carrier-carrier collisions dominate over carrier-lattice collisions and carriers begin to behave collectively to form plasma. Here, we apply a short-pulse laser to generate third-harmonic radiation from a semiconductor plasma (electron-hole plasma) in the presence of a transverse wiggler magnetic-field. The process of third-harmonic generation of an intense short-pulse laser is resonantly enhanced by the magnetic wiggler, i.e., wiggler magnetic field provides the necessary momentum to third-harmonic photons. In addition, a high-power laser radiation, propagating through a semiconductor imparts an oscillatory velocity to the electrons and exerts a ponderomotive force on electrons at the third-harmonic frequency of the laser. This oscillatory velocity produces a third-harmonic longitudinal current. And due to the beating of the longitudinal el...

Enhanced focusing of laser beams in semiconductor plasmas

The beating of two copropagating laser beams (having frequency difference Δω≈ωp, where ωp is the plasma frequency) can resonantly excite a large amplitude plasma wave in a narrow-gap semiconductor [V. I. Berezhiani and S. M. Mahajan, Phys. Rev. B 55, 9247 (1997)]. The higher ponderomotive force on the electrons due to the plasma beat wave makes the medium highly nonlinear. As a result, the incident laser beams become self-focused due to the nonlinearity by the ponderomotive force. In this paper, we show the self-focusing and spot size evolution of the laser beams in semiconductor plasmas.

Electron acceleration and electron-positron pair production by laser in tunnel ionized inhomogeneous plasma

A high intensity laser short pulse causes rapid tunnel ionization of an inhomogeneous gas. The tunnel ionization of the gas causes a defocusing of the laser pulse. The electron experiences an unequal ponderomotive force due to the trailing and rising part of the laser pulse, hence, gains net energy. The net acquired electron energy is reduced due to the inhomogeneity in gas density. If the accelerated electrons are targeted to a low-Z material nucleus, the electron-positron pair will be created via a trident process.

Pulse width effects on Raman backward laser amplification

The effects of the longitudinal pulse width were studied in Raman backward amplification (RBA) of laser pulses in plasmas. In RBA systems, which utilize Raman backscatter in plasmas, it is generally required that a long pump laser pulse, whose longitudinal pulse duration is typically tens of picoseconds, propagates more than a few millimetres of gas jet or capillary plasmas. However, from the experimental point of view, a short propagation distance of the pump pulse is more desirable to avoid various technical difficulties and physical instabilities. In this paper, the change in the characteristics of RBA systems for varying widths of the pump pulse is studied by fluid and kinetic simulations. The effects of the initial width of the seed pulse are also addressed.