D. N. Gupta

Laser wakefield acceleration of electrons from a density-modulated plasma

This research reports the increased electron energy gain from laser wakefield acceleration in density-modulated plasma with an external magnetic field. Periodic plasma density- modulation can excite higher harmonics of different phase velocities of fundamental wakefield that can assist in improving the self-trapping of pre-accelerated electrons to accelerate them for higher energy. Furthermore, the applied magnetic field assisted self-injection can also contribute in electron energy enhancement during the acceleration. The physical mechanism is described with a theoretical formulation for this scheme. Results of two-dimensional particle-in-cell simulations are reported to understand the proposed idea.

Laser pulse distortion in a plasma of the weakly relativistic regime

We present particle-in-cell simulations to demonstrate laser pulse distortion by incorporating the role of plasma electron temperature, where the laser pulse propagates in the weakly relativistic regime. A high-intensity laser pulse gives rise to a relativistic ponderomotive force on the electrons and redistributes the plasma density, so the refractive index of the plasma changes. The electron temperature also contributes to the nonlinearity and it results in distortion of the laser pulse. As the laser pulse becomes self-focused, the front part of the pulse acquires higher group velocity compared to the tail part. As a result, the laser pulse shape changes. Our simulation shows that the plasma electron temperature plays a key role in the laser pulse distortion phenomena in the weakly relativistic regime.

Laser pulse propagation in inhomogeneous magnetoplasma channels and wakefield acceleration

Wakefield excitation in a preformed inhomogeneous parabolic plasma channel by an intense relativistic (≃1019 W/cm2) circularly polarized Gaussian laser pulse is investigated analytically and numerically in the presence of an external longitudinal magnetic field. A three dimensional envelope equation for the evolution of the laser pulse is derived, which includes the effect of the nonparaxial and applied external magnetic field. A relation for the channel radius with the laser spot size is derived and examines numerically to see the external magnetic field effect. It is observed that the channel radius depends on the applied external magnetic field. An analytical expression for the wakefield is derived and validated with the help of a two dimensional particle in cell (2D PIC) simulation code. It is shown that the electromagnetic nature of the wakes in an inhomogeneous plasma channel makes their excitation nonlocal, which results in change of fields with time and external magnetic field due to phase mixing ...

Relativistic Third-Harmonic Generation of a Laser in a Self-Sustained Magnetized Plasma Channel

Relativistic third-harmonics of a laser in a magnetized plasma channel is studied. Due to relativistic self-focusing in plasma, a high-intensity laser pulse pushes the plasma electrons radially outward to create a plasma channel. The relativistic mass effect and the magnetic field contribute to the nonlinear dielectric response of plasma and create a plasma channel due to the laser self-focusing. Self-sustained plasma channel is very important and may affect the efficiency of harmonic generation of the interacting laser beam. The velocity and density perturbation associated with the self-focused laser beam can generate a nonlinear current at triple fold frequency of the fundamental laser. Our results show the significant effect of the self-sustained magnetized plasma channel on the efficiency of third-harmonic generation of the laser beam.

Cyclotron resonance effects on electron acceleration by two lasers of different wavelengths

Cyclotron resonance effects on electron acceleration by two lasers of different wavelengths in the presence of a magnetic field have been investigated. Beating of two high-intensity lasers of different wavelengths, propagating in opposite direction to each other, can produce a high accelerating field gradient. An electron can be accelerated by such accelerating field to a sufficiently higher energy level. Additional energy gain has been observed due to the applied magnetic field. The magnetic field turns down the electrons to the acceleration region to extract more energy from the accelerating field produced by the beating of the lasers. At resonance, when the Larmor frequency is comparable to the laser frequency, this effect becomes more pronounced. Using some reasonable experimental parameters, we estimate the electron energy gain for this mechanism.

Suppression of stimulated Brillouin instability of a beat-wave of two lasers in multiple-ion-species plasmas

Stimulated Brillouin instability of a beat-wave of two lasers in plasmas with multiple-ion-species (negative-ions) was studied. The inclusion of negative-ions affects the growth of ion-acoustic wave in Brillouin scattering. Thus, the growth rate of instability is suppressed significantly by the density of negative-ions. To obey the phase-matching condition, the growth rate of the instability attains a maxima for an appropriate scattering angle (angle between the pump and scattered sideband waves). This study would be technologically important to have diagnostics in low-temperature plasmas.

Large-scale magnetic field generation by asymmetric laser-pulse interactions with a plasma in low-intensity regime

We propose a way to enhance the strength of self-generated magnetic field from laser-plasma interactions by incorporating the combined role of pulse asymmetricity and plasma inhomogeneity. The pulse asymmetry combined with the plasma inhomogeneity contributes for strong nonlinear current within the pulse body; consequently, a stronger magnetic field can be produced. The nature of self-generated magnetic field is “Quasistatic” that means the self-generated magnetic field varies on the time scale of the period of laser radiation. Our results show that the magnetic-field generated by a temporally asymmetric laser pulse is many-folds higher than the magnetic-field generated by a symmetric laser pulse in plasmas. The present study predicts the generation of magnetic field of the order of 15 T for the laser intensity of ∼ 1014 cm−2. Our study might be applicable to improve the accelerated bunch quality in laser wakefield acceleration mechanism.

Efficient second- and third-harmonic radiation generation from relativistic laser-plasma interactions

We propose an idea to enhance the efficiency of second- and third-harmonic generation by considering the amplitude-modulation of the fundamental laser pulse. A short-pulse laser of finite spot size is modeled as amplitude modulated in time. Amplitude-modulation of fundamental laser contributes in quiver velocity of the plasma electrons and produces the strong plasma-density perturbations, thereby increase in current density at second- and third-harmonic frequency. In a result, the conversion efficiency of harmonic generation increases significantly. Power conversion efficiency of harmonic generation process is the increasing function of the amplitude-modulation parameter of the fundamental laser beam. Harmonic power generated by an amplitude modulated laser is many folds higher than the power obtained in an ordinary case.

Evolution of laser pulse shape in a parabolic plasma channel

During high-intensity laser propagation in a plasma, the group velocity of a laser pulse is subjected to change with the laser intensity due to alteration in refractive index associated with the variation of the nonlinear plasma density. The pulse front sharpened while the back of the pulse broadened due to difference in the group velocity at different parts of the laser pulse. Thus the distortion in the shape of the laser pulse is expected. We present 2D particle-in-cell simulations demonstrating the controlling the shape distortion of a Gaussian laser pulse using a parabolic plasma channel. We show the results of the intensity distribution of laser pulse in a plasma with and without a plasma channel. It has been observed that the plasma channel helps in controlling the laser pulse shape distortion. The understanding of evolution of laser pulse shape may be crucial while applying the parabolic plasma channel for guiding the laser pulse in plasma based accelerators.

Space-Charge Field Assisted Electron Acceleration by Plasma Wave in Magnetic Plasma Channel

When the duration of a laser pulse is comparable to the plasma wave period, a large-amplitude plasma wave can be driven by the laser. In the presence of a guiding magnetic field, the large-amplitude plasma wave of finite transverse extent and large phase velocity resonantly accelerates the electrons to a higher energy level. For a small laser spot size, the laser exerts an axial as well as the radial ponderomotive force on the electrons that creates a density depression on the laser axis. This electron-depleted channel also creates a radial electric field (ion space-charge field). The acceleration of electrons in this channel is investigated, where the effect of ion space-charge field is considered, by solving the single particle dynamical equations. This paper shows that the space charge field plays an important role in electron energy gain during acceleration by a plasma wave in a magnetic plasma channel. This paper may be crucial in understanding the dynamics of particle motion and improving the quality of accelerated electron beam in the laser wakefield accelerators.