Asymmetric laser-pulse based magnetic field enhancement in a plasma

Abstract

Self-modulation of a laser pulse in a plasma takes place because of the nonlinear properties of the medium, which leads to laser pulse asymmetricity. Magnetic-field generation is an important feature of the interaction of ultra-intense pulses with plasmas. Effect of Asymmetric laser pulse on the magnetic field generation is investigated in this work. One may expect a strong magnetic field from laser-plasma interactions, where a large-amplitude Langmuir wave is produced via linear mode conversion and sharp density gradients. When the plasma density has a gradient perpendicular to the ponderomotive force giving rise to an irrotational current density, a quasistatic magnetic field is generated. This mechanism is important only when laser intensity variation is faster than the ion sound time scale, otherwise ions and electrons move together via ambipolar diffusion and drift velocity would be small. An analytical expression of the magnetic field generated by an asymmetric laser pulse is put forwarded for the problem and validated by particle-in-cell simulations. It is shown that the asymmetricity of the pulse influences the magnetic field generation during plasma interactions and greatly enhanced the strength of this magnetic field. This work may be crucial to understand the physics of plasma based accelerations, where the shape of the pulse seriously affects the particle energy gain during accelerations.