Non‐linear interaction of a q‐Gaussian laser beam in a plasma channel created by the ignitor‐heater technique

Abstract

This paper presents a theoretical investigation of the propagation characteristics of a q-Gaussian laser beam propagating through a plasma channel created by the ignitor-heater technique. The ignitor beam creates the plasma by tunnel-ionization of air. The heater beam heats the plasma electrons and establishes a parabolic channel. The third beam (q-Gaussian beam) is guided in the plasma channel under the combined effects of density non-uniformity and non-uniform ohmic heating of the plasma channel. Numerical solutions of the non-linear Schrodinger wave equation (NSWE) for the fields of laser beams are obtained with the help of the moment theory approach. Particular emphasis is placed on the dynamical variations of the spot size of the laser beams and the longitudinal phase shift of the guided beam with the distance of propagation. KEYWORDSoptical guiding, phase shift, plasma channel, q-Gaussian, self-focusing INTRODUCTIONWith the rapid development of laser technology, fueled by the advent of the chirped pulse amplification (CPA) technique, [1] ultra-intense lasers have become available, leading to a continuously extending research interest on laser-matter interactions. The laser intensity is always a key factor that decides the physical phenomena and intrinsic schemes. Currently, since the enhancement of laser power has gotten into a bottleneck at the order of 10 PW, secondary optical modulation and tight focusing of the laser beams have become important approaches to achieve higher power densities. However, the laser intensities and spot sizes coming from conventional solid-state optics are limited by the damage threshold and the diffraction effects, respectively. To overcome these limitations, pre-formed plasma channels have been proposed as a promising scheme, which can support high intensities and compensate diffraction. [2,3] As with an optical fibre, a plasma channel can provide optical guidance if the index of refraction peaks on the axis, which can be achieved with a plasma density profile that has a local minimum on the axis. To guide highly intense laser beams, plasma channels must be produced in deeply ionized gases, where the density profile cannot be changed by the guided beam through further ionization. An increase in the density on the axis would lead to ionization-induced refraction and hence negate the guiding. To meet these requirements, Volfbeyn et al. [3] developed a novel technique known as the ignitor-heater technique to create plasma channels for the long-range propagation of laser beams. The physics of guiding the laser beam is as follows: the ignitor beam creates a plasma by tunnel-ionization of an ambient gas. The heater beam heats the plasma and thereby creates a concave parabolic electron density profile so that the plasma density becomes minimum on the axis compared to that at the edges of the channel where the density is maximum. Therefore, the refractive index becomes a maximum on the axis and decreases towards the edges.When a third laser beam is passed through the plasma channel, ...