Relativistic quasi-solitons and embedded solitons with circular polarization in cold plasmas

The existence of localized electromagnetic structures is discussed in the framework of the 1-dimensional relativistic Maxwell-fluid model for a cold plasma with immobile ions. New partially localized solutions are found with a finite-difference algorithm designed to locate numerically exact solutions of the Maxwell-fluid system. These solutions are called quasi-solitons and consist of a localized electromagnetic wave trapped in a spatially extended electron plasma wave. They are organized in families characterized by the number of nodes p of the vector potential and exist in a continuous range of parameters in the ω−V plane, where V is the velocity of propagation and ω is the vector potential angular frequency. A parametric study shows that the familiar fully localized relativistic solitons are special members of the families of partially localized quasi-solitons. Soliton solution branches with p>1 are therefore parametrically embedded in the continuum of quasi-solitons. On the other hand, geometric arguments and numerical simulations indicate that p=0 solitons exist only in the limit of either small amplitude or vanishing velocity. Direct numerical simulations of the Maxwell-fluid model indicate that the p>0 quasi-solitons (and embedded solitons) are unstable and lead to wake excitation, while p=0 quasi-solitons appear stable. This helps explain the ubiquitous observation of structures that resemble p=0 solitons in numerical simulations of laser-plasma interaction