Spontaneous formation of magnetic dipoles by interaction of intense laser with overdense plasma

When a laser field is incident on an overdense plasma it is unable to penetrate inside it. Nevertheless, a part of its energy gets transferred to the electrons through a variety of mechanisms (e.g. vacuum and $\vec{J}\times \vec{B}) heating [1, 2]). The dynamics of these energetic electrons inside the plasma is a field of great interest. It is demonstrated here using 2-D PIC (Particle-In-Cell) simulation that when a high intensity laser pulse is incident on an overdense target, energetic electrons get generated which spontaneously organize themselves to form coherent structures. These coherent structures are observed to have similar dynamical traits as displayed by the solutions of the EMHD (Electron Magnetohydrodynamic) model [3]. This is noteworthy that EMHD is an approximate model and is applicable for the dynamics of non-relativistic electrons. However, in these simulations, the electron energy is in the relativistic regime as the laser intensity is significantly high. Thus, it shows that the relativistic dynamics also permits the existence of robust coherent structures. Interesting kinetic behaviour at particle level is observed in the simulation which shows that the coherent structures take background electrons in its fold and subsequently emit them at a higher energy.