We will address several problems in astrophysical plasma and in laboratory plasma. Astrophysical and Space plasma is usually collisionless because its particle number density is low. It can be as low as a few tens of particles per cubed centimetre in the Solar wind near the Earth. The mean-free path of particles in this plasma is comparable to the distance Sun-Earth. The Earth's bow shock has, however, a thickness of just a few tens of kilometres. It is evident that this shock is not mediated by binary collisions between particles. Laboratory plasma is collisionless if it is hot and moderately dense like in the case of a plasma that is created when a solid target is ablated by a laser pulse with a ultra-high intensity. In such a plasma, the particle's kinetic energy is large compared to variations of its energy in the electric potential of neighboring particles; hence binary interactions are unimportant. We will pursue two projects:
Discontinuities in energetic astrophysical plasma: Collisionless discontinuities have not yet been covered much in spite of their potential relevance for magnetic field generation and radiation generation. We have recently found a discontinuity that develops between an electron-positron pair plasma (jet material) with a high thermal pressure and an ambient magnetized electron-proton plasma. The expanding pair plasma piles up the ambient magnetic field ahead of it. Eventually it becomes strong enough to expel ambient electrons. Their current generates an electric field that accelerates the protons and pushes them out of the way of the jet material. Such discontinuities form whenever macroscopic pair clouds encounter a magnetized electron-ion plasma. We expect that this is the case in the coronal plasma surrounding the accretion discs of black holes. These discontinuities are characterized by strong magnetic fields that are in contact with relativistically hot electrons and positrons. Their synchrotron emission could contribute to the radio emissions of the relativistic jets that leave this plasma. We will continue to examine this discontinuity and its stability by means of PIC simulations. We will examine its stability by means of parametric studies in 1D that vary the plasma temperature and magnetic field strength. We will also examine effects due to a mix of different ions. We will examine plasma configurations, which lead to interesting results in 1D, with 2D and possibly 3D simulations.
We will examine with PIC simulations collisions between two electrostatic electron-ion shocks. Such collisions can be achieved in the laboratory by ablating two separate targets with ultra-intense laser pulses. We will set up simulations, in which two circular plasma clouds with a large density expand into a dilute ambient plasma. The distance between the cloud centers will be set such that the expanding front has turned into a shock before both clouds collide. Our simulations will reveal structures that form when the shocks collide. We will compare our results to experimental ones.