A plasma is an ionized gas. The presence of mobile charges allows it to interact with electromagnetic fields. Plasma clouds, which flow at a high speed relative to a cloud of background plasma, are observed in a wide range of space-, astrophysical- and laboratory environments. Interactions between colliding plasma clouds give rise to structures and to the emission of energetic electromagnetic radiation.
Some structures form on time scales that are short compared to the time interval between particle collisions and they are thus sustained by electromagnetic forces rather than by binary collisions. We call such structures collisionless. Particle-in-cell (PIC) simulation codes can capture the evolution of collisionless plasma structures.
I will examine with PIC simulations collisionless shocks in electron-ion plasma that resemble those predicted by (collisional) magneto-hydrodynamic (MHD) models. Remarkably, MHD models describe well some of the collisionless shocks found in the collisionless plasma of the Solar wind, in the plasma close to planets and at the boundary of the heliosphere.
Our recent PIC simulations have found collisionless shocks, which have the same structure as slow and fast magnetosonic (MHD) shocks. The parameters we used to set these shocks up are realistic for energetic astrophysical plasma (e.g. the blast shells ejected by supernova explosions) and for the blast shells that are created when an energetic laser pulse ablates a solid target.
I will continue to study with PIC simulations collisionless shocks in magnetized plasma and compare my results to the findings of my collaborating group (Prof. Marco Borghesi, Dr Gianluca Sarri) at the Queen's University Belfast, which specializes on laser-generated plasma and on its interactions with a surrounding plasma. I will examine shocks, which propagate at various angles relative to a background magnetic field and at speeds that are larger than those we modeled before. My aim is to determine the speed, at which the shocks start to behave differently from MHD-type shocks.
I will also examine in more detail the interaction of a cloud of electrons and positrons with an electron-proton plasma. The presence of two positive charge carriers with different masses introduces novel physics.
I and my collaborating group at Belfast have recently succeeded in detecting the first current filamentation instability between a pair plasma and a background plasma in the laboratory. We have published our results in the Physical Review Letters. My PIC simulation showed that the number of electrons and positrons, which could be generated with the available laser energy, was just enough to drive this instability. I have started to perform simulations with the pair number expected for forthcoming lasers. Several structures emerged in these simulations. Examples are magnetic discontinuities between the pair plasma and the electron-proton plasma, the emergence of solitary ion acoustic waves and of magnetized jets. I will continue to study these structures, which are relevant for jet formation and particle acceleration by jets ejected by accreting black holes and can probably be observed in forthcoming laser-plasma experiments.