Title:	Collisionless shocks and discontinuities in electron-ion-positron plasma
DNr:	SNIC 2019/3-413
Project Type:	SNIC Medium Compute
Principal Investigator:	Mark Eric Dieckmann <Mark.E.Dieckmann@liu.se>
Affiliation:	Linköpings universitet
Duration:	2019-10-01 – 2020-10-01
Classification:	10303 10305
Homepage:	http://www.itn.liu.se/~mardi06
Keywords:

Abstract

A plasma is an ionized gas. Plasma particles can interact with each other via their Coulomb fields (binary interactions) or through the electromagnetic fields, which are generated by the current of all particles (collective interactions). We call a plasma collisionless if collective interactions dominate over binary ones. Currents due to the collective motion of particles are strong and they can affect the properties of electromagnetic waves in plasma. A plasma thus supports wave modes and structures that do not exist in vacuum. Their description is difficult in particular if they are nonlinear. Particle-in-cell (PIC) simulation codes can capture the physics of collisionless plasma and resolve all of its waves and plasma structures even in the nonlinear regime. Plasma clouds, which stream relative to a background plasma or collide with other plasma clouds, are observed in a wide range of space-, astrophysical- and laboratory environments. If the interacting clouds are composed of collisionless plasma then their interaction gives rise to waves and nonlinear structures and to the emission of electromagnetic radiation. We will examine with PIC simulations the instabilities and nonlinear structures that emerge when fast clouds of electrons and positrons interact with a plasma, which consists of electrons and protons. Their study is important for two reasons. The lasers based at the recently inaugurated Extreme Light Infrastructure (ELI) in Prague have enough power and energy to create electron-positron clouds that contain so many particles that they start to behave like a pair plasma. These pair clouds have relativistic mean speeds and their interaction with an electron-ion plasma can give rise to plasma instabilities and nonlinear structures that have so far remained unexplored. We will perform simulations that model the interaction of plasma with pair clouds that have a size that is similar to the one that can be created in laboratory experiments. Our simulations will provide theoretical support for forthcoming experiments of my collaborators. We have recently found a novel plasma structure that develops if a large and dense pair cloud interacts with an electron-proton plasma (M.E. Dieckmann et al., Astronomy and Astrophysics, 621, A142, 2019). It is a magnetic boundary that can separate a hot pair cloud from an electron-proton plasma. Its magnetic pressure is comparable to the pressure of the pair cloud. This magnetic discontinuity could form the boundary that separates the relativistic jets of electrons and positrons, which are emitted by the accreting black holes known as (micro-)quasars, from the ambient medium (the stellar wind of a companion star or interstellar medium). Its strong magnetic fields are in contact with the relativistically hot particles of the pair cloud and their radio-synchrotron emissions could contribute to the observed electromagnetic emissions of astrophysical jets. We will continue to study with PIC simulations the properties of this magnetic boundary and its robustness relative to changes in the initial conditions of our PIC simulations.