Variation of particles distribution function in expansion of plasma slab into vacuum

In this paper, the variations of particles distribution function (DF) in a one-dimensional collisionless plasma expansion are studied. It is shown that, for the self-similar cases the initial Maxwellian distribution function of ions gradually converse to a δ-like one. So, the dynamic of the ions could be considered with fluid equations. On the other hand, for the effects of charge separation that occurs assuming the expansion of finite plasma, the process of plasma expansion imposes variations on electrons DF which causes it to change from Maxwellian to a non-Maxwellian one. In order to investigate the electrons DF at each moment, a particle simulation is used. In this simulation, the electrons dynamic is determined by Vlasov equation. Ions dynamic is given by fluids equations. Finally, it is indicated that, the higher energy tails of electrons DF in the main body of plasma are descended due to the motion of high energy electrons into vacuum and the electrons DF transforms to a super-Maxwellian DF. In the opposite side, at ion front zone which is the place of high energy electrons presence, electrons DF transform from Maxwellian to Lorentzian case with the high energy tails