Expansion of a collisionless hypersonic plasma plume into a vacuum

: Both fully kinetic and hybrid particle-in-cell (PIC) simulations are performed to investigate the two-dimensional (2D) expansion of a collisionless, hypersonic plasma plume into a vacuum. The fully kinetic PIC simulations are carried out using the real ion-to-electron mass ratios of H^{+}, Ar^{+}, and Xe^{+}, while the hybrid PIC model assumes the electrons to be a massless, isothermal fluid. We find that the hypersonic plasma plume exhibits four distinct regions, the unperturbed, quasisteady expansion, self-similar expansion and electron front regions. The behavior of electrons is strongly anisotropic, causing considerably different expansion characteristics between the plume direction and the transverse direction. Along the plume direction, the expansion dynamics is similar to that of the classical one-dimensional (1D) semi-infinite plasma expansion and the electrons are almost isothermal. In the transverse direction, the expansion process can be considered analogous to the 1D expansion of a finite plasma where the effect of electron cooling is important. This anisotropic characteristic is attributed to the amount of electron thermal energy available from the source in different directions. A direct comparison between the hybrid and full PIC simulations shows that the widely used equilibrium isothermal electron fluid model is in general not valid for modeling the expansion of a collisionless plasma plume.