Analytical expressions of the front shape of non-quasi-neutral plasma expansions with anisotropic electron pressures

An analytical expression is proposed to describe the front shape of a non-quasi-neutral plasma expansion with anisotropic electron pressures. It is of significance in the study of ultrashort plasma expansions generated from laser-foil interactions and anisotropic astroplasma expansions in space science. It is found that the plasma front shape depends on the relationship between the ratio of the longitudinal and the transverse temperature of hot electrons kappa(2) and the electron-ion mass ratio mu. For kappa(2)is an element of(mu,1], the ion front is a part of an ellipse and the major axis is in the lower-temperature axis. For kappa(2)<mu, the ion front is composed by a part of a hyperbolic and a small pointed projection at the center. In the strongly anisotropic region, there is an ultrashort anomalous plasma emission of tens of femtoseconds at the angle of near 90 degrees. The ion-velocity distribution and angular-energy distribution at the ion front have also been given. Particularly, anomalous positron emissions exist in the electron-positron plasma anisotropic expansion.