  Relativistic unmagnetized cloud-plasma interaction is analyzed by performing
linear analysis and particle-in-cell simulation. This course consists of an
electron-ion cloud injected into a stationary ambient plasma and has long been
a favorite topic in laboratory and space plasmas. An oblique electromagnetic
instability dominates the unstable spectrum. In the interaction with the
generated electromagnetic fields, the cloud electrons are entirely mixed with
the ambient ones and form a hot electron population. The velocity of the cloud
ions, however, has not changed significantly from the initial bulk velocity. As
this ion cloud propagates into the plasma, it derives an electrostatic field
which can accelerate the electrons up to energy equipartition between electrons
and ions. The electrostatic field is amplified at the expense of the kinetic
energy of ions, and its spatial scale is in order of the electron skin depth.
The electron acceleration in such an electrostatic field is, therefore, a
likely process for pre-acceleration of electrons in unmagnetized plasmas.
