Collisionless expansion of a Gaussian plasma into a vacuum

The collisionless expansion into a vacuum of a one-dimensional Gaussian plasma bunch of size much larger than the Debye length is studied both analytically and with a hybrid Lagrangian code. Both the case of an isothermal plasma fed by an external energy source and the case of an energy-conserving plasma are studied. It is shown that charge-separation effects are responsible for deviations with respect to known self-similar solutions. In particular, wave breaking occurs because the inner parts of the expanding plasma acquire faster velocities than the outer parts. As a result an ion front appears at the edge of the plasma. In the isothermal case the self-similar solution progressively applies to the whole plasma, while the fastest ion's velocity asymptotically reaches the same values as in the sharp boundary case. In the energy-conserving case the self-similar description only applies to a part of the plasma, which approximately corresponds to the part which was initially quasineutral, and the final velocity of the fastest ions is significantly smaller than in the sharp boundary case. (C) 2005 American Institute of Physics.