Geometrically self-similar ion acceleration in collisionless plasma beam expansion.

Fully kinetic particle-in-cell simulations are carried out to study ion acceleration in a geometrically expanding plasma beam. The ion acceleration is demonstrated to be essentially self-similar. An analytical model is developed for ion acceleration evaluation. The plasma plume is found to be fully expanded after about 20 times of the beam radius (R₀) downstream of the beam exit, where the ion velocity reaches the maximum. The maximum ion velocity increase is about twice of the initial ion acoustic speed. Hence, to minimize the facility effects on plasma plume experiments in a vacuum chamber, the chamber needs to provide a plume expansion region of at least 20R₀ along the beam direction so the expansion process is not terminated prematurely. [ABSTRACT FROM AUTHOR]