Prof. Jan Trulsen: Kinetic instabilities associated with injection of a plasma beam into a neutral background

Kinetic effects associated with a neutralized plasma beam injected into a neutral background is investigated by numerical methods, using a hybrid particle-in-cell code. We assume the dominant interaction between the ions of the beam and the background neutral atoms to be charge exchange collisions. Ions removed from the beam by collisions are replaced by a stationary ion population. A two-stream situation is thus created. Under proper conditions this situation is unstable, leading to the formation of ion phase space holes and a modified beam energy spectrum. The situation is unusual in that it is a collisional prosess that is responsible for the generation of the free energy necessary to drive the instability, normally collisions tend to stabilize systems. Other types of collisions may produce somewhat similar results. The effect may have importance for reaction rates in industrial gaseous chemistry, but also for astrophysical applications.