Suppression and excitation condition of collision on instabilities of electrostatic plasmas
Two-stream (TS) and Bump-On-Tail (BOT) electron distributions can induce instabilities in collisionless plasmas, which is closely related to phenomena in space and fusion plasmas. Collisions can lead to unexpected plasma behavior, especially in dense and/or low temperature plasmas. In this work, the Vlasov-Poisson system with Krook collisions are used to study the effect of collisions. Normally, the collision can dissipate the system energy which causes the suppression of the instabilities. Against the traditional suppression effect of collision on the instability, it is found in our simulation that the collision can also excite the instability even in the forbidden beam velocity range predicted by the cold-beam theory. With collision, the beam velocity range can be divided into suppression area [vth/2, vc + vth], transition area [vc - vth, vc + vth], excitation area [vc + vth, 2vc] and forbidden area [2vc, +infinity] for TS instability. where vc is the critical velocity from cold-beam theory and vth is thermal velocity or the beam width in our simulation. The collision dissipation effect and the excitation of beam instability can compete with each other, which evoked the excitation of collision on TS instability. The collision can change the suppression and excitation condition from beam theory. However, for BOT instability, there is only suppression effect of collision on the instability. These results can expand the view of collision effect on instability of electrostatic plasmas.