Relaxation effect of electron inertial delay in an ion-beam plasma system

Abstract

In this article an interesting result has been reported to highlight the physical implication of finite nonzero electron inertial delay in acoustic oscillations of an ion-beam plasma system. A simple fluid model has been used to demonstrate the excitation of relaxation type resonant instability on the electron inertial delay time scale. Physical arguments have been included to understand and explain the driving mechanism of the instability and its utility to comprehend the ion-beam driven oscillations in plasma sheath experiments. The physical nature and origin of the potential relaxation instability has been correlated to the electron inertial delay effect in the ion current carrying plasma system.

Effect of electron inertial delay on Debye sheath formation

“…Similar effect, due to weak but finite electron inertial delay, is reported to occur in an ion beam plasma system where the ions are drifting with flow velocity exceeding the phase velocity of the ion acoustic wave [15]. As a result, the fluid mode of ion acoustic wave fluctuations resonantly couples with the ion beam mode to produce relaxational growth of the acoustic fluctuations at down shifted wave frequency (Doppler effect), which other wise is not possible in the asymptotic limit of m e /m i → 0, i.e.…”

“…De for weak dispersion effect [15]. Let us further argue that the reduction in Mach number threshold for Debye sheath formation i.e.…”

“…For example, let us assume that the sheath edge is susceptible to weakly dispersive mode of acoustic fluctuations due to electron inertia induced instability [15] as one of the possible turbulence mechanisms. This has less threshold value than that of the dispersionless acoustic speed and the reduction in excitation threshold is proportional to k 2 λ 2…”

“…inertialess electrons. In the light of inertia induced frustration of the Debye shielding [15], it is therefore important to review the Bohm condition of DSP formation as because the transonic point satisfies the condition for its physical realization.…”

Response to “Comment on ‘Graphical analysis of electron inertia induced acoustic instability' ” [Phys. Plasmas 13, 104701 (2006)]

“…2,3 He argues that the fluid acoustic limit as defined by v ti Ͻ v p Ͻ v te is violated for the reported unstable ion sound mode. Here v ti , v p , and v te , respectively, represent for the ion thermal speed, phase velocity of the ion sound mode, and the electron thermal speed.…”