Plasma response to pulsed ion acoustic wave excitation

Experimental investigations into the plasma’s response to a pulsed ion acoustic wave excited via a grid have been carried out in a quiescent, multi-dipole confined hot cathode discharge. A frequency limit at ∼1/140 of the ion plasma frequency f_pi has been found in the plasma’s ion acoustic response to the excitation wave. This limiting response frequency is much lower than a plasma’s expected ion acoustic resonance frequency, which previous computational and experimental investigations revealed to be >f_pi/10. The corresponding wavelength at ∼860 times the Debye length λ_Debye also mismatches both the plasma resonance wavelengths, the device dimensions and the grid dimensions. It was found that multi-cycle pulses do not drastically change the frequency but only increase the response amplitude, which closely reflects the increase in transmitted total pulse energy. These findings show that the preferred plasma response to an excitation pulse might not reflect its wave resonance characteristics and other plasma parameter related effects might be at play. Experiments also show an inverse relationship between plasma density and excited wave amplitude with identical excitation parameters, and a strong inverse correlation between the amplitude of the excited wave and the expected sheath thickness near the launch grid, suggesting that the fundamental process of exciting ion acoustic waves mirrors that of capacitively coupled plasma heating, i.e. via sheath fluctuations. The change of the ion acoustic wave damping length is also found to reflect the change of neutral pressure but only up a certain limit, which could be either due to a cone expansion of the launched waves and/or an additional damping mechanism other than ion-neutral collisions. The implications of these findings for other wave-related plasmas, i.e. pulsed rf-plasmas, are discussed.