Saturation of the Ion-Acoustic Instability in a Weakly Ionized Plasma

The spatial saturation and the frequency dependence of the energy spectrum of ionacoustic waves in a low-pressure discharge in helium in the pressure range 0.03-0.2 Torr has been investigated. In the high-pressure region the spectrum is peaked around the most unstable frequency predicted by linear theory [i.e., ωPeak~(1√3)ωᵢ], and the wave energy saturates at the modest value of WnTₑ10^-5. At low pressures the wave energy increases and varies with frequency according to W(ω)∝ω^-α, with α assuming values between 2.2 and 2.8.