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Self-modulation of the oscillating electric field is observed when its frequency is near the electron plasma frequency. The localized field due to the self-modulation is found to make depressions in the plasma density, and to move in unison with the density dip with approximately the ion-acoustic speed. The maximum amplitudes of the high-frequency field and the density dip are linearly proportional to each other, but the former shows a phase jump at the bottom of the density dip. All these features are qualitatively explained by the results of the recent theory of coupled electron-plasma and ion-acoustic solitary waves. The self-modulation occurs in the overdense region, the parametric decay instability is observed in the underdense region.

An RF probe experiment was carried out with a low density plasma, the resonances of the probe impedance were studied. The resonances were observed in a frequency range between the sheath-plasma resonance and the plasma frequencies. The mechanism of these additional resonances is discussed. It is shown that a finite electron temperature will introduce these resonances in addition to the sheath-plasma resonance. Choosing some artificial density distributions in front of the probe, the frequency dependence of the resonances is explained qualitatively.

The probe noise in quiescent plasmas has been studied experimentally. The observing frequency f normalized to the electron plasma frequency f p e is between 10 -3 and 1. In the low frequency range, f / f p e ≪1, the probe noise consists of the shot noise for the electron collecting region of the Langmuir probe, even when the probe potential is higher than the space potential. The shot noise arising from the probe electron current suffers a reduction due to the transit time effect near the electron plasma frequency. In this frequency range the thermal noise is observed instead of the shot noise. Resonant increases of the noise current are found at the sheath-plasma resonance and at the plasma wave resonance frequencies. On the other hand the noise current arising from the probe ion current is smaller than the full shot noise level. The reduction factor for the noise is found to depend on f / f p i , where f p i is the ion plasma frequency. A mechanism for the reduction of the ion shot noise is proposed.