Ion sheath effects on RF plasma probes—Experimental results in laboratory plasmas

In this paper, laboratory work dealing with the frequency characteristic of the plasma impedance of spherical and cylindrical electrode systems is reported. The influence of the ion sheath on various features of the impedance characteristic is considered as the main point of interest throughout this work. Those features are the series and parallel resonance as well as additional resonances due to the excitation of electroacoustic and cyclotron harmonic waves. The dependence of the series and parallel resonances on dc biasing leads to a method of determining the ion sheath capacity Ci for a cylindrical electrode system. These Ci values agree fairly well with those obtained from a theoretical model for the density and potential distribution in the sheath of a cylindrical sensor aligned with a supersonic plasma flow. The amplitude of resonances due to excitation of longitudinal plasma waves (electroacoustic and cyclotron harmonic) reduces or even vanishes for sufficiently negative dc bias. Positive bias first leads to an increased amplitude up to a certain dc bias value above which, however, the amplitude decreases again due to electron absorption at the sensor surface. It is verified experimentally that large RF amplitudes exist across both the ion sheath and the plasma body in the regime of the series resonance, leading to a strong rectification effect. In this regime cyclotron harmonic waves are effectively excited, even if a strong negative dc bias voltage is applied.