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

Abstract

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 C i 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.

Kinetic damping in the spectra of the spherical impedance probe

“…The three matrices in (15) are: the identity matrix I, the collision matrix T S , and the Vlasov matrix T V . e is the explicitly expanded excitation vector.…”

“…k T 00 1 2 Since all matrices and the excitation vector in equation (15) are defined, the explicit expansion of the admittance can be calculated to compute different spectra of the sIP.…”

“…In subsequent years, the RP was intensively investigated both experimentally and theoretically. Many researchers have attempted this task, especially the RP with a spherical electrode [4][5][6][7][8][9][10][11][12][13][14][15]. The theoretical works of the cited papers share a common feature in that the underlying models are based on an electrostatic approximation, but the plasma description is of a different complexity.…”

Terminal properties of a spherical RF electrode in an isotropic Vlasov plasma: Results of a computer simulation

“…The consequences of this effect for current collection and impedance of an antenna in the ionosphere were discussed by Lafrarnboise et al [1975]. Positive floating potentials on an RF probe were observed in the laboratory by Kist [1977] at frequencies above the plasma frequency. Below plasma frequency, rectification dominated and the floating potential was driven more negative.…”