Transient Ion Sheath Effects on Probe Admittance

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An experimental investigation of the impedance characteristic of a spherical plasma probe has been carried out for a quantitative assessment of theoretical work, and of hydrodynamic or fluid theory in particular. A stable, low temperature (T ~ 500øK) plasma was produced in nitrogen by a cold cathode discharge, with a plasma frequency of the order of 15 MHz and with an electron collision frequency in the range of 100 to 107 sec-•. The probe consisted of two hemispheres, one serving as a guard to eliminate the effects of the, connecting leads; this geometry provided an almost purely radial electric field over the test hemisphere. Probe diameters ranged from 7 to 20 mm ( 10 to 30 Debye lengths). Hydrodynamic probe theory was tested against the experimental results for the specific cases corresponding to the probe at floating potential and at space potential. Good agreement was obtained for both the real and imaginary parts. The real part of the impedance shows a peak near the plasma frequency, a small shift being explicable in terms of realistic sheath profiles. Electron density can thus be deduced rather accurately, and electron temperature approximately, on the basis of hydrodynamic theory. Accurate values of the electron-neutral collision frequency were also obtained for plasmas where the collision frequency was larger than 0.4 times the radian plasma frequency. At lower pressures the presence of collisionless or Landau damping was clearly established, and the more accurate kinetic theory is necessary to explain the experimental results. 493 494 TARSTRUP AND HEIKKILA SPHERICAL PROBE IMPEDANCE 495

The Impedance Characteristic of a Spherical Probe in an Isotropic Plasma

Tarstrup¹,

Heikkila²

1972

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

Kist¹

1977

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.

Low‐frequency sheath admittance of a sphere in a collisionless plasma

Godard¹,

Laframboise²

1986

A systematic theoretical study of the low-frequency sheath admittance of a spherical electrode immersed in a collisionless, nonflowing, Maxwellian plasma with no magnetic field is done for the following ranges of plasma parameters: nondimensional electrode dc bias potential -25 _< eckp/kT e _< 25, ion to electron temperature ratio 0 _< Tilt e • 1, electrode radius to Debye length ratio 0 _< rp//•a g 200. A surprising feature of the results is the existence of double and triple peaks in the sheath capacitance as a function of dc bias potential at certain values of rv//•a. The same capacitance results also show discontinuous behavior at space potential. Various plasma-diagnostics applications are suggested.