Resonance behavior of the ion-sheath capacitance near the plasma ion frequency

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Abstract
A slight minimum in the measured rf capacitance of the ion sheath surrounding negatively biased planar and cylindrical Langmuir probes in a collisionless plasma has been observed. The position of the minimum, which is near the plasma ion frequency ωi, and which closely agrees with that predicted on the basis of simple physical arguments, is studied both as a function of plasma density and probe potential. The experimental results, for the planar probe, are compared with a recent theory by Rosa. The plasma was formed in the positive column of a helium dc discharge at a pressure of 0.3–0.5 Torr, with plasma electron densities and temperatures ranging between 1015 and 1016 m−3 and between 5×105 and 7×105°K, respectively. Measurements of the sheath capacitance were made using a tunnel‐diode oscillator circuit whose frequency could be adjusted approximately an order of magnitude above or below ωi.