Comparison of Langmuir Probe and rf Capacitance Probe Electron Density Measurements in Low-Density Plasmas

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Abstract
Electron densities calculated from data obtained in the laboratory by dc and ac probes simultaneously exposed to low-density low-pressure air plasmas are compared. With electron densities between about 1011 and 1012 m−3 and collision frequencies on the order of 108 sec−1, data from a rf capacitance probe operating near 5 MHz and data from the ion saturation region of a dc probe each yield electron density values that generally agree within a factor of 2. By the use of a differential pressure cell, lower electron densities, ∼109 m−3, and reduced collision frequencies, <105 sec−1, were obtained. Under the latter conditions the dc probe operation was erratic but the ac probe operated satisfactorily, and operated as expected theoretically even under the laboratory limitations.

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