The orbital-motion-limited regime of cylindrical Langmuir probes

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Abstract
An asymptotic analysis of electron collection at high bias Φp serves to determine the domain of validity of the orbital-motion-limited (OML) regime of cylindrical Langmuir probes, which is basic for the workings of conductive bare tethers. The radius of a wire collecting OML current in an unmagnetized plasma at rest cannot exceed a value, Rmax, which is found to exhibit a minimum as a function of Φp; at Φp values of interest, Rmax is already increasing and is larger than the electron Debye length λDe. The breakdown of the regime relates to conditions far from the probe, at electron energies comparable to the ion thermal energy, kTi; Rmax is found to increase with Ti. It is also found that (1) the maximum width of a thin tape, if used instead of a wire, is 4Rmax; (2) the electron thermal gyroradius must be larger than both R and λDe for magnetic effects to be negligible; and (3) conditions applying to the tether case are such that trapped-orbit effects are negligible.

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