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Physical processes responsible for the nonlinearities of probe characteristics and their implications in space plasma measurements

Recent studies emphasize the significance of antenna‐plasma coupling consideration when analyzing the results of space plasma measurements, obtained with the Langmuir probe technique. We investigate the current‐voltage (I−V) and charge‐voltage (Q−V) curves characterizing antenna‐plasma coupling and calculate probe impedance and harmonic magnitudes under different physical conditions. The effects of absorption or reflection of charged particles by the antenna surface are taken into account. Impedance dependence on the bias probe voltage V0 and the RF signal amplitude V1 is studied. We give simple analytical formulas and numerical results for either a big or a small (with respect to the Debye length) spheric probe in isotopic plasma. I−V curve nonlinearities due to electron current exponential growth and saturation and photoelectron current contribution are considered. The main attention is paid to Q−V curve calculation: a problem which is poorly studied up to now. It is shown, in particular, that the electric capacitance of the absorbing spheric probe at low probe‐plasma voltage equals C = 1.25a(a/λD)2/3 (where a and λD are the probe radius and Debye length, respectively) instead of the commonly used expression C = a(a/λD). Q − V curve nonlinearity due to the complicated space charge distribution around the absorbing probe (which could be described as an effective increase of the probe sheath thickness at high voltage) is investigated. It is shown that the role of particles trapped by the probe near field can be substantial for a small‐probe capacitance estimation.