Variation of the r.f. response of a Langmuir probe with amplitude and frequency

The r.f. response of a double Langmuir probe immersed in a laboratory plasma has been measured over a wide range of signal amplitude, signal frequency and plasma electron density. A theory is developed which predicts (in direct contradiction to the theory of other workers) a decrease in the probe sheath capacitance with increasing frequency in the neighbourhood of the ion plasma frequency, and a probe response above this frequency which at high amplitudes obeys a current is proportional to (voltage)^¼ dependence. While no significant change of capacity with frequency is observed, possibly owing to experimental limitations, the high-amplitude results are in good agreement with theory and do show the appropriate current is proportional to (voltage)^¼ dependence. This is in contrast to earlier step waveform (i.e. low-frequency) measurements which showed a current is proportional to (voltage)^¾ dependence.