The contribution of charge exchange ions to cylindrical Langmuir probe current

A theory for the collection of ion current by cylindrical Langmuir probes is presented which includes the effect of charge exchange collisions. These collisions replace ions accelerated into the probe sheath with slower ions having a much greater probability of collection, thus increasing the probe current. The probe ion current increase is obtained from the volume integral of the charge exchange rate weighted by the collection probability. The increase is significant even for plasma with a mean free path of ∼103 Debye lengths. The validity of the model is verified experimentally in an Ar plasma of density ∼107 cm−3 created with a hot cathode discharge. The plasma density measured by microwaves is held constant as the pressure and collision frequency are varied. The charge exchange current, obtained by subtracting the calculated orbit-motion-limited current from the measured net ion current, increases nearly linearly with probe bias and with neutral gas pressure.