Four parameter data fit for Langmuir probes with nonsaturation of ion current

The four parameter data fit for Langmuir probes is here generalized within the classical single Langmuir probe theory. The data fit accounts for the variation of the Debye sheath thickness of the probe with the applied voltage which results in a nonsaturation of the ion current. The new fitting formula has been applied to the RFX experimental data. With respect to the use of the simple exponential fit, in general a correction of the order of 20% on electron temperature and ion saturation current has been obtained.