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An rf probe is placed within a plasma and an rf signal from a network analyzer for a given dc bias voltage Vp is applied The frequency applied by the network analyzer, ω, is less than the plasma frequency, ωpe, and therefore is not in the resonant absorption range (ω=ωpe) used to determine electron density in typical rf impedance probe operation. Bias voltages at the applied frequency are applied to the probe in a series of voltage steps in a range which includes the plasma potential. At each bias step, a value of Re(Zac), the real part of the plasma's complex impedance, is returned by the analyzer. A local minimum in the real part of the impedance Re(Zac) occurs where the applied bias voltage Vp equals the plasma potential φp. The plasma potential φp can be found by taking the first derivative of Re(Zac) with respect to Vp, ⅆ(Re(Zac)ⅆVp, and finding the value of Vp at which ⅆ(Re(Zac)ⅆVp=0 within error tolerances.