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Typical double probe I-V characteristics in a nearly Maxwellian plasma (low discharge voltage, high pressure).

Evaluation of the I-V trace obtained with a double Langmuir probe can be performed using the following recipe:

1. Fit a straight line to the ion "saturation" current regimes. It's intersection with the ordinate yields the ion saturation current, I_i,sat.
   
2. Subtract the difference between the linearly rising ion current and Iisat from the measured current so as to obtain a corrected I-V curve.
   
3. The slope of the corrected curve at the origin should be dI/dV = I_i,sat[e/2kT_e]. Hence, 2kT_e/e is the voltage at which a straight line, tangent at the origin, assumes the value Iisat.
   
4. From the electron temperature and ion saturation current one can calculate the density: n = I_i,sat/A e sqrt[kT_e/m_i]. However, this value may underestimate the density since in the presence of primaries the tail electron temperature exceeds the bulk electron temperature.
   

Evaluation of a double probe I-V Characteristic via the above method.

The ion saturation current is found to be I_i,sat = 34 µA and the electron temperature kT_e = 2.65 eV. For a probe area of 0.633 cm², this yields a density n = 1.3 × 10⁹ cm^-3, close to the value 1.5 ×10⁹  cm^-3 obtained from the single Langmuir probe.

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