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Single Langmuir probe traces dominated by primary electrons.

The above shown traces were taken at very low neutral gas pressures where ionization effects, hence secondary electrons become negligible.

The onset of current collection occurs close to the "discharge" voltage, i.e., primary electron energy.

No ionization can occur for voltages below the ionization potential (15.4 eV in Argon).

Current collection for Vdis=0 can be due to electrons accelerated by the probe's electric field and/or the field between both ends of the filaments (Vfil=7V).

The I-V curves are nearly straight lines. Theoretically, the second derivative of the I-V curves yields the isotropic electron energy distribution. It would indicate a group of electrons at Vdis. When ionization is possible there appears a second group of "degraded" primaries at Vdis - 15 eV.

Double probe characteristics dominated by primary electrons.

Note the straight-line I-V dependence near the origin.

Standard evaluation yields a high electron temperature (6.8eV) which is more characteristic of the energy spread of primaries than of the secondary electrons.

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