Home| Syllabus| Notes| Assignments

Single Langmuir probe trace in the vicinity of the floating potential.

This region is useful to determine the floating potential and the tail electron temperature.

Only in a Maxwellian plasma one can obtain the plasma potential from the floating potential via the relation Vp=Vf + (kT/e)ln(Iesat/Iisat). Then, from the extrapolated ion current at the plasma potential one can also obtain the density, ne=Iisat/A e sqrt(kT/mi).

The presence of energetic primary electrons in a discharge plasma results in a higher tail electron "temperature" and a more negative floating potential than expected on the basis of the colder bulk electrons. Hence, caution must be applied in inferring plasma parameters from this part of the I-V curve.

Evaluation of the Langmuir probe characteristic near the floating potential.

As in the earlier case, one first subtracts the linearly extrapolated ion current from the total current so as to obtain the electron current.

Then, the electron current is plotted on a logarithmic scale and a straight-line fit is made. The voltage difference for a change of one on the vertical scale (lnI) yields the electron temperature. A value of kTe=2.52eV is found which is larger than the bulk electron temperature of 1.47eV.

For kT=2.52eV the plasma potential should be at Vp=Vf+kT/e[ln sqrt(mi/me)]=-7.3+2.52 ln271=6.8V. However, the measured plasma potential is only 2.8V. This example shows influence of the primary electrons on the data analysis.

Home| Syllabus| Notes| Assignments