Langmuir and Emissive Probes
Due to the plasma sheath formation, the potential at which Langmuir probes float is below the
plasma potential. By varying the bias that is applied to a probe, one can measure ion density,
electron temperature and plasma potential from the I-V characteristic. In order to get an accurate
measure of the plasma potential from a Langmuir probe, a large portion of the voltage range
must be swept over and the derivative of the I-V curve must be taken. This causes issues in
measuring potential fluctuations faster than a possible sweep time and in measuring the plasma
potential in real-time without post-processing of the data. Also, since a derivative is being taken,
noise in the data can be quite problematic and must be filtered out in such a way as to not
strongly affect the peak value in dI/dV which is often identified as the plasma potential.
One technique for overcoming these issues is the emissive probe. An emissive probe is heated to
a temperature such that it can begin thermionically emitting electrons into the plasma which is
equivalent to collecting additional ions, causing the probe to float at a higher voltage and closer
to the plasma potential.