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.

Pulsed heating is often used on the emissive probe to remove heating voltage offsets that occur during constant DC heating. This feedback-controlled heating accounts for changing resistivity of the hot wire vs. the cold wire, which requires fast samples (~10kHz) and real-time calculations. When DC heating is used, the approximation that the floating point is at the midpoint of the voltage of the two ends of the probe is often used. When heating voltages are large (<10V), using this approximation is dubious and error could easily be a few Volts.

The floating potential of a strongly emitting probe is about one electron temperature below the plasma potential due to a virtual cathode that forms around the emitter from to the space charge buildup of slow electrons emitted from the surface. This virtual cathode forms in order to reflect some of the emitted electrons back to the emitter so that it collects zero net current.