Relationship between the number density and the phase shift in microwave interferometry for atmospheric pressure plasmas

Diagnosing atmospheric pressure discharges requires more sophisticated techniques than for low pressure plasmas. The plasma number density is a crucial parameter in several applications. Langmuir probe as a number density measuring technique is not applicable at high pressures because the electron mean free path is shorter than the Debye distance. Microwave interferometry appears to be an effective diagnostic technique in this case. However, because of the high collisionality of atmospheric pressure plasmas, the relationship between the phase shift, as measured by a microwave interferometer, and the plasma number density is not straightforward, as is the case in collisionless plasmas. For the special case of a uniform discharge, the plasma number density is found to depend on the square root of the phase shift.