Millimeter Wave Radar-based Plasma Measurements

A new approach for contactless in-situ plasma measurements, based on phase evaluation of a high precise FMCW radar is presented in this paper. Assuming the plasma to act like a frequency dependent dielectric material, its dependency on the time-of-flight and therefore, the phase is analyzed. 3D electromagnetic field simulations are applied for the investigation of the measurement effect, using the Drude model to describe the dielectric properties of the plasma. It can be shown, the longer the propagation length the stronger the change in phase, which is also directly affected by the plasma electron frequency. Final measurements with an 80 GHz FMCW radar in an excited argon plasma prove the applicability of this new approach with a maximum phase difference of 54°.