influence of sheath thickness on cutoff probe diagnostics in low-pressure plasmas

The cutoff probe is known to be a highly precise diagnostic method that measures the electron density through the transmission microwave frequency (TMF) spectrum of the plasma. In particular, the plasma density measured by the cutoff probe is reported to be unaffected by the sheath thickness formed around the antennas of the cutoff probe. However, studies on the TMF spectrum of the cutoff probe with respect to the sheath thickness have focused primarily on theoretical calculations, while experimental validation remains relatively insufficient. In this study, we establish an experimental setup that enables the control of the sheath thickness around the antennas of the cutoff probe by applying a DC voltage. Plasma density is measured using the developed setup, and the results are compared with electromagnetic wave simulations, where the sheath thickness is systematically varied from 28.57% to 100% of the gap between the two probe tips. Consequently, we confirm that the plasma density measured by the cutoff probe is independent of the sheath thickness.