Crossing Frequency Method Applicable to Intermediate Pressure Plasma Diagnostics Using the Cutoff Probe

Although the recently developed cutoff probe is a promising tool to precisely infer plasma electron density by measuring the cutoff frequency ([Formula: see text]) in the S [Formula: see text] spectrum, it is currently only applicable to low-pressure plasma diagnostics below several torr. To improve the cutoff probe, this paper proposes a novel method to measure the crossing frequency ([Formula: see text]), which is applicable to high-pressure plasma diagnostics where the conventional [Formula: see text] method does not operate. Here, [Formula: see text] is the frequency where the S [Formula: see text] spectra in vacuum and plasma conditions cross each other. This paper demonstrates the [Formula: see text] method through three-dimensional electromagnetic wave simulation as well as experiments in a capacitively coupled plasma source. Results demonstrate that the method operates well at high pressure (several tens of torr) as well as low pressure. In addition, through circuit model analysis, a method to estimate electron density from [Formula: see text] is discussed. It is believed that the proposed method expands the operating range of the cutoff probe and thus contributes to its further development.