Floating harmonic probe measurement through thick dielectric material for noninvasive plasma diagnostics*

 The floating harmonic method is a well-known technique in plasma diagnostics. It is founded on applying an AC voltage to an electric probe immersed within a chamber. This method allows for measuring plasma parameters, such as electron temperature and ion density, by analyzing the ratio of harmonic current, even in cases where the probe is contaminated or coated by the thin dielectric film. However, there has been increasing interest in noninvasive diagnostic methods in recent years. Because when the probe is immersed in the chamber, it can distort the plasma and contribute to the unwanted generation of particles caused by ion bombardment. We propose a plasma diagnostic using an floating probe via thick dielectrin material in the viewport to address this drawback. Despite the presence of thick dielectric material, we were able to measure harmonic currents by applying a high frequency in the MHz range. To compensate for the phase delay caused by the viewport, we analyzed a non-linear equivalent circuit. The measurements obtained from this method exhibited good agreement with those obtained using the traditional immersed probe technique. Our research is expected to provide an effective approach to noninvasive plasma diagnostics.