Optically measured plasma density using Phase Resolved Optical Emission Spectroscopy in low-pressure capacitive discharges

As plasma applications continue to advance technologically and grow in complexity, precise plasma diagnostics are crucial for optimizing plasma processes. However, diagnosing plasma properties remains both technically and physically challenging, requiring non-intrusive methods that maintain uninterrupted plasma operation while also incorporating a fundamental understanding of plasma physics. In this study, we measure electron density using a non-invasive optical measurement in low-pressure capacitive discharges. This optical approach provides nanosecond time-resolved insights into the dynamics of energetic electrons, particularly the multiple electron beams that are launched into the plasma during a single phase of sheath expansion adjacent to an electrode. As the time interval between two consecutive beams corresponds to the inverse instantaneous electron plasma frequency, the absolute electron density can be determined by measuring this time interval. The measured electron density is found to oscillate within the radio-frequency period under low-pressure discharge conditions. The dynamics of this density oscillation is analyzed using the continuity equation, accounting for the contribution of the electron beams.