Characterization of nonlinear sheath dynamics in high-pressure asymmetric RF discharge

This study investigates the nonlinear behavior of asymmetric radio frequency (RF) discharges at high pressures implemented in a compact coaxial connector geometry, with a particular focus on sheath dynamics. We present a novel experimental approach that enables accurate, non-intrusive measurements of the temporal evolution of voltage and current waveforms. The experimental results reveal strongly nonlinear, anharmonic current waveforms, while driving voltage is kept nearly sinusoidal in the experiments. Increasing the pressure increases from 10 to 40 Torr leads to stronger anharmonicity in the current response. Furthermore, we demonstrate that a simple theoretical model, representing the RF discharge cell as a time-varying capacitor associated with the oscillating sheath near the central discharge electrode, closely matches experimentally measured voltage and current waveforms. The measured plasma capacitance shows strong agreement with theoretical predictions, thereby validating the model. Fast Fourier transform analysis confirms a substantial rise in the second harmonic amplitude at higher pressures and frequencies, highlighting the growing role of nonlinearities. These findings have significant implications for plasma capacitors and reconfigurable RF devices, where controlling nonlinearity is crucial for optimizing circuit performance. This study extends the operational understanding of RF plasmas to higher pressures, bridging the gap between low-pressure theoretical models and practical high-pressure applications.