Acoustic standing wave modulation of capacitively coupled plasmas
This work presents a concept of using acoustic standing waves to modulate plasmas. A 1D self-consistent fluid model combined with an acoustic standing wave model has been established to investigate the strong coupling effects between acoustic standing waves and capacitively coupled argon plasmas. The modulation effects are revealed by comparing the spatiotemporal distributions of electron density and excited species number density with and without the acoustic standing waves in one acoustic period, as well as the electric field, the electron temperature, the ionization and excitation rates, and the power density in one radio-frequency period. Under the nonlinear acoustic standing waves, the plasmas oscillate in between the electrodes, which indicates a strong modulation effect beyond conventional regimes. This modulation effect is primarily attributed to the neutral flux friction to the plasma species and secondarily to the variation of the neutral gas density. A pulsed excited species flux with a maximum value up to twice of the minimum can be achieved at the electrodes as a result of the acoustic standing wave modulation. The distributions of the ionization and excitation rates in one RF period are significantly influenced by the acoustic standing waves, due to the variation of the neutral gas density and electron temperature. This study initiates the effort to understand the mechanisms and characteristics of plasma discharges in a high-intensity acoustic standing wave field. Using acoustic waves to modulate plasmas has the potential to create many new applications and promote plasma-materials interactions.