Discussion on energetic electron–sheath interactions and collisionless heating in low-pressure capacitively coupled plasmas

Sub-mode transitions and bistability within a single discharge mode are observed in low-pressure capacitively coupled plasmas through kinetic particle-in-cell simulations. We demonstrate that these phenomena are fundamentally driven by energetic electron–sheath interactions, rather than by specific external control parameters. The emergence of such behavior relies on strongly non-local electron kinetics, facilitated by low pd values and high f/p ratios, which correspond to extremely low collision events across both spatial and temporal scales. Under these conditions, adjusting the external parameters can shift impingement phase of energetic electrons at the opposing sheath from collapse to expansion, thereby enhancing the energetic electron confinement and significantly increasing the plasma density.

Meanwhile, the combination of extremely low-frequency electron–neutral collisions and high-frequency sheath oscillations makes collisionless electron power absorption and heating possible. By incorporating the kinetic and thermal energy transport equations, we find that although electron power absorption is greatly enhanced in the high-density sub-mode, the conversion of kinetic energy into thermal energy due to collisions remains relatively unchanged. Instead, viscous dissipation induced by electron anisotropy emerges as the dominant mechanism for electron heating.