Unified mechanism of sub-mode transitions and bistability induced by electron-sheath interactions in 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. Independent variations of different external control parameters (magnetic field, gas pressure, electrode gap, driving frequency, etc.) can drive the sheath-bulk structure to a critical state, where the impingement phase of energetic electrons at the opposing sheath shifts from collapse to expansion, triggering transitions from low to high plasma density within the same discharge mode. In the high-density sub-mode, the spontaneously increased plasma density further moves the impingement phase towards the sheath expansion stage, preventing an immediate reduction of the plasma density when external control parameters are altered in reverse, thus leading to bistability and hysteresis. These findings identify a unified mechanism underlying sub-mode transitions and bistability, independent of specific external control parameters, and fundamentally driven by energetic electron–sheath interactions.