Transient Self-Organized Striations During Gas Breakdown Under RF Excitation

A transient self-organized striated structure is experimentally observed for the first time during the ignition phase of a pulse-modulated capacitively coupled RF plasma with a sufficiently long pulse-off duration by using time-resolved optical emission spectroscopy. The experimental observation is well reproduced and analyzed using PIC/MCC simulations. During the ignition phase, the externally applied RF field is able to penetrate into the central region, accelerating electrons and generating ionization and excitation. Due to much larger electron mobility compared to ion, alternating positive and negative charge regions are formed across the central region where the ion and electron density exhibit their maximum and minimum. The electric field caused by the space charges enhances or attenuates the local drift electric field, resulting in a spatially-modulated electric field profile. This electric field distribution leads to a spatially modulated profile of ionization rate, and particularly, the ionization maximum occurs exactly at the location where the charge density peaks. So, the maximum of charge density is further increased, resulting in further enhancement of spatially-alternating net space density and the striated electric field due to a positive feedback. This self-amplified effect persists during the ignition process until the drift electric field inside the central region is gradually shielded out as the charge density grows between the electrodes. When the charge density is sufficiently high, the striated structures diminish and finally vanish. By increasing the gas pressure or the excitation frequency, the striation gap (defined as the distance between two excitation rate peaks) narrows, and namely, the electrode gap can be filled by more striations. We also present a phase diagram as a function of the pressure and the electrode gap to provide a parametric window for the presence of the striations.