Effects of external control parameters on mode transitions in capacitively coupled oxygen discharges

In capacitively coupled radio frequency discharges, plasma parameters such as the densities of reactive radicals, ions, and electrons are strongly affected by the spatio-temporally resolved electron power absorption dynamics and the Electron Energy Distribution Function. These, in turn, are determined by the electron power absorption mode of the discharge. In oxygen gas, various modes of operation, e.g., the α-, Drift-Ambipolar (DA), and γ-mode, as well as mode transitions among them induced by external control parameters such as driving voltage and pressure are known. In 2012, Dittmann et al. [1] discovered another electron power absorption mode, where electrons are generated via detachment from O⁻ ions inside the sheaths, accelerated towards the bulk by the sheath electric field and collisionally multiplied, similar to secondary electrons. This new electron heating mode was called Detachment-Induced (DI) mode in a later computational study [2]. In this work, kinetic plasma simulations are performed to investigate the effects of pressure, voltage, and secondary electron emission coefficients of the electrodes on mode transitions into and out of the DI-mode. Increasing the pressure is found to induce transitions from the DA- to the α-, and to the γ- and DI-mode. Increasing the driving voltage amplitude leads to transitions from the DA- to the DI- and finally to the γ-mode. Higher secondary electron emission coefficients suppress the DI-mode. These mode transitions are explained at the kinetic level and found to affect the ion flux to the electrodes and the atomic oxygen generation in the plasma volume.