Discharge Characteristics and α–γ Mode Transition Law of CO 2 Radio Frequency Capacitively Coupled Plasma at Different Gap Distances

ABSTRACT The discharge structure of CO 2 Radio Frequency Capacitively Coupled Plasma (RF‐CCP) under Martian pressure includes two parts, sheath and plasma bulk, so whether the gap distance is sufficient to ensure the complete existence of the sheath and plasma bulk is the key to whether the discharge can occur. This article investigates the discharge characteristics of RF‐CCP under varying discharge gaps and discusses the transition laws and mechanisms between the two common discharge modes (α and γ modes) at different gap distances. The results show that when the gap distance is greater than twice the sheath thickness, both the gap electric field strength and the average density of positive ions in the plasma bulk increase as the gap distance decreases, leading to a decrease in the transition voltage of the α–γ mode; When the gap distance is less than twice the sheath thickness, the plasma bulk is absent and α mode discharge cannot occur, resulting in a rapid decrease in the number of electrons and positive ions, which in turn increases the γ mode generation voltage. However, with further reduction in gap distance, the gap‐averaged electric field strength is enhanced, leading to a subsequent decrease in the γ mode generation voltage. The results of this article provide a reference in terms of gap selection for future RF‐CCP devices to be built in the Martian environment.