Mode transition (α–γ) and hysteresis in microwave-driven low-temperature plasmas

Abstract

We discovered a hysteresis in a microwave-driven low-pressure argon plasma during gas pressure change across the transition region between α and γ discharge modes. The hysteresis is manifested in that the critical pressure of mode transition depends on the direction of pressure change. As a corollary, the plasma would attain different discharge properties under the same operating parameters (pressure, power, and gas composition), suggesting a bi-stability or existence of memory effect. Analysis of the rotational and vibrational temperatures measured from the OH (A–X) line emissions shows that the hysteresis is mainly due to the fast gas heating in the γ-mode leading to a smaller neutral density than that of the α-mode. When increasing the gas pressure, the γ-mode discharge maintains a relatively higher temperature and lower neutral density, and thus, it requires a higher operating pressure to reach the α-mode. On the other hand, decreasing the pressure while maintaining α-mode, the transition to γ-mode occurs at a lower pressure than the former case due to a relatively higher neutral density of α-mode discharge. This interpretation is supported by the fact that the hysteresis disappears when the plasma properties are presented with respect to the neutral gas density instead of pressure.