Transition of low-temperature plasma similarity laws from low to high ionization degree regimes

Abstract

Similarity laws are often employed when characteristics of two or more discharge plasma systems are compared. The classical similarity laws were previously validated and applied for weakly ionized plasma discharges. However, the classical similarity laws will not be valid for all plasma regimes. Especially for strongly ionized regimes, scaling laws are not well elaborated. In this study, transition characteristics of low-temperature plasma similarity laws are evaluated from low to high ionization degree regimes. The similarity relations of plasma density and ionization degree are obtained for geometrically similar gaps. The deviations from classical similarity laws are observed, which are gradually enlarged as the ionization increases from low to high degrees. The transition characteristics are affected by the significance of nonlinear reaction processes (such as three-body collisions), resulting in that the classical similarity relations hold up to a higher ionization degree at low pressure than that at high pressure. The time-dependent scaling characteristics of species densities and electron kinetic behaviors in geometrically similar gaps are also examined. The results are beneficial for utilizing the similarity laws in a wide range of ionization degree regimes, which is essential for correlating plasma characteristics in geometrical similar vessels of various scales.

Similarity properties in capacitive radio frequency plasmas with nonlinear collision processes

“…More recently, Sharma et al [32][33][34] studied the very high-frequency capacitive discharges and found that driving frequency has a considerable influence on the energy distribution functions of the electrons and ions. Fortunately, similarity laws (SL) are effective methods for reducing the independent variables for gas discharge plasmas, which usually determine the discharge characteristics with combined parameters, such as the reduced gap length pd (gas pressure times gap distance) and reduced electric field E/p (electric field divided by gas pressure) [35][36][37]. The pioneering work on similarity theory was contributed by Paschen [38], Townsend [39], Holm [40], von Engle [41] and many others [42][43][44].…”