Studies On Pattern Characteristics Of Radio Frequency Atmospheric Pressure Discharges

Radio frequency atmospheric pressure discharges (RF APDs) is widely used in the material surface treatment, biological medicine, thin film deposition and so on due to the advantage of low breakdown voltage, high electron density and simple equipment etc. Gas temperature in continuous wave RF discharge is relatively high compared with low-frequency discharge. On the other hand, the problem of gas temperature rising will be more serious as atmospheric pressure RF reactor's size, power and the content of adding chemical reactive gas increase in continuous wave RF APDs, which will limit the application field of discharge. While it helps to control the gas temperature by changing pulse duty ratio, pulse-modulated radio frequency atmospheric pressure discharges (PM RF APDs) generate new discharge characteristics:pattern discharge and glow-pattern discharge transformation. In order to get uniform PM RF APGDs without patterns, there are still a lot of discharge characteristics of the PM RF APDs is yet to be studied.To this end, the following works are done in this paper:I. Dependence of real plasma-absorbed power on net power of RF amplifierFirstly, we studied the dependence of real plasma-absorbed power on net power of RF amplifier to obtain the factual plasma-absorbed power to observe and analyze the evolution of pattern discharge with plasma-absorbed power in RF APDs. The results show that factual plasma-absorbed power is almost linear with net power of RF amplifier for constant matching capacitance.II. Studies on pattern characteristics of PM RF APDThe effects of discharge parameters on pattern characteristic in PM RF APDs are studied at a specific carrier frequency 13.56MHz. In the low pulse duty ratio, the quantity, the distance and the brightness of discharge patterns changed over the net power of RF amplifier, and manifest irreversible characteristic of hysteresis, that is, the characteristic of the pattern is related to the history of achieved this discharge power. In the high pulse duty ratio, the number and the brightness of pattern not only are maintained the characteristic of hysteresis in the low pulse duty ratio, but also pure pattern discharge can transform into glow discharge under the same low net power of RF amplifier. We obtained the partition map of glow discharge, glow-like discharge and pattern discharge by changing the pulse period and pulse duty ratio. We also studied the positive and negative feedback regions of these three kinds of discharge modes were proved to be different by changing the series capacitor Cs. Research also suggests that argon flow rate has an effect on the pattern discharge. With the increase of gas flow rate, the discharge region of pattern is far away from the inlet port and the discharge area of two dimensions is reduced one dimension. Study in pattern discharge of argon mixed with oxygen and helium, under the same peak to peak voltage, the breakdown voltage decreases, the discharge area increases and discharges gradually far away from the intake with increase of the oxygen content, while the breakdown voltage decreases, the discharge area increases and discharges gradually near the inlet with the increase of helium content. We also studied the pattern discharge in the wedge electrode, the position of small gap was vulnerable to a breakdown and the first pattern appeared. Create new patterns in large gap, which area is larger and higher brightness with RF power increased.The patterns at small gap is the first to disappear as RF power decreased.III. Studies on the pattern characteristics of continuous wave RF APDs(CW RF APDs)Adding different content of water vapor in argon, CW RF APDs will have evolution of glow, glow-pattern and pattern discharges over electron voltage. With the increase of water vapor content, glow discharge region becomes narrow and the non-uniform pattern discharge region is widened. When the water vapor content high to a certain value, once breakdown is non-uniform pattern discharge. We also studied the gap voltage as a function of conduction current at different content of water vapor. Research suggests that the slope of gap voltage-current conduction curve transit from positive value to negative value in the transition of alpha mode to gamma mode at the low content of water vapor. The negative resistance region of the gap voltage-conduction current curve increased under higher content of water vapor. When the moisture content is higher than the critical value, the positive resistance region of gap voltage-conduction current curve is disappeared.