Abstract
The expanding cascaded arc Ar/N2 plasma has been investigated by both the active and passive optical diagnostic technologies. In the investigation, the laser Thomson scattering (LTS) and optical emission spectroscopy (OES) have been adopted to measure electron temperature (Te) and electron excitation temperature (Texc), respectively. The LTS measurements show that a remarkable nonlinear behavior of Te as a function of the N2/(Ar + N2) ratio is found, which is caused by the collective interaction between the superelastic collision and the electron-impact excitation. The superelastic collisions by the highly excited vibrational nitrogen molecules can effectively heat the free electrons, while the electron kinetic energy can be transferred to N2 molecular internal energy via the electron impact with N2 in the ground state. The difference between Texc and Te demonstrates that the cascaded arc Ar/N2 plasma significantly deviates from the local thermodynamic equilibrium. This would be useful for improving our further understanding of nonequilibrium plasma and extending applications of the cascaded arc Ar/N2 plasma.
Highlights
INTRODUCTIONCascaded arc plasma source can generate high electron density (up to 1021 m−3), low electron temperature, and steadystate plasma and has widely been applied in many fields, such as chemical vapor deposition, etching, material surface modification, and linear plasma simulator. the physical mechanism of the cascaded arc discharge under the mixture gases conditions, especially for the reactive gases (such as N2, O2), is still not fully understood
Cascaded arc plasma source1 can generate high electron density2,3, low electron temperature, and steadystate plasma and has widely been applied in many fields, such as chemical vapor deposition,4,5 etching,6,7 material surface modification,8,9 and linear plasma simulator.10,11 the physical mechanism of the cascaded arc discharge under the mixture gases conditions, especially for the reactive gases, is still not fully understood
When the optical emission spectroscopy (OES) measurement was carried out, the head of fiber bundle connected to Triple Grating Spectrometer (TGS) in the laser Thomson scattering (LTS) experiment was switched to a commercial spectrometer (Andor Shamrock 750) which has been calibrated by a broadband light source
Summary
Cascaded arc plasma source can generate high electron density (up to 1021 m−3), low electron temperature, and steadystate plasma and has widely been applied in many fields, such as chemical vapor deposition, etching, material surface modification, and linear plasma simulator. the physical mechanism of the cascaded arc discharge under the mixture gases conditions, especially for the reactive gases (such as N2, O2), is still not fully understood. The electron temperature (Te) which relates to the electron kinetic energy is one of the most important physical parameters of plasma. This concept seems to be confusing sometimes because different “temperatures” can be obtained when using different diagnostic approaches.. As an active plasma diagnostic technology, LTS allows one to obtain Te accurately without any prior assumption on the state of departure from the equilibrium of plasma.. As an active plasma diagnostic technology, LTS allows one to obtain Te accurately without any prior assumption on the state of departure from the equilibrium of plasma.13 This technique has been developed as one of the most accurate approaches and utilized to diagnose various types of plasmas.. We analyzed the deviation from LTE in Ar plasma with adding N2 gas via the comparison between the “electron temperatures” measured by these two approaches
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