Aiaa 99-3604 Diagnostics of an Atmospheric Pressure Plasma Jet

Broadband ultraviolet absorption spectroscopy is employed to measure absolute number density of C2 and CH radicals in the boundary layer of an atmospheric DC plasma assisted chemical vapor deposition (CVD) system. High spatial resolution (125 μm) is achieved by using a spatial filter and a ultra-bright argon mini-arc light source. The spectra of D Σu←X Σg electronic transition of C2 and C Σ←X ∏ electronic transition of CH are simulated by summing the related rovibronic lines. The rotational temperature determined from the peak separation of C2 P-R branches is compared with a one dimensional numerical model. The superequilibrium number densities of C2 and CH in the plasma freestream indicate that chemical nonequilibrium exists in the thermal plasma due the limited flow reaction time. The measured C2 concentration agrees well with the numerical simulations in the boundary layer near the substrate. Introduction In recent years a number of CVD techniques for diamond thin film synthesis have been successfully developed. Amongst these, atmospheric pressure CVD is one of the most promising methods due to its ability to produce large volume, high reactant density plasma, and thus attain high grow rates. In addition, positive substrate biasing can be used to promote nonequilibrium gas-phase chemistry in the boundary layer near the substrate. A factor of seven increase in Research Assistant, Member AIAA. ** Senior Research Scientist, Member AIAA. † Professor Mechanical Engineering, Member AIAA. Copyright © 1999 by Maosheng Zhao. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. growth rate has been demonstrated with an applied bias as compared to an unbiased substrate case. Until now the growth mechanism and the species contributing to diamond growth have not been fully understood, partly due to the lack of information about species concentrations in the boundary layer near the substrate. Simple hydrocarbon species such as C2, CH, CH2, CH3, C2Hx are believed to play an important role in diamond growth. In the freestream of a high enthalpy thermal plasma, the only species with significant mole fractions are H, C, C2 and CH. Measurements of C2 and CH can provide important information about the gas phase. Due to high pressure and large flow velocity, the boundary layer thickness of atmospheric thermal plasmas is usually very thin, on the order of one millimeter. Single pass ultraviolet absorption spectroscopy with its high spatial resolution provides an excellent way to probe species concentrations inside the thin boundary layer. In this article we report measurements by ultraviolet absorption spectroscopy of the C2 and CH radical density in an atmospheric DC plasma jet with input gases of CH4 and H2 diluted in argon. The concentrations of C2 and CH are monitored both in the freestream and in the boundary layer. It is found that the freestream absolute concentrations of C2 and CH are higher than their equilibrium value at the same gas temperature. Numerical modeling of the reaction boundary layer is also employed to investigate the reaction mechanism and the simulation results are compared with the spectroscopic measurements.