Plasma diagnostics and modeling of direct current microplasma discharges at atmospheric pressure.

摘要

High pressure (100s of torr) microplasma (length scale 100s of micrometer) non-equilibrium discharges have potential applications as chemical microreactors, sensors, microelectromechanical systems (MEMS), and excimer radiation sources. Experimental and theoretical studies of these microplasmas can provide critical information on fundamental discharge characteristics, and help extend the window of stable discharge operation. Spatially resolved measurements (resolution ∼ 6 mum) were taken across a 200 mum slot-type microdischarge in atmospheric pressure helium or argon. Small amounts of actinometer gases were added to the flow for optical emission spectroscopy measurements. Gas temperature profiles were determined from N2 emission rotational spectroscopy. Stark splitting of the hydrogen Balmer-beta (Hbeta ) line was used to investigate the electric field distribution in the cathode sheath region. Electron densities were evaluated from the analysis of the spectral line broadenings of Hbeta emission. The measured gas temperature was in the range of 350--650 K in He, and 600--1200 K in Ar, both peaking near the cathode and increasing with power. The electron density in the bulk plasma was in the range (3-7)x1013 cm -3 in He, and (1-4)x1014 cm-3 in Ar. The measured electric field in He peaked at the cathode and decayed to small values over a distance of ∼50 mum (sheath edge) from the cathode.;The experimental data were also used to validate a self-consistent one-dimensional plasma model. By a combination of measurements and simulation it was found that the dominant gas heating mechanism in DC microplasmas was ion Joule heating. Simulation results also predicted the existence of electric field reversals in the negative glow under operating conditions that favor a high electron diffusion flux emanating from the cathode sheath. The electric field adjusted to satisfy continuity of the total current. Also, the electric field in the anode layer was self adjusted to be positive or negative to satisfy the "global" particle balance in the plasma. Gas heating was found to play an important role in shaping the electric field profiles both in the negative glow and the anode layer.;Furthermore, the effect of gas flow on gas temperature in these microplasmas was investigated again by a combination of experiments and simulation. A plasma-gas flow simulation of the microdischarge, including a comprehensive chemistry set, a compressible Navior-Stokes (and mass continuity) equation, and a convective heat transport equation, was performed. The gas temperature was found to decrease with increasing gas flow rate, more so in argon compared to helium. This was consistent with the fact that conductive heat losses dominate in the helium microplasma, while convective heat losses play a major role in the argon microplasma. The experimental measurements of gas temperature spatial distributions as a function of gas flow rate were in good agreement with the simulation predictions.