Nonthermal atmospheric rf plasma in one-dimensional spherical coordinates: Asymmetric sheath structure and the discharge mechanism

Journal Article · Sun Apr 01 00:00:00 EDT 2007 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.2715745· OSTI ID:20982789

Sakiyama, Yukinori; Graves, David B ^[1]

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1. Department of Mechanical Engineering, University of Tokyo, Tokyo 113-8656 (Japan)

We present one-dimensional simulations of atmospheric pressure rf-excited plasma with two concentric spherical electrodes and the inner electrode powered. The gas used is helium with 0.1% nitrogen addition. The gap distance between the inner and outer electrodes is 1 mm. The coupled continuity equations and electron energy equation are solved with Poisson's equation using the finite element method. A mode transition is observed in the discharge power-voltage curve between 1 and 1000 mW. In the low power mode, ionization rate peaks only near the inner electrode. The electron-impact excitation and ionization rates peak in the local cathodic phase. In the high power mode, the rate of ionization peaks near the outer electrode as well as the inner electrode. The inner sheath significantly shrinks and the direct electron-impact ionization is the primary ionization reaction near the inner electrode. The ionization rate near the outer electrode is due to Ohmic sheath oscillation heating of electrons, resulting in a peak in metastable helium creation. Penning ionization is the major ionization reaction near the outer electrode. Thus, two different ionization mechanisms coexist near the inner and outer electrodes. Electron heating near the outer electrode may have implications for surface processing in atmospheric pressure microdischarges. The local field approximation (LFA) in high power mode fails to predict the ionization rate peak near the outer electrode due to its inability to properly account for electron diffusion in the presence of both a strong electric field and electron density gradient. However, use of the LFA is adequate to model the low power mode and it correctly predicts the existence of the mode transition.

• OSTI ID: 20982789
• Journal Information: Journal of Applied Physics, Vol. 101, Issue 7; Other Information: DOI: 10.1063/1.2715745; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
APPROXIMATIONS
ASYMMETRY
ATMOSPHERIC PRESSURE
CONTINUITY EQUATIONS
COORDINATES
ELECTRIC FIELDS
ELECTRIC POTENTIAL
ELECTRODES
ELECTRON DENSITY
ELECTRONS
EXCITATION
FINITE ELEMENT METHOD
HELIUM
HIGH-FREQUENCY DISCHARGES
IONIZATION
NITROGEN ADDITIONS
PLASMA
PLASMA SHEATH
PLASMA SIMULATION
POISSON EQUATION