Measurement and modeling of electric field and space-charge distributions in obstructed helium discharge

• Published in: Physics of plasmas 2015-08, Vol.22 (8)
• Main Authors: Fendel, Peter, Ganguly, Biswa N., Bletzinger, Peter
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ANODES; AXIAL SYMMETRY; CATHODES; Charge density; Charge distribution; Collision dynamics; CURRENT DENSITY; DIELECTRIC MATERIALS; ELECTRIC FIELDS; Electrodes; Electron trajectories; ELECTRONS; FINITE ELEMENT METHOD; FLUORESCENCE; Gas density; HELIUM; Modelling; Parallel plates; Plasma physics; POISSON EQUATION; Simulation; SPACE CHARGE; SPECTRA; STARK EFFECT
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.4928114

Axial and radial variations of electric field have been measured in dielectric shielded 0.025 m diameter parallel plate electrode with 0.0065 m gap for 1.6 mA, 2260 V helium dc discharge at 1.75 Torr. The axial and radial electric field profiles have been measured from the Stark splitting of 21S→11 1P transition through collision induced fluorescence from 43D→23P. The electric field values showed a strong radial variation peaking to 500 kV/m near the cathode radial boundary, and decreasing to about 100 kV/m near the anode edge, suggesting the formation of an obstructed discharge for this low nd condition, where n is the gas density and d is the gap distance. The off-axis Stark spectra showed that the electric field vector deviates from normal to the cathode surface which permits longer path electron trajectories in the inter-electrode gap. Also, the on-axis electric field gradient was very small and off-axis electric field gradient was large indicating a radially non-uniform current density. In order to obtain information about the space charge distribution in this obstructed discharge, it was modeled using the 2-d axisymmetric Poisson solver with the COMSOL finite element modeling program. The best fit to the measured electric field distribution was obtained with a space charge variation of ρ(r) = ρ0(r/r0)3, where ρ(r) is the local space charge density, ρ0 = 6 × 10−3 Coulomb/m3, r is the local radial value, and r0 is the radius of the electrode.