AVS 53rd International Symposium
    Plasma Science and Technology Wednesday Sessions
       Session PS2-WeA

Paper PS2-WeA9
Finite Element Analysis of Atmospheric Pressure RF-excited Plasma Needle

Wednesday, November 15, 2006, 4:40 pm, Room 2011

Session: Atmospheric and Microplasmas
Presenter: Y. Sakiyama, University of Tokyo, Japan
Authors: Y. Sakiyama, University of Tokyo, Japan
D.B. Graves, University of California, Berkeley
Correspondent: Click to Email
The atmospheric pressure RF-excited plasma needle is a non-thermal discharge powered at 13.56 MHz with a localized plasma sustained at the sharp tip of a thin cylindrical conducting electrode. Using a finite element solution to the governing fluid equations, we identify two discharge modes of the plasma needle as well as the transition mechanism. The gas used is helium with 0.1% nitrogen addition. The needle has a point-to-plane geometry with a radius of 3 µm at the tip, 150 µm at the base and an inter-electrode gap of 1 mm. The plasma needle operates as a corona discharge at low power and a glow discharge above a critical power. The discharge power increases but the discharge voltage drops abruptly by a factor of about 2 in the corona-glow transition. In corona-mode, the peak plasma density and ionization is confined near the needle tip. Penning ionization of the trace nitrogen gas is the dominant ionization reaction and displacement current dominates over the conduction current. On the other hand, the plasma spreads back along the needle surface in glow-mode. Direct ionization of helium prevails over Penning ionization and conduction current accounts for 80% of the total current in glow mode. The corona-glow transition is also characterized by a dramatic decrease in sheath thickness and an order of magnitude increase in plasma density and volume-averaged ionization. The transition is observed whether or not secondary electron emission is included in the model. The influence of species such as oxygen and water vapor, and the role of forced convection towards a surface to be treated, will be also presented. Experimental validation of the model predictions is discussed.