Finite element analysis of an atmospheric pressure RF-excited plasma needle

We report results from a two-dimensional, axisymmetric numerical simulation of the plasma needle, powered at 13.56 MHz. The atmospheric pressure discharge is simulated in helium with a small amount of nitrogen. The needle has a point-to-plane geometry with a radius of 30 mu m at the tip and an inter-electrode gap of 1 mm. We employ the one-moment fluid model with local field approximation. The coupled continuity equations for electrons, ions and metastables are solved with Poisson's equation using the finite element method with an unstructured grid. The discharge voltage - power characteristic demonstrates a region in which multiple solutions exist for a given applied RF voltage. This mode transition in the plasma needle resembles an alpha-gamma transition from a lower plasma density regime with a relatively thick sheath at the needle tip to a higher plasma density regime with a relatively thin sheath at the needle tip.