We demonstrate that a balance between convective helium flow and ambient air diffusion creates a unique ring-shaped light emission profile by means of finite element analysis of the atmospheric pressure RF-excited plasma needle. The plasma needle has a point-to-plane geometry with a radius of 30 νm at the tip and an inter-electrode gap of 1 mm. We employ a coupled model between time-dependent plasma dynamics based on a fluid model and steady state neutral gas flow in two-dimensional cylindrical coordinates. When the mean inlet gas velocity is 1.5 m s-1 and the discharge is in high-power glow mode at 200 mW, the concentration of air drastically increases near a treated surface being away from the needle tip. As a result, Penning ionization by helium metastables and air (nitrogen) peaks at an off-axis position, corresponding to the ring-shaped emission profile in cylindrical coordinates. The off-axis ionization peak leads to an off-axis flux peak of nitrogen ions onto the treated surface. The 'ion wind' and gas heating have only minor effects on the discharge structure under the conditions considered here.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics