Three distinct modes in a surface micro-discharge in atmospheric pressure He + N2 mixtures

A surface micro-discharge in atmospheric pressure He + N-2 mixtures is studied in this paper with an emphasis on the discharge modes. With the N-2 admixture increasing from 0.1% to 20%, the discharge evolves from a spatially diffuse mode to a filamentary mode during positive half-cycles of the applied voltage. However during the negative half-cycles, an additional patterned mode emerges between the diffuse and the filamentary modes, which has not been reported before to exist in surface micro-discharges. In the diffuse and patterned modes, the plasmas cover almost the entirety of the mesh area during one cycle after plasma ignition in all mesh elements, and the discharge power increases linearly with the applied voltage. In contrast, plasma coverage of the mesh area is only partial in the filamentary mode and the plasma is more unstable with the discharge power increasing exponentially with the applied voltage. As the surface micro-discharge evolves through the three modes, the density of excited species changes significantly, for instance, the density of N-2(+)(B) drops by similar to 20-fold from [N-2] = 0.2% to 20%. The N-2(+) (B) is predicted to be generated mainly through successive processes of Penning ionization by helium metastables and electron-impact excitation of N-2(+)(X), the latter is most responsible for the density decrease of N-2(+)(B) because much more N-2(+)(X) is converted to N-4(+)(X) as the increase of N-2 fraction. Also, the electron density and electron temperature decrease with the discharge mode transition. (C) 2015 AIP Publishing LLC.