Impact of Nitrogen Addition on Spatial Profiles of Electron Density and Mode Transition in Microwave-Driven Ar – N₂ Plasmas

The spatial profile and time-resolved density of metastable Ar species and distribution of electron density in microwave-driven Ar – N₂ plasmas operating at 2.45 GHz are explored using Laser-induced fluorescence and probe diagnostics. Radially resolved electron density is conducted alongside imaging of the plasma diameter using an ICCD camera. The study examines the effects of gas pressure and microwave power, in the range of 70 mTorr to 1 Torr, and 25-150 W. The transitions between the two operating regimes (underdense and overdense), driven by variations in delivered power and operating gas pressure, have been characterized. An abrupt transition between the two modes occurs near the critical power. The critical power required for this mode transition decreases as the pressure increases. This transition phenomenon is further corroborated by analyzing plasma diameter changes across various pressure and power settings. The study extends to mapping the radial profile of electron density and plasma size by introducing nitrogen admixture (ranging from 0 to 20 %) into the argon plasma environment. The influence of the percentage of nitrogen in Ar – N₂ plasma on the mode transition is investigated. Additionally, the experimental findings are compared with simulations conducted using CRANE and ZAPDOS plasma simulation applications in the Multiphysics Object-Oriented Simulation Environment (MOOSE) platform.