High resolution two-dimensional TDLAS diagnostics of helium metastable density in structured electrode atmospheric pressure RF He/N₂ jets driven by tailored voltage waveforms.

Radio-frequency driven micro atmospheric pressure plasma jets (µAPPJs) are widely used in medical, surface, and synthesis applications due to their ability to generate reactive oxygen and nitrogen species (RONS) at atmospheric pressure and near-room temperature. In helium-based plasmas, helium metastable species play a critical role in sustaining the discharge via Penning ionization and generating RONS, and their densities are typically measured using tunable diode laser absorption spectroscopy (TDLAS). Accurate spatial resolved diagnostics of helium metastables are essential, as they provide critical insight for optimizing discharge efficiency, but they remain challenging due to the small plasma volume. To address this, we developed an improved TDLAS method that enables high-resolution two-dimensional mapping of helium metastable densities. By expanding the laser beam with a collimator and diffuser and detecting the absorption with a camera-based optical system, the need for mechanical scanning was eliminated while preserving spatial accuracy. This method was applied to measure the two-dimensional distribution of helium metastable densities in jet discharges driven by tailored voltage waveforms with structured electrodes. The results show excellent agreement with both previous measurements and simulations, demonstrating the method's accuracy and efficiency.