Microwave air plasmas in capillaries at low pressure II. Experimental investigation

Abstract

This work presents an experimental study of microwave (2.45 GHz excitation frequency) micro-plasmas, generated in dry air (N2 80%: O2 20%) within a small radius silica capillary (345 µm inner radius) at low pressure (300 Pa) and low powers (80–130 W). Experimental diagnostics are performed using optical emission spectroscopy calibrated in absolute intensity. Axial-resolved measurements (50 µm spatial resolution) of atomic transitions N(3p4S) → N(3s4P); O(3p5P) → O(3s5S) and molecular transitions N2(C,v′) → N2(B,v″); (B,v′) → (X,v″) allow us to obtain, as a function of the coupled power, the absolute densities of N(3p4S), O(3p5P), N2(C), N2(B) and (B), as well as the gas (rotational) temperature (700–1000 K), the vibrational temperature of N2(C,v) (7000–10 000 K) and the excitation temperatures of N2(C) and N2(B) (11 000 K). The analysis of the Hβ line-width gives an upper limiting value of 1013 cm−3 for the electron density; its axial variation (4 × 1011–6 × 1012 cm−3) being estimated by solving the wave electrodynamics equations for the present geometry, plasma length and electron–neutral collision frequency. The experimental results were compared with the results from a 0D model, presented in companion paper I [], which couples the system of rate balance equations for the dominant neutral and charged plasma species to the homogeneous two-term electron Boltzmann equation, taking the measured gas temperature and the estimated electron density as input parameters. Good qualitative agreement is found between the measurements and calculations of the local species densities for various powers and axial positions. The dissociation degree of oxygen is found above 10%. Moreover, both the measurements and calculations show evidence of the non-equilibrium behavior of low-temperature plasmas, with vibrational and excitation temperatures at least ten times higher than the gas temperature. These observations confirm that low-pressure microwave micro-plasmas exhibit high-reactivity, and the present study shows that these features are controlled by electron kinetics, as expected.