Microwave micro-plasma sources based on microstrip-like transmission lines*

We study three sources based on a planar transmission line configuration, corresponding to linear resonators, which use a 2.45 GHz (1-50 W) continuous excitation to produce stable micro-plasmas at atmospheric pressure in air, Ar and He. In all sources, micro-plasmas are produced within the 50-200 $\mu $m gap created between two metal electrodes placed at the open-end of a microstrip-like transmission line. The sources design and optimization uses the numerical tool CST Microwave Studio{\textregistered} and an analytical model of the transmission line, in a complementary approach that also measures the return loss. Plasma diagnostics, based on optical emission spectroscopy measurements, enable to obtain (i) the rotational temperature ($T_{rot})$ and the vibrational temperature ($T_{vib})$, using the N$_{2}$ (in air) and the OH (in Ar and He) rovibrational spectra; (ii) the excitation temperature ($T_{exc})$ and the electron density ($n_{e})$ in Ar, using atomic line transitions and the Stark broadening of H$_{\beta }$, respectively. Typically, we obtain $T_{rot}\sim $1000 K in air, $\sim $600 K in Ar and $\sim $400 K in He; $T_{vib}\sim $5000 K in air; $T_{exc}\sim $6000 K in Ar and $\sim $4000 K in He; and $n_{e}\sim $10$^{14}$ cm$^{-3}$ in Ar.