Microwave microplasma sources based on microstrip-like transmission lines

Abstract.

In this paper, we study two microwave sources based on a planar transmission line configuration, corresponding to linear resonators. In both 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 study of the sources follows a complementary approach that uses simulation and experiment. Simulations analyze the electromagnetic behavior of the sources, using the commercial tool CST Microwave Studio^®, and characterize the plasmas produced, using a fluid-type code to describe the dynamics of charged particles. Experimentally, the return loss of the sources (hence their quality factors) is measured without and with plasma. Plasma diagnostics (in air and in argon), based on optical emission spectroscopy measurements, enable to obtain the typical plasma temperatures and the electron density (using Stark broadening measurements of the H$_{\beta}$ line-emission profile). Results reveal that the sources have similar quality factors (~15–20), yielding high-density (~10¹⁴ cm$^{-3})$, low-power (~10–50 W), non-equilibrium micro-plasmas (with rotational temperatures of ~950–1400 K in air and ~550–630 K in argon, vibrational temperatures of ~5200–5800 K in air and excitation temperatures of ~5800 K in argon), over volumes of ~10^-4–10^-3 cm³.