Integrated optical emission spectroscopy and imaging diagnostics of atmospheric-pressure argon microwave-induced plasma

Abstract

Evaluating the ionization capability of microwave-induced plasma (MIP) ion sources remains challenging due to the lack of rapid, straightforward diagnostics. A rapid analysis platform integrating optical emission spectroscopy (OES) diagnostics with CCD-based plasma imaging was established in this study. An atmospheric-pressure argon MIP was characterized under varying microwave power (50–250 W) and argon flow rates (200–1000 SCCM). The results showed that T_exc ranged from approximately 4500 to 6340 K, T_rot from 758 to 1234 K, and N_e from 3.21 × 10¹⁴ to 5.55 × 10¹⁴ cm⁻³ across the tested conditions, in agreement with previously reported OES values for similar plasmas. Notably, the thermal and ionization characteristics of the plasma can be effectively regulated through optimization of the input power and gas flow rate. By analyzing the concomitant changes in plasma morphology, we further illustrate how discharge power and gas flow influence the stability and ionization behavior of the MIP. Finally, based on the calculated T_exc and N_e, the Saha framework was used to semi-quantitatively estimate the apparent ionization tendency of selected elements and compounds. The results indicate that the ion source exhibits a high ionization tendency for species with first ionization energies below 6 eV, whereas species with first ionization energies above 10 eV are difficult to ionize. Overall, the proposed integrated spectroscopic and imaging diagnostic platform provides a practical, rapid strategy for evaluating and improving MIP ion source performance. It also offers insights into further optimization of the ion source to enhance analytical capabilities.