A new measurement technique for non-equilibrium plasmas

A better understanding of non-equilibrium plasma properties is relevant to a variety of aerospace, biomedical and other applications. Currently, there are two common optical techniques for measuring such properties: tunable diode laser spectroscopy (TDLAS) and two-photon absorption laser-induced fluorescence (TALIF). However, optical techniques can be difficult to employ due to the light emitted by the plasma, as well as the physical configurations of existing test facilities.

This article reports the development of a spatially-resolved method for measuring neutral species inside non-equilibrium plasmas. The researchers demonstrate the utility of coherent microwave Rayleigh scattering for detection of resonance-enhanced multiphoton ionization — a technique called radar REMPI — to measure atomic oxygen in a pulsed plasma discharge.

As opposed to TDLAS and TALIF, radar REMPI can be performed with microwave scattering rather than optical imaging or light collection. The technique uses a multiphoton process to liberate electrons from selected energy levels within the target atoms or molecules, in this case the atomic oxygen. A coherent, homodyne microwave scattering system then counts the number of electrons liberated by the ionization process. The researchers measured atomic oxygen within a pulsed discharge at 100 Torr in a molecular oxygen and helium gas mixture. The data matched well with known TALIF results.

The researchers also argue that because radar REMPI is immune to background emission from the plasma, it is likely to provide better signal quality than techniques such as TALIF.

Radar REMPI provides a robust, practical method for measurement of non-equilibrium environments. It gives results consistent with more established techniques, but is easier to implement into a non-laboratory setting, which could open up the study of non-equilibrium plasmas for new applications.

Source: “Radar resonance-enhanced multiphoton ionization for measurement of atomic oxygen in non-equilibrium pulsed plasmas,” by Mark Gragston, Jordan Sawyer, Steven F. Adams, Yue Wu, and Zhili Zhang, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5091854 .