Radical probe system for in situ measurements of radical densities of hydrogen, oxygen, and nitrogen

In this study, an in situ catalytic radical probe system together with a software platform is developed to measure concentrations of reactive species in low-temperature plasmas with high spatial resolutions. The radical probes can be used to determine radical densities of hydrogen, nitrogen, and oxygen independently, in pairs and often simultaneously in any continuous plasma source in a vacuum environment. The basic principle and advantage of a probe array is the capability to distinguish between different gas species due to several sensitive elements acting as recombination catalysts. Radical densities of hydrogen, nitrogen, and oxygen were measured in a helicon plasma source. Generally, it is observed that radical densities increase with respect to pressure and power. Additionally, the electron density and electron temperature were measured by Langmuir probes. The electron density increased with increasing power and pressure. Electron temperature increased with power but decreased with increasing pressure. The key to getting absolute numbers of radical densities is based on knowing the recombination coefficient of the given gas on the catalytic surface. The probe system measures densities in a broad range of reactive species’ concentrations varying from about 1013 to 1015 cm−3.