Enhanced Catalytic Probe Design for Mapping Radical Density in the Plasma Afterglow

The electrification of chemical processes using plasma generates an increasing demand for sensors, monitoring concentrations of plasma-activated species such as radicals. Radical probes are a low-cost in situ method for spatially resolved quantification of the radical density in a plasma afterglow using the heat from the exothermal recombination of radicals on a catalytic surface. However, distinguishing recombination heating from other heat fluxes in the system is challenging. In this study, we present a heat flux analysis based on probe measurements inside the reactor, with simultaneous IR imaging monitoring of the temperature of the reactor wall. The impact of radiation heat on a single thermocouple as well as the advantage of a dual thermocouple setup (using a catalytic unit together with a reference thermocouple) is shown. We add a heat sink with a monitored temperature to the dual thermocouple setup, allowing the determination of conductive and radiative heat fluxes. The heat sink gives more information on the measurement and reduces ambiguities in the evaluation used by others. The probe was tested by mapping N atom densities throughout the plasma afterglow of our reactor, enabling evaluation of the recombination kinetics of the radicals in the gas phase. Three-body recombination was shown to be the main pathway of recombination, with a recombination rate of <i>k</i>_rec = (2.0 ± 0.9)·10^-44 m⁶/s, which is in line with the known literature findings, demonstrating that the measured species are N radicals and the probe did not influence the plasma or recombination reactions in the afterglow.