Temporally resolved cavity ring-down spectroscopy in a pulsed nitrogen plasma

Cavity ring-down spectroscopy (CRDS) has enabled temporally resolved measurements of the N2+ ion concentration in a pulsed atmospheric pressure nitrogen plasma. A 10 ns voltage pulse is applied to a dc-sustained plasma to change the ionization fraction rapidly. Our measurements show that the pulse increases the N2+ ion concentration from 3.9×1012 to more than 1.5×1013 cm−3, and that the N2+ concentration returns to the dc level in about 10 μs. We also determine the electron density by measuring the electrical conductivity of the plasma. Because N2+ is the dominant ion, the good agreement between electrical and CRDS measurements provides validation of the temporally resolved CRDS technique.