Absolute Number Density Measurement of OH Radicals in Low Temperature Atmospheric Pressure Plasmas using Cavity Ringdown Spectroscopy
CommitteeMonts, David L.
Arnoldus, Henk F.
Reese, Robert B.
Embargo TypeMSU Only Indefinitely
Low-temperature non–thermal plasmas are of growing interest due to their applications in various fields, such as plasma-assisted combustion, plasma medicine, material processing, etc. Hydroxyl radical (OH) is one of the key agents and most important reactive species generated in plasmas. We employ cavity ringdown spectroscopy (CRDS), both a pulsed laser and a continuous wave (cw) laser to measure absolute number densities of OH radicals in low-temperature plasmas. A 2.45 GHz microwave plasma source was used to excite two different types of plasma cavities: an atmospheric plasma jet and microwave plasma torch (MPT). The atmospheric microwave plasma jet was thoroughly explored and operated with different plasma gases. Plasma jets with argon (Ar), helium (He), Ar/N2, Ar/O2, He/N2, He/O2 and Ar/H2O were investigated. The absolute number densities of OH radicals were measured along the jet axis in all of plasma jets using pulsed CRDS. Effects of plasma power and gas flow rates on OH radical generation were also studied. We have reported for the first time that OH radicals exist in the far downstream region of a plasma jet axis. The far downstream is a location where the ratio of distance from the plasma jet orifice over the plasma jet column length is larger than 3. For an Ar plasma jet length of 3 mm, OH radicals were detected at a farthest distance ratio of 7.6. The OH density profiles along the axis in all the plasma jets indicate that OH radicals have the highest number density in the vicinity of the jet tip and gradually decreases in the downstream. Optical emission spectroscopy and digital imaging were simultaneously employed to identify the different radicals generated in plasma jets and to study the fine structures of the plasma jets. Pulsed CRDS was also employed to measure OH radical density in an Ar MPT. By using high temporal resolved imaging, it was observed that the widely reported converging point in Ar MPT is actually a time-averaged visual effect. Absolute number densities of OH radicals and water molecules were measured in an alternating current (AC) glow discharge using near infrared cw CRDS.