Characterization of periodic changes in an inductively coupled plasma using emission spectroscopy

Inductively coupled plasma (ICP) torches are used in material processing, propulsion applications, and the testing of thermal protection systems (TPS). In most cases, stable and steady operation of the plasma stream is desirable. However, properties of the power circuit, swirl stabilization of the plasma core, and the interaction between fluid mechanics and plasma kinetics can lead to instabilities and time-periodic behavior. Our ICP torch operates with argon and air plasmas at atmospheric pressure, at flow rates of a few grams per second and input powers in the range of 30 to 60 kW. Using optical emission spectroscopy, circuit measurements, and analytic estimates of plasma plume properties, we investigate the origin and impact of several periodic changes in the plasma plume on temperature and excited species. One dominant periodic mode is caused by the imperfect rectification of the AC power supply. It occurs at 180 Hz and, at atmospheric pressure, seems to cause negligible changes in the plasma temperature. Another much slower variation appears at <1 Hz, causing significant changes in the plasma temperature and warranting further characterization. Finally, we discuss the possible impact of the periodic behavior on ICP applications.