Time Dynamics of an Inductively Coupled Plasma Torch

Inductively coupled plasma torches are important instruments in material processing and the testing of heat-shield materials. Properties of the supply circuit, swirl stabilization of the plasma core, and the interaction between fluid mechanics and plasma kinetics can lead to instabilities and time-periodic behavior. Using high-speed imaging and phased averaged optical emission spectroscopy, we investigate time dependent variations in plasma temperature in both the exit plasma plume and the plasma core, for both argon and air plasmas. The results show that the variations originate in the core of the torch and then transfer to the plume. Based on line-of-sight averaged optical emission spectroscopy, temperature fluctuations are, at most, on the order of 2-12% for argon and 2% for air. The dominant fluctuation frequency of ∼ 180 Hz does not strongly depend on input power, mass flow rate, or working gas used, over the range of operating conditions investigated. The source and nature of the fluctuations is discussed in more detail, as is their impact on the heat load experienced by material samples in the plasma plume.