Abstract
Spatially resolved noise amplitude spectra of emission signals from an inductively coupled plasma (ICP) were obtained by moving the ICP source along the vertical and horizontal axes. In the noise amplitude spectra, an audio frequency (AF) noise was observed at a frequency of ca. 280Hz. The noise frequency was-independent of the spatial positions of the ICP and coincided with that shown in high-speed motion pictures of the plasma. By putting a 100mm extension tube on the plasma torch, the AF noise disappeared in high-speed motion pictures as well as in the noise amplitude spectra. These results confirm that the AF noise observed under our experimental conditions is due to vortices formed at the boundary region as the plasma gas flows from the ICP torch into the surrounding static atmosphere.
