Spatial Dynamics of Oscillations in a Thermal Plasma Jet

Summary form only given. Optical radiation of a D.C. plasma torch working with argon at various flow rates was observed by an optical system consisting of arrays of optical fibers and photodiodes (each array formed by 15 elements) arranged at 2 levels above the nozzle and at 4 directions separated by angle intervals 45deg. The sample rate of the electronic equipment recording the optical data was 468 kHz/channel and the data length was 1 Msample/channel. The distribution of the plasma jet radiation in each of 2 cross sections was evaluated from 4 measured profiles using the inverse Radon transform. The previous results based on experimental data acquired in the same experimental conditions from just one direction by a CCD camera have shown that various oscillation modes in the plasma jet are characterized not only by various spatial distributions of intensities but also by various levels of temporal stability. The evaluations of radial distributions of oscillation amplitudes acquired by wavelet analysis from the present date at various scales (corresponding to frequencies which may be obtained by FFT) completed the previous results by new findings concerning distributions of oscillations and their 2D stability in planes perpendicular to the jet axis. The results depend on the gas flow rate and oscillation frequency. For the gas flow rate 30 slm, the oscillations coming from the basic residual ripple modulation of the arc current at 300 Hz have their maxima at the central part of the plasma jet and their distribution in the plane perpendicular to the jet axis is unstable. On the other hand, the maximal amplitudes of the first harmonic of this frequency at 600 Hz form a very stable ring around the jet axis. The hydrodynamic oscillations at higher frequencies (1-4 kHz) are characterized by highly unstable distributions with maxima located mostly at the boundary regions of the jet. The analysis of the records based on complex wavelets may be also used for evaluations of phases showing the advancement of oscillation waves. Some of these motions have a rotational character. The current results together with the previous ones acquired by CCD cameras show a highly diverse range of grounds and consequences responsible for instabilities in thermal plasma jets.