Spatial dynamics of oscillations in a thermal plasma jet

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 4 directions and at 2 levels above the nozzle.The evaluations of radial distributions of oscillation amplitudes acquired by wavelet analysis from the present data 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.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.