Scheme of laser beam deflection within the gradient field of the refractive index n caused by an axially symmetric plasma filament. The notation of trajectory parameters used in the equations is shown in the diagram. The trajectory is plotted schematically.
Proposed refractive index profile and related temperature profile describing an axial symmetric plasma filament (a). Corresponding deflection angle of a laser beam depending on the impact parameter (b). The absolute value of the deflection rises sharply to a maximum with ρ approaching zero and falls to zero (no deflection) for the exact center of the filament.
Schematic drawing of the non-thermal atmospheric pressure plasma jet. The gas flows through the quartz capillary with ring electrodes in between the filamentary plasma is ignited. Indicated are the locations where the diagnostics are applied: effluent (LSD) and active discharge zone (OES).
Experimental set-up of LSD used for diagnostic of the plasma jet. The cross-section of the jet displays three rotating plasma filaments (LM3). The laser beam passes the plasma effluent perpendicularly to the jet axis. The movement of the laser spot behind the jet is detected either with a fast camera or with a photodiode. The coordinates and intensity profile of the spot are evaluated by means of a PC or by an oscilloscope using real-time FFT mode.
Typical spectrum of the measured LSD signal with the fundamental frequency f 1 corresponding to the LM3, where 3 filaments rotate with frequency of 70/3 Hz (a). Correlation diagram for the optical emission signal and the fundamental frequency of the LSD signal (b).
Photograph of the laser spot on the screen (a). The result of the video spot tracking (b). The square points show the various positions of the laser beam centre on the screen during the measurement. The maximal deflection δ1 and error interval are marked.
Effect of the filament thickness on the resulting frequency spectrum. LSD signal produced with a thin profile of refractive index (small FWHM) (a) generates a large frequency spectrum (b). Broader FWHM of the profile (c) causes the dominant amplitude of the fundamental frequency in a FFT spectrum (d). The proportion of the fundamental frequency and higher harmonic frequencies is a significant marker for the thickness of the profile.
Measurement of amplitudes for the fundamental and second harmonic frequency as a function of the axial position of the LSD measurement in the effluent. The amplitude ratio has been calculated and the linear fit of the result is plotted, too (a). The effect of the axial position on the profile width. Two typical regions are indicated regarding the results of the frequency analysis and calculation of profile width (b).
The widths of the refractive index profiles and the corresponding ratios of FFT amplitudes for second harmonic frequency a 2 and fundamental frequency a 1.
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