Power modulated microwave plasma jet operating in argon at atmospheric pressure was studied by spatio-temporally resolved optical emission spectroscopy (OES) in order to clarify the influence of modulation on plasma parameters. OES was carried out in OH, NH, N 2 and + N 2 spectral regions using a spectrometer with intensified CCD detector synchronised with 10 1 -10 3 Hz sine modulating signal. A special software, able to fit even the overlapping spectra, was developed to batch process the massive datasets produced by this spatio-temporal study. Results show that studied species with the exception of + N 2 have balanced rotational and vibrational temperatures across the modulation frequencies. Significant influence of modulation can be clearly observed on temperature spatial gradients. Whereas for low modulation frequencies where the temperatures reach sharp maxima upon discharge tip, the high frequency modulation produces thermally homogeneous plasma.

The effects of amplitude modulation (AM) on an atmospheric pressure microwave argon jet is investigated using time-resolved optical emission spectroscopy, passive acoustic diagnostic and digital camera imaging. These techniques show significant changes of the effluent plasma properties with varying AM frequency. Operation in AM mode can enhance the plasma jet length or width over continuous-wave mode with the same mean power, which could be advantageous in many practical applications of plasma jets.

Microwave plasma jet operating in atmospheric pressure argon was power modulated by audio frequency sine envelope in the 10 2 W power range. Its efluent was imaged using interference ilters and ICCD camera for several different phases of the modulating signal. The combination of this fast imaging with spatially resolved optical emission spectroscopy provides useful insights into the plasmachemical processes involved. Phase-resolved schlieren photography was performed to visualize the gas dynamics. The results show that for higher modulation frequencies the plasma chemistry is strongly inluenced by formation of transient low perturbation resembling a vortex during each period. The perturbation formation and speed are strongly inluenced by the frequency and power variations while they depend only weakly on the working gas low rate. From application point of view, the perturbation presence signiicantly broadened lateral distribution of active species, effectively increasing cross-sectional area suitable for applications.

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Abstract-This paper describes the gliding arc operated in four different noble gases (helium, neon, argon, and krypton) under normal gravity and hypergravity conditions up to 18 g. We studied the influence of gas flow, gas properties, and gravity-dependent buoyancy on the gliding arc behavior.Index Terms-Atmospheric-pressure plasma, buoyancy, gas flow, gravity. GLIDING arc (GA) [1] has been studied for decades, as it is not only a scientifically compelling demonstration of dynamic plasma phenomena, but also a useful plasma source for many applications [2]. In these, the GA is typically operated in air at atmospheric pressure. However, less attention has been paid to the GAs in noble gases.A typical feature of the GA is its movement along slanted electrodes. The discharge ignites at the shortest distance between the electrodes, then moves upward and finally at maximum elongation it extinguishes owing to insufficient power. The upward movement is due to two phenomena: 1) the GA can be dragged up with upstream gas flow and 2) a thermal buoyancy caused by the difference in mass density between the heated plasma column and the cold surrounding atmosphere is also directed upward. In case of a low or zero gas flow, the buoyancy can be the dominant force moving the GA. As the buoyancy is naturally related to the gas mass density, it increases with rising atomic mass of the used gas and the gravity level.The effect of gravity on plasmas has been investigated before [3], but to our best knowledge, nobody has studied the GA in hypergravity which tends to emphasize the thermal buoyancy over the gas flow. We did so, using Large Diameter Centrifuge (LDC) [4] in ESA/ESTEC facility in the range of 1 g-18 g. We used nearly quarter-elliptic copper electrodes with a minimum distance of 4.5 mm and an initial angle between them of 36°[5], [6]. These electrodes were connected to a current limiting high-voltage transformer supplying 0-10 kV at 50 Hz ac. The configuration was enclosed in a nonconductive discharge chamber with 2 dm 3 inner volume. Front and back walls were made of a heat-resistant glass allowing direct optical observation by digital cameras (Nikon D3100, Canon Power Shot A460). Four noble gases-He, Ne, Ar, and Kr-were selected to cover a wide range of gas mass densities. Their atomic masses are 4, 20, 40, and 84 a.m.u., respectively, giving the mass ratios 1:5:10:21.Long exposure images of the GAs in four noble gases at 1 g are shown in Fig. 1. The gas flow and discharge voltage are slightly different for each gas to maintain stable gliding movement of the arc. Operating in ac, the arc filament periodically brightens and dims, which in combination with upward motion, forms a typical striped structure in the images. This allows a direct visualization of spatiotemporal GA evolution with 10 ms time intervals. The main difference in operation of the GA in different noble gases is, besides the apparent color due to dominant spectral lines, the minimum gas flow needed for the arc to glide (0.4 slm for He, 0.15 slm for Ne, and...