Ruediger Foest

Laser schlieren deflectometry for temperature analysis of filamentary non-thermal atmospheric pressure plasma

The heat convection generated by micro filaments of a self-organized non-thermal atmospheric pressure plasma jet in Ar is characterized by employing laser schlieren deflectometry (LSD). It is demonstrated as a proof of principle, that the spatial and temporal changes of the refractive index n in the optical beam path related to the neutral gas temperature of the plasma jet can be monitored and evaluated simultaneously. The refraction of a laser beam in a high gradient field of n(r) with cylindrical symmetry is given for a general real refraction index profile. However, the usually applied Abel approach represents an ill-posed problem and in particular for this plasma configuration. A simple analytical model is proposed in order to minimize the statistical error. Based on that, the temperature profile, specifically the absolute temperature in the filament core, the FWHM, and the frequencies of the collective filament dynamics are obtained for non-stationary conditions. For a gas temperature of 700 K inside the filament, the presented model predicts maximum deflection angles of the laser beam of 0.3 mrad which is in accordance to the experimental results obtained with LSD. Furthermore, the experimentally obtained FWHM of the temperature profile produced by the filament at the end of capillary is (1.5 ± 0.2) mm, which is about 10 times wider than the visual radius of the filament. The obtained maximum temperature in the effluent is (450 ± 30) K and is in consistence with results of other techniques. The study demonstrates that LSD represents a useful low-cost method for monitoring the spatiotemporal behaviour of microdischarges and allows to uncover their dynamic characteristics, e.g., the temperature profile even for challenging diagnostic conditions such as moving thin discharge filaments. The method is not restricted to the miniaturized and self-organized plasma studied here. Instead, it can be readily applied to other configurations that produce measurable gradients of refractive index by local gas heating and opens new diagnostics prospects particularly for microplasmas.

Visualization of Revolving Modes in RF and MW Nonthermal Atmospheric Pressure Plasma Jets

The high-speed sequence imaging of two different atmospheric pressure plasma jets illuminates the low-frequency dynamics of discharge filaments. Usually, atmospheric plasma jets consist of several contracted filaments occurring in the active zone of the plasma randomly in space and stochastically in time. However, particular plasma excitation schemes support collective regimes where the filaments are self-organized at large spatial and temporal scales, even at atmospheric pressure. Moreover, self-organized dynamics can act in a similar manner at different types of plasma. In this paper, evident similarities in filament patterning are pointed out for radio-frequency and microwave atmospheric pressure jets by publishing images of short exposure time (6 ms) from which the filament movement can be derived.