Visualization of the vortex structure formation of a long-lived afterglow arising in a symmetrical pulsed discharge

A visualization of the emerging toroidal vortex in the long-lived afterglow of a high-current symmetrical pulse discharge with an electrolyte anode has been carried out. The visualization of the emerging vortex flow was carried out using the laser knife method using a powerful 10 W laser, which formed a flat divergent beam, which was reflected from dust particles in the atmosphere. For greater contrast, water vapor was let in into the region of the discharge and the emerging afterglow, which moved synchronously with the emerging afterglow vortex. The laser beam reflected from small water vapor droplets made it possible to visualize the gas flow, which was then filmed with a high-speed video camera. The presented data clearly show that the emerging afterglows are able to retain their volume for a long time precisely due to the formation of a vortex structure.


Introduction
For a long time now, the problem of the natural phenomenon of ball lightning has still remained unexplored, despite the large number of hypotheses and experiments performed that make it possible to reproduce only some properties of this very rare and unique natural phenomenon.One of the most interesting experiments that make it possible to obtain remotely resembling analogues of natural ball lightning with a lifetime of up to one second is a high-current pulsed discharge of a symmetrical configuration with an electrolyte electrode.This discharge is ignited by a pre-charged capacitor bank between the surface of a highly conductive solution (anode) and a centrally located metal or carbon electrode (cathode).A distinctive feature of this discharge is that before the discharge current is turned off, a luminous formation of a round shape with clear boundaries separates from it, which exists for quite a long time after the discharge current break off.This discharge was first described in [1,2] more than two decades years ago, where it's basic characteristics and the resulting long-lived afterglow were described.In a large number of subsequent works, this discharge was studied in detail by various scientific groups around the world [3][4][5][6][7][8][9][10][11][12].In [3][4][5][6], an analysis was carried out in order to establish the relationship between the shape, size, speed and time of autonomous existence of the discharge afterglow with energy.In [7,8], the spectral characteristics of the discharge in the infrared region were measured, and the discharge products were analyzed using mass spectrometry.
Since in most works the purpose of studying this gas discharge was to model some properties of the natural phenomenon of ball lightning, many of them studied the dynamics of an emerging autonomous luminous formation in order to understand the reason for its ability to maintain its volume for a long time.For example, in [9], a high-speed video shooting of the discharge was carried out in order to conduct a more detailed analysis of the dynamics of the formation of a long-lived afterglow.In [10], using observations in the infrared spectrum, it was noted that the afterglow exists for more than 1 second and has a characteristic ring shape, from which it was concluded that a toroidal vortex structure is formed inside the afterglow.The idea of the presence of a toroidal vortex in the forming long-lived afterglow was developed in [11,12], where a gas-dynamic model of this nonstationary discharge was proposed.Such a toroidal vortex structure may well be the reason for the stable existence of afterglow in free space, just as, for example, this is the case with smoke rings.For this reason, further study of the formation mechanism of such a toroidal vortex structure can be of great importance both for understanding the stable existence of the emerging long-lived afterglow and for finding a way to increase its lifetime and make its shape more stable.The purpose of this work is to study the dynamics of the formation of the vortex structure of the emerging long-lived afterglow by visualizing the gas-dynamic discharge flow using the laser light sheet flow visualization system [13].

Experimental setup
The gas-discharge system used is in many respects similar to those described in the works cited above.The main element of the experimental setup (Figure 1) was a cylindrical vessel 300 mm in diameter and 300 mm high, which was filled with a strong solution of baking soda.An electrode made of thick copper wire in the form of a ring was immersed at the bottom of this vessel, which provided electrical contact between the highly conductive solution and the positive pole of a 600 μF storage capacitor bank.A relay was connected in series between the ring electrode and the positive pole of the capacitor bank, which ensured the ignition of the discharge.The negative pole of the capacitor bank was connected to a carbon electrode, which was located in the center of the vessel and was isolated from the conductive solution using a dielectric tube.The tip of the central electrode was installed 1-1.5 mm above the surface of the conductive solution.To limit the discharge current, a ballast resistance was connected in series between the central electrode and the negative pole of the capacitor bank.The capacitor battery was charged from an external source of high voltage up to 5-7 kV, which was quite sufficient for the breakdown of the air gap between the central electrode and the surface of the conducting solution.The time dependences of the voltage on the discharge cell and the discharge current were taken using a highvoltage probe and a non-contact current probe.Current and voltage data were recorded on a digital oscilloscope.
To visualize the gas flows formed in the discharge, a powerful 10 W green laser (532 nm) was used, the beam of which was directed to a horizontally located cylindrical glass rod 6 mm in diameter.This made it possible to simultaneously form a sufficiently wide and bright laser light sheet, which was directed through the axis of a cylindrical vessel and captured almost the entire active region of the discharge and the forming long-lived afterglow.The light, reflected from the dust in the air, formed a clear visual picture of the flows arising in the discharge, which was filmed on a video camera.When conducting a high-speed survey, the intensity of the light scattered from dust particles in the air was insufficient.In this case, water vapor was additionally admitted into the discharge region, which provided a higher scattered light intensity.In order to separate the green light of the laser from the glow of the discharge itself and to minimize the effect of the latter, a special light filter was used, which transmitted only the green light of the laser.

Formation of a vortex structure in a long-lived autonomous afterglow of a pulsed discharge
The video filming of vortex flows clearly confirmed the results of previous studies, in which the presence of a toroidal vortex structure of the emerging long-lived formation was noted.Figure 2 shows images of the discharge and the resulting vortex flow at different times without using a light filter and without the injection of contrast water vapor into the discharge region.As can be seen from Figure 2 immediately after the start of the discharge (at the active stage), the particles of atmospheric dust illuminated by the laser are uniformly distributed and no significant gas movement is visible.After the active stage of the discharge, an afterglow of an almost spherical shape is formed approximately 200 ms after its onset, and at the same time, ordered lines are already observed that start approximately from the middle of the afterglow (a toroidal vortex structure begins to form).Due to the very high brightness of the intrinsic glow of the long-lived formation, the vortex structure is not clearly visible.However, in subsequent moments of time, it can be seen that the shape of the afterglow acquires a characteristic ring shape, and particles of atmospheric dust swirl around this ring.From Figure 2 it can also be seen that there is no significant erosion from the central electrode and the water surface at the stage of afterglow formation.Only at the last moments of time is an erosive ejection, which occurs after a long-lived afterglow.
To observe a clearer picture of the formation of a toroidal vortex structure, water vapor was let into the discharge region, and the video recording of the discharge was carried out at a high speed (1000 frames per second) and through a light filter, which minimized the effect of the discharge glow.An example of such a video is shown in Figure 3. Since in this case the video recording was carried out at a higher speed, the initial stage of the discharge can be clearly observed here.After this stage, a luminous formation of a rounded shape begins to form, which then separates from the discharge.As in the case above, the formation of a vortex toroidal structure is also observed here, as evidenced by the appearance of characteristic two dark rings in the presented video.Unlike Figure 2, it can be seen here that the toroidal vortex structure is clearly observed already after about 100 ms.

Conclusion
Observations of vortex flows carried out using a laser light sheet showed that an autonomous long-lived afterglow formed in a symmetrical atmospheric discharge with an electrolyte anode has a vortex toroidal structure.With increasing time, this afterglow takes on the characteristic shape of a ring.Largely due to this structure, the emerging luminous formations exist for a long time.In addition, the observations showed that a significant erosive ejection from the central electrode and liquid anode occurs much later than the long-lived luminous formation.

Figure 1 .
Figure 1.Experimental setup.The main element of the experimental setup (Figure1) was a cylindrical vessel 300 mm in diameter and 300 mm high, which was filled with a strong solution of baking soda.An electrode made of thick copper wire in the form of a ring was immersed at the bottom of this vessel, which provided electrical contact between the highly conductive solution and the positive pole of a 600 μF storage capacitor bank.A relay was connected in series between the ring electrode and the positive pole of the capacitor bank,

Figure 2 .
Figure 2. Discharge and vortex formation images without water vapor at different points in time.As can be seen from Figure2immediately after the start of the discharge (at the active stage), the particles of atmospheric dust illuminated by the laser are uniformly distributed and no significant gas

Figure 3 .
Figure 3. Discharge and vortex formation images with water vapor at different points in time.