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FORMATION OF VORTEX PLASMOIDS USING A PULSED ELECTROTHERMAL
ACCELERATOR
Fedun V.
Associate Professor of the Department of Physics Pryazovsky State Technical University, Mariupol, Ukraine
Abstract
This paper presents the results of the formation of vortex long-lived plasma formations (LLPF) using a plasma gun - an ablation pulsed plasma accelerator. Discharge duration ~ 0.6 ms. LLPF evolution was recorded using a high-speed camera. The vortex lifetime reached 30 ms. The conditions for the formation of an LLPF have been determined. It has been found that an increase in the discharge power reduces the stability of vortices and leads to a decrease in the life of plasma formations.
Keywords: long-lived plasma formations, plasma gun, vortex, discharge, capacitive energy storage.
Introduction. The obtaining of long-lived plasma formations (LLPF), both in a free atmosphere [1,2] and in laboratories [3,4], is of great scientific and applied interest. LLPF researches are relevant in terms of studying the fundamental properties of plasma, determining the mechanisms of various structures in plasma, energy transport in plasma, as well as elucidating the nature of ball lightning, which currently has no generally accepted explanation.
Previous data. LLPs were obtained in the laboratory conditions using: current pulse over the surface of an aqueous electrolyte [5,6], pulsed plasma jets in the atmosphere [7,8], microwave fields [9,10], an electric discharge in flammable hydrocarbon gas [11,12], electrical explosion of conductors [13], lightning strike into the ground [14], etc.
Toroidal vortices are a configuration that provides stability for a low-temperature plasma at atmospheric pressure [15,16]. Toroidal plasmoids have a number of specific properties. In particular, this is a relatively long lifetime and a high degree of isolation from the environment, which allows maintaining the high adiaba-ticity of the toroidal plasma formation after the termination of the supply of electrical energy.
The mechanism for the formation of high-temperature (plasma) vortices and low-temperature vortex rings produced by ejecting pulsed subsonic plasma/gas jets into air was investigated experimentally. A toroidal vortex forms due to the interaction between a pulsed jet
with the flow induced by this jet in the ambient medium [].
The aim of the study. This article presents the results of the formation of vortex plasmoids using a plasma gun - an ablative pulsed plasma accelerator, in which the plasma is formed as a result of the evaporation of an insulator surrounding the discharge gap. Plasma parameters can be varied over a fairly wide range by varying the voltage of the power source and the pulse duration [17], which makes it possible to form plasma jets in high-pressure media.
Materials and methods. Laboratory studies of LLPF generation were carried out on an experimental installation, which consisted of plasma gun (PG), a capacitive energy storage, a storage power source, a discharge initiation block, a contactless switch - magnetic key, voltage and current measuring instruments in an electrical circuit (voltage divider and Rogowski coil), high speed camera (Nikon 1s1).
The schematic diagram of the installation is shown in Fig. 1a. The power supply for the plasma gun PG was provided by a capacitive energy storage Ch with a capacity of 1500 ^F, which was pre-charged to U0 <5 kV. The discharge was initiated by a high voltage pulse (~50 kV) of microsecond duration, which was applied to the PG electrodes. This pulse was formed by the secondary winding L2 of the pulse transformer PT after the controlled spark gap P was triggered (discharge of the capacitance Cb through the primary winding L1). A short circuit of the high-voltage pulse to the capacitance Ch was prevented by using the magnetic key MK. To prevent the discharge of the capacitor bank Ch through the L2 winding a decoupling capacitor Cp is used in the circuit.
a) b)
Fig. 1. Principle electrical diagram of the experimental installation (a) and plasma gun system (b)
The industrial tubular arrester RTF-6-0.5 / 10 U1 was used as a plasma gun. The basis of the arrester (PG) is made up of fibro-bakelite tube 1 (Fig. 1b). The outer diameter of the tube was D = 43 mm, the length of the spark gap was L = 505 mm, and the distance between the electrodes inside the tube during the studies was 30 mm. The inner hole diameter is 10 mm. The tip - electrode 2 in the form of a ring was placed at the open end of the tube. The other end of the tube was plugged with a metal cover 3, on which an internal rod electrode 4 was fixed.
The principle of operation of a plasma gun is as follows. The gap between the electrodes is punctured by a high-voltage pulse, and a conductive channel is formed. This channel is able to close a section of the circuit with a capacitive energy storage (see Fig. 1a). In this case, the spark discharge turns into a high-current arc discharge. Gas heating occurs when current passes through the channel and, therefore, the channel cross-section increases. When the channel overlaps the section of the inner hole of the tube, intensive
evaporation of bakelite occurs under the action of a pulsed arc current and the gas pressure increases greatly. The gases form a jet when they exit through the open end of the tube (ring electrode).
Results and discussion. Figure 2 shows typical oscillograms of the discharge current I and voltage U (between electrodes 2 and 4) at a voltage U0 = 1500 V. The oscillograms show that the voltage on the electrodes after the initiation of the discharge drops sharply (~1 ^s) to almost zero. This change is caused by the presence of a magnetic key in the circuit. Then the capacitive storage begins to discharge through the discharge channel, and the discharge current and voltages increase simultaneously. The discharge current increases at first, and then decreases, and the discharge voltage changes slightly. When the current reaches zero, the discharge is interrupted, and the voltage between the electrodes decreases sharply. In all experiments the discharge time was 600 ^s.
a) b)
Fig.2. Oscillograms of current (a) and voltage (b) of discharge (U0 = 1.5 kV)
In the course of the experiment, it was found that in the case when the discharge power per unit volume is less than 5 MW / cm3, then the plasma jet is capable of forming a bright long-lived vortex plasmoid. An example of such an object is shown in Fig. 3, which is a frame-by-frame scan of high-speed video recording made with a Nikon 1s1 camera at a shooting speed of
1200 frames per second. The plasmoid is formed after preliminary charging of the capacitive storage to a voltage of 1.5 kV, when the rate of release of the volumetric energy density was about 1.5 MW / cm3. If the charging voltage of the capacitor bank is increased to 2 kV, then the discharge power per unit volume approaches the critical value, and although the plasma jet forms a
vortex structure, its lifetime is noticeably (4 times) of [7]. Analysis of Fig. 3 made it possible to establish
shorter than in the previous case. the dependence of the path traveled by the vortex on
Note that the critical value of the discharge power time, which is shown in Fig. 4a. In per unit volume is in good agreement with the results
Fig.3. Formation and motion (sequence offrames at intervals of1/1200 s) of toroidal vortex (U0 = 1.5 kV)
addition, the time dependence of the vortex ascent Further deceleration of the LLPF was not recorded due
velocity was also obtained (Fig. 4b). As seen from Fig. to the disappearance of the plasmoid. These depend-
4b, the vortex velocity monotonically decreases with ences do not contradict the results of work [7,8]. time from several tens of meters per second to 5 m/s.
a) b)
Fig.4. Dependences ofpath (a) and lifting speed (b) of the toroidal vortex on time (U0 = 1.5 kV)
Conclusions.
1. The work demonstrates the possibility of creating long-lived plasma formations by creating a high-current pulsed arc discharge in a limited narrow dielectric channel at atmospheric pressure. An axial electrothermal plasma accelerator operating in a gas-dynamic mode was used.
2. The design of the plasma accelerator used in the experiments makes it possible to obtain long-lived plasma vortices in the atmosphere. The vortex lifetime reached 30 ms.
3. The conditions under which the formation of long-lived plasma objects is possible are determined. It was found that an increase in the power of the arc discharge reduces the stability of the vortices and leads to a decrease the life-time of plasma formations.
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