CONDITIONS FOR NON-STATIONARY DISCHARGE INITIATION

INTO "BURNING-THROUGH" MODE

S. E. Emelin

Scientific-Research Institute of Radiophysics at St.Petersburg State University

Sergei.Emelin@pobox.spbu.ru

Abstract

The special form of an electric discharge - "burn-through" in regard to the ball lightning problem has been considered. Having been carried out within closed volume, this mode of erosive discharge has shown the series of clear differences consisting in the high gas density, formation of the metal-polymer aerogel, the percolating current and unusual features of plasma's exhaust. The conditions for its stable origination and the main stages of the discharge process have been ascertained.

1. Introduction. One of the key questions on "energy-dense ball lightning" simulation consists in determination of conditions for the originating of metastable state of substance satisfying two principal requirements - of high specific energy and prolonged relaxation time. From the point of view of the approach developed in the framework of structural - energy self-organization, the fundamental feature of the mentioned state and the process of its creating is the highest level of their non-equilibrium. The complex and multivariate character of high nonequilibrium development forces to pay attention to conditions of such electric discharges, directed onto reproducing of energy-dense ball lightning, which, in contrast to powerful single-impulse ones, combine durational retention of active medium under pumping with suppression of equilibrium regime to achieve the selective and deep energy-content. In connection with solving of the considered problem the attention to a similar conditions, having been observed at electric burning of wood, had been paid for the first time in paper [1]. The analogous condition had been detected independently by author in 1992 in so-called closed capillary discharge, but, having been complex and occuring irregularly in rather small volume the respective process had remained little-studied [2]. In the present work the investigation of considered discharge of pressure less 100 atm in enlarged volume has been carried out to clarify the initial stage of process and to find out the means of their sure reproduction.

2. Closed capillary discharge. In [2] the polyethylene pipe by diameters 9 mm/1.5 mm by a length about 45 mm with two symmetric rod steel electrodes with thread M2 screwed into its endings up to magnitude of an interelectrode gap ~6 mm, had been used as a discharger. After a firing up the short discharge from a source by voltage about 1.8 kV through the resistor 100 Ohm had been changing by a multiple decrease of a current, and after a fraction of second the ejection of autonomous fluorescent object through a side wall of the pipe had been accompanying the switching-off of the current. The study of contents of a finished off discharger had shown presence of mixture of oligomers, metallic fine aerosol and the structured particles of major size. The attempts undertaken to regularize this form of the discharge had revealed its extreme instability to the firing up energy, to the heterogeneities of the thread canals for throttling of an overpressure of gas, to the starting temperature, to the pipe material etc. It had been remarked that absence of iron fine aerosol had been never accompanied by formation of the objects, but led to the early burst-like ejection out of the discharger.

3. Experimental setup. The discharger (fig. 1) differed from the one used in [2] and represented a pipe by diameters 25 mm to 20 mm and by a length 80 mm. The steel electrodes by a diameter 20 mm had a thread by a pitch 0.8 mm with conic end chamfer and were inserted into the pipe up to depth 25 mm to form an interelectrode gap 30.0 mm. The assembled discharger was placed in semicircular grooves of a squeezing consisting of two plates of polymethylmethacrylate by thickness 40 mm, which were tightened by six bolts by a diameter 10 mm. For the discharge products exhaust one of plates of the squeezing was supplied with a conic aperture having minor diameter 6 mm and a smooth conjugation with a groove. The assembly was erected between stops preventing knockout of electrodes. The construction eliminated the throttling of the overpressure along the electrode thread and, accordingly, the heating of the endings of massive electrodes and the melting of the polymer which took place in [2], but also entirely eliminated the causes of the pointed above instability using the possibility of exchange of a prime-aerosol making procedure considered in [3].

Fig. 1. The discharger assembly

The electrical part represented facility for simulation of action of lightning impulses onto protective apparatus of a high-voltage transmission line. It included the isolating discharger which allowed to accompany a breakdown of an studied discharger with simultaneous connection to it of a lightning equivalent by duration 50 µs with current up to 30 kA and of the source of voltage up to 15 kV with inductance L_d = 7.6 mH and resistance R_d up to 200 Ohm.

4. Electrical discharge properties. After breakdown of the discharger with the help of a pulse capacitor C_f = 1200 pF x 80 kV there was an entire discharge of a capacitor C_l = 25 µF x 16 kV via an inductance 5 µH with a magnitude of the current 24 kA. On its completion the conduction of the discharger was temporarily disappearing and the discharger voltage drop was increasing together with charging of the capacitor C_l from a high-voltage source via inductance L_d and resistor R_d; being charged, the capacitor C_l was disconnected from the discharger circuit. At this juncture the "burning-through" current was becoming stabilized and at the constant resistor R_d = 60 Ohm it could continue from tens milliseconds to one second in dependence on magnitude of the applied voltage within the range 1.6 - 1.1 kV with approximate conservation of magnitude of the transmitted charge Q_d. Being initiated by the ejection of the discharger contents, the cut-off of the current was accompanied by disconnection of the discharger circuit from the high-voltage source. The procedure was providing the stable initiation of the "burning-through" mode in all range of magnitudes of the resistor R_d.

5. Main stages of the discharge process. The simulated interruption of process-flow in the different moments had allowed to detect presence of some process constituents which had formed the series of sequential stages. At the first stage under influence of the impulse of large current the evaporation of the polymer and metal was entailing appearance of major pressure which could destroy the pipe without the squeezing, and also formation of a magnetic fine aerosol depositing onto the lower electrode by a layer a millimeter thick and more, onto the upper electrode less and onto the hot surface of the pipe wall. In outcome of it the resistance of a discharger was descending up to several kiloohms. The smooth voltage growth under the increased pressure wasn't invoking an arc discharge, and the current within this stage was resulting from the conduction of the pipe wall and the percolating discharge of a fluidized metal aerosol if the lower electrode was positive.

Fig. 2. A preparation of the metal-polymeric aerogel (resolution 1 µm)

The second stage was characterized by passing of the wall current under the gas pressure close to the critical value for an arc discharge. Under action of the heating by the current the polymer was melted and mixed with an aerosol, and in a layer of this mixture the percolating discharge was arising. It was originating the rapid growth of an aerogel on the basis of disrupted polymer absorbing metallics (fig. 2). The growth rate, the growth duration, the admittance and structural properties of the aerogel were depending on magnitudes of the voltage, pressure and parameters of elements of the electric circuit. Together with filling of the pipe by the aerogel the part of the current which was flowing through the discharger, was growing, and the wall current was diminishing.

At the following stage the full discharge current was concentrating on an aerogel, invoking its "burning-through" under high pressure of gas. When the conditions was optimal, the " burning through " current was distributed on the average almost along all pipe cross-section. This were testified by the electrode spots (fig. 3), the size of which was unusually large with the count of magnitudes of pressure and current. The metal in zone of a spot was having a characteristic brilliance and strong swelling, which was permitting it to be the effective source of fine metallics for "burning-through". At a closing stage the polyethylene wall was hogging into the exhaust port of the squeezing, forming a funnel-shaped recess; in its center the canal was appearing and creating out the exhaust of a discharge emission (fig. 4).

Fig. 4. Outlet canal (cross-section layer by thickness 0.4 mm perpendicular to radius of the pipe).

6. Some features of the discharge. The shape of a canal cross-section was representing several radial breakages outgoing from the common center and inscribing in a circumference by a diameter less 1 mm. In cases of most durational discharges the circumradius wasn't exceeding 200 µm. Under non-optimum condition the piece of a pipe wall in the form of a disk by a diameter 6 mm and thickness 2.8 mm was extruded into the canal.

When the discharge was durational the plasma ejection was carried out in two acts, moreover only " former " was taking the form of a globe, and " latter " was having the form of a jet (fig. 5). The tens of blisters by a diameter about 200 µm were remaining on an interior pipe wall. It was very likely that the obtained energy-dense substance was showing instability to the generation of the slender beam-jet pulses of an energy and substance transfer - interchange, and with its density was decreasing the nonlinear properties were relaxing considerably.

For comparison with processes in the case considered in [2], it is necessary to take into account that in the present experiment the density of the energy input didn't exceed 200 J/cm³. The ratio of the wall thickness to the diameter of the canal was 0.14, and in [2] it was sixteenfold - 2.3. Under the action of more strong radiation in the region of the punched hole the polymer should be more hot and have other shock-viscosity properties. Therefore both the form of the outlet canal and the character of interaction of the polymer with plasma would be different.

Fig. 5. The parted ejection

7. Conclusions. Effective for "metastable substance" formation the transition of the erosive discharge into "burning-through" mode comes from the breaking of the arc current as a result of a short-time voltage decrease at sufficient high density of gas and is carried out via conductivity of the partially disrupt dielectric with smallest particles of metal of the electrodes. The raise of efficacy of this transition is reached by means of the creating of the high fractal metal aerosol of the electrodes material and side wall with the help of a short impulse of a large current starting the process.

References.

1. N. M. Slyusarev, Ball Lightning [in Russian], IVTAN, Moscow (1990), pp. 18-29.

2. S. E. Emelin et al., Tech. Phys. 42 (3), March 1997, pp. 269-277.

3. Yu. V. Sokolov, V.S. Zhelezniy, Tech. Phys. Lett. 29 (8), August 2003.