Fig. 1. Evolution of LLPF-MW.
A.L. Pirozerski1 and S.E.Emelin2
1 Scientific-Research Institute of Physics and 2 Scientific-Research Institute of Radiophysics at Saint-Petersburg State University, Saint-Petersburg, 198904, Russia
WWW: http://balllightning.narod.ru
Abstract. We propose a method of generation of
long-living plasma objects on the base of low-temperature plasma with condensed
dispersed phase in the form of microscopic metal particle and investigate
their main characteristics.
Investigation of long-living plasma formations (LLPF), arising from different types of erosional discharge [14] is of doubtless interest both for understanding principle mechanisms responsible for metastability and structure-energy self-organization processes in low-temperature plasma with condensed dispersed phase and for laboratory modeling of such rare and transitory natural phenomena as ball lightning, sprites, blue jets, etc.
The present paper is devoted to the experimental study of LLPF, arising from metal wires burning by electric current of medium magnitude (20100 A) (LLPF-MW). Erosional plasma, appeared at wire burning, maintains the discharge. The plasma volume undergoes a series of structural changes, resulting in formation of a mushroom-shaped object, which changes then into a quasi-spherical plasmoid and then into a torus. LLPF-MW are characterized by anomaly long lifetime (about 0.20.3 s, that is three orders of magnitude greater than relaxation time of equilibrium plasma at given conditions) and by considerable spatial non-homogeneity (e.g., the presence of fiber- and filament-like structures and of envelopes). They can kindle some dielectric materials, for example the cotton wool.
2. Experimental setup and dischargers construction
The experimental setup consists of a pulse storage capacitor C0 = 0.6mF x 5kV, inductance L= 40mkH 7.6mH, integrated with pulse ignition transformer (resonance frequency 0.03 0.3MHz, pulse voltage up to 50kV), current limiting resistor (R = 10 400 Ohm ), protecting spark gap (4 mm in length) and main discharger.
The main discharger consists of a base in the form of a narrow dielectric plate, in the middle of which dielectric supports or partitions were installed. . A wire to burn, shaped as inverted V letter, was extended over the top edge of the partition, its ends being fixed by clamp placed at the edges of the base.
We used a camcoder Sony DCR-TRV11E, sensitive in the near infrared range, for videorecording.
In some experiments the erosional water discharger, whose construction was similar to the one from [3,4], was connected in series with the spark gap and the main discharger.
3. Discharge modes and main characteristics of LLPF-MW
In our experiments we used wires made from different metals, in particular, from copper, iron and nichrome. The best results were obtained with copper wire of 0.1mm in diameter.
At too low (<30 Ohm ) or too high (>600 Ohm ) values of current limiting resistor LLPF did not appear. In the first case the wire exploded with formation of a dust cloud, luminosity of which decreased rapidly and extinct after about 20 ms. In the second case only a small segment of the wire burned out followed by discharge breaking. For copper wire 0.1 mm in diameter at storage capacitor voltage range 1 5 kV optimal value of the resistor was 70 150 Ohm.
Fig.1 presents selected frames from videorecording showing different stages of LLPF-MW evolution.
Fig. 1. Evolution of LLPF-MW.
Below each frame its number is given starting from the discharge beginning, time interval between successive frames is 40 ms. Initial storage capacitor voltage was 3.1 kV, the residual one 1.0 kV, the resistor value 86 Ohm.
As is seen from Fig.1, the LLPF-MW form changes gradually from spherical to toroidal. LLPF luminescence lasts about 0.3-0.4 s, while the residual dust toroid lives another 510 s with relative stability of the form; for example, it can pass around a barrier placed on its way by means of increasing of its horizontal diameter.
It should be noted that LLPF-MW form and dynamics of its evolution heavily depends on aerodynamic conditions, in particular, on size and shape of the partition installed in the discharger, and on the presence of nearby objects affecting convective motion of air.
Oscillograms of the discharge current and voltage (the latter was measured on the connected in series protection gap and main discharger) are shown on Fig.2.
Fig. 2. Discharge current and voltage.
As may be seen from Fig.2, total time of the discharge
is about 90 ms, therefore, the LLPF-MW shown on Fig.1 exist already after
the current break. Oscillations of the current and voltage evidence about
occurrence of undulatory processes in the erosional plasma appeared at
wire burn, and are related, more probably, with acoustic waves in dust
skeleton formed by metallic particles of micron and submicron sizes.
Fig. 3. LLPF-MW with envelopes.
LLPF-MWs are characterised by expressed spatial non-homogeneity, in particular, by the presence of weakly luminous envelopes (Fig. 3) and filaments (Fig. 4). It should be noted that similar spatial structures were observed in LLPF, appearing in erosional capillary discharge on the water surface (LLPF-WD) [5].
For direct comparison of these two types of LLPF the corresponding dischargers were connected in series, that enabled simultaneous observation of the both objects (Figs. 46).
Fig. 4. Filament-like structures of LLPF-MW (left).
Fig. 5. Simultaneous generation of LLPF-MW and LLPF-WD.
The both LLPF had a similar dynamics, but the LLPF-WD lifetime was 10-20% greater, which is connected, seemingly, with lower temperature of the latter.
To clarify the role of oxidation processes we carried out an experiment on LLPF-MF generation in CO2 atmosphere, see Fig.6.
Fig. 6. A LLPF-MF in CO2-filled column (on the right).
The corresponding LLPF were smaller and float up more slowly, which can be explained by air-dynamics conditions change due to increasing of the gas density and due to the presence of the nearby column walls. Decreasing of LLPF luminosity and change of its color to green, characteristic for copper vapors, are may be connected with the absence of the nitrogen, which has many bright spectral lines both in the red and the blue spectral range. It should be noted also that LLPF-MF lifetime in CO2 atmosphere was shorter.
4. Discussion
Finding out main mechanisms of metastability and determination of the energy storage form are of principal importance for understanding the LLPF physical nature. At the present several models were proposed (see [610] and references therein), but the available experimental data do not allow to choose undoubtedly only one of them.
In the paper [4], devoted to studying LLPF-WD, it was proposed, following the model from [10], that the base of LLPF-WD is cold plasma consisting of clusters of hydrated ions, which the main part of the object energy stored in. The hydrate envelopes prevent approach and recombination of ions, that, by opinion of the writers [4], determines the metastability of the plasmoid.
Studied in the present paper LLPF-MW have similar basic characteristics, of which in this case the most important ones are anomalously long lifetime and the presence of non-equilibrium energy store (the latter is confirmed also by that fact, that LLPF-MW even at last stages of the existence ignites cottony cotton, though mean gas-kinetic temperature calculated from on speed of a up-floating (see [4]) exceeds only a little the room one). As the formation of the plasmoid occurs in conditions, when the concentration of water vapour is negligible, the metastability LLPF-MW can not be explained by the hydration mechanism. The similarity of the parameters LLPF-MW and LLPF-WD allows to suppose, that this mechanism can not be determining nor for LLPF-WD.
That fact, that formation LLPF-MW can occur in the CO2 atmosphere, evidences that the process of oxidation of the metal nor can serve the main power source of the plasmoid.
The main features of LLPF-MW can be explained by the hypothesis, that the base of the objects is highly non-ideal plasma with condensed disperse phase in the form of metal particles, of micron and submicron (presumptively about tenshundreds nanometers) sizes arising at vaporisation of the wire. The influence of the discharge radiation and current leads to electron excitation and ionization of the particles. At the same time the free electrons are fast bounded by electronegative gas, that results in the charge separation, the positive charges being located on the metal particles. The charge of a particle can attain thousands e, that implies high Coulomb non-ideality of the plasma, and, therefore, results in the capability of appearance of collective effects, that is indirectly sustained by the oscillations of discharge current and voltage. The strong collective interaction of the particles leads to development of plasma instability of different types and to formation of filament-like spatial structures. The envelopes consist, apparently, of metal oxides formed on the plasmoid boundary.
5. Conclusions
The burning of metal wires by electric current of medium magnitude, at proper choice of the discharge conditions, results in generation of long-living plasma formations. The plasmoids live 0.2-0.3 s after discharge breaking, their shape evolves from quasi-spherical to toroidal. The LLPF-MW are high non-homogeneous spatially that manifested by the presence of fiber- and filament-like structures and of envelopes. The main features of LLPF-MW may be explained by the model of cold non-ideal plasma with condensed disperse phase in the form of positively charged metallic particles of micron and submicron sizes.
6. References