Spark discharge. Concept of a spark discharge This discharge is characterized by an intermittent form

Such a discharge usually occurs at pressures on the order of atmospheric pressure and is accompanied by a characteristic sound effect- “crackling” spark. The temperature in the main channel of the spark discharge can reach 10,000. In nature, spark discharges often occur in the form of lightning. The distance “pierced” by a spark in the air depends on the tension electric field at the surface of the electrodes and their shape. For spheres whose radius is much larger than the discharge gap, it is considered equal to 30 kV per centimeter, for needles - 10 kV per centimeter.

Conditions [ | ]

A spark discharge usually occurs when the power of the energy source is insufficient to support a steady-state arc discharge or glow discharge. In this case, simultaneously with a sharp increase in the discharge current, the voltage across the discharge gap for a very short time (from several microseconds to several hundred microseconds) drops below the extinction voltage of the spark discharge, which leads to the termination of the discharge. Then the potential difference between the electrodes increases again, reaches the ignition voltage, and the process repeats. In other cases, when the power of the energy source is sufficiently large, the whole set of phenomena characteristic of this discharge is also observed, but they are only a transient process leading to the establishment of a discharge of another type - most often an arc one.

Nature [ | ]

A spark discharge is a bunch of bright, quickly disappearing or replacing each other thread-like, often highly branched stripes -. These channels are filled with plasma, which in a powerful spark discharge includes not only ions of the source gas, but also ions of the electrode substance, which intensively evaporates under the action of the discharge. The mechanism for the formation of spark channels (and, consequently, the occurrence of a spark discharge) is explained by the streamer theory of electrical breakdown of gases. According to this theory, from electron avalanches arising in the electric field of the discharge gap, under certain conditions, streamers- dimly glowing thin branched channels that contain ionized gas atoms and free electrons split off from them. Among them we can highlight the so-called leader- a faintly luminous discharge, “paving” the path for the main discharge. Moving from one electrode to another, it closes the discharge gap and connects the electrodes with a continuous conductive channel. Then the main discharge passes in the opposite direction along the laid path, accompanied by a sharp increase in the current strength and the amount of energy released in them. Each channel rapidly expands, resulting in a shock wave at its boundaries. The combination of shock waves from the expanding spark channels generates a sound perceived as the “crack” of a spark (in the case of lightning, thunder).

The ignition voltage of a spark discharge is usually quite high. The electric field strength in the spark decreases from several tens of kilovolts per centimeter (kV/cm) at the moment of breakdown to about 100 V/cm after a few microseconds. The maximum current in a powerful spark discharge can reach values of the order of several hundred kiloamperes.

A special type of spark discharge - sliding spark discharge, which occurs along the interface between a gas and a solid dielectric placed between the electrodes, provided that the field strength exceeds the breakdown strength of air. Areas of a sliding spark discharge, in which charges of one sign predominate, induce charges of a different sign on the surface of the dielectric, as a result of which spark channels spread along the surface of the dielectric, forming the so-called Lichtenberg figures.

Processes close to those occurring during a spark discharge are also characteristic of a brush discharge, which is a transition stage between

7. Spark discharge

A spark discharge, unlike other types of discharge, is intermittent even when using a constant voltage source. By appearance a spark discharge is a bunch of bright zigzag stripes, constantly replacing one another. Luminous stripes - spark channels - spread from both electrodes. The discharge gap in the case of a spark is non-uniform, so a quantitative study of processes in a spark discharge is difficult. One of the main methods for studying spark discharge is photography.

The ignition potential of a spark discharge is very high. However, when the gap has already been broken, its resistance decreases sharply, and a significant current passes through the gap. If the source power is low, the discharge goes out. After this, the voltage across the discharge gap increases again and the discharge can be ignited again. This process is called relaxation oscillations of the discharge. If the discharge gap has a large capacity, the spark channels glow brightly and give the impression of wide stripes. This is a condensed spark discharge.

If there is any obstacle between the electrodes, the spark breaks through it, forming a more or less narrow hole. It has been established that the gas temperature in the spark channel can increase to very high values (10000-12000 K). The formation of high-pressure areas and their movement in the gas are explosive in nature and are accompanied by sound effects. This may be a slight crackling sound (with slight excess pressure) or thunder.

A special type of spark discharge is a sliding discharge that occurs along the interface between a solid dielectric and a gas around a metal electrode (tip) touching this surface. If you use a photographic plate as a dielectric, you can make this picture visible to the eye. The shapes obtained using a spark discharge on the surface of a dielectric are called Lichtenberg figures. Lichtenberg figures can be used to determine the polarity of the discharge and to determine the high voltage, since maximum voltage discharge pulse is directly proportional to the radius of the surface occupied by the figure. Instruments for measuring very high voltages - clinodographs - are based on this principle. If the distance between the electrodes is small, then the spark discharge is accompanied by destruction of the anode - erosion. This effect is used for spot welding and cutting metals.

Based on numerous observations of spark discharge in 1940, Mick and independently of him Rether put forward a theory of spark discharge, which was called the streamer theory. A streamer is a region of gas with a high degree of ionization, propagating towards the cathode (positive streamer) or towards the anode (negative streamer). The streamer theory is a theory of single-avalanche breakdown. According to this theory, an avalanche of electrons passes between the electrodes. After the avalanche passes, electrons fall on the anode, and positive ions, having significantly lower speeds, form a cone-shaped ionized space. The ion density in this space is not sufficient for breakdown. However, under the influence of photoelectrons, additional avalanches occur. These avalanches will move towards the trunk of the main avalanche if its space charge field is commensurate with the applied voltage. Thus, the space charge continuously increases, and the process develops as a self-propagating streamer. When the voltage applied to the discharge gap exceeds the minimum breakdown value, the space charge field generated by the avalanche will be commensurate with the magnitude of the external field even before the avalanche reaches the anode. In this case, streamers appear in the middle of the gap. Thus, for the emergence of a streamer, two basic conditions must be met: 1) the avalanche field and the field created by the voltage applied to the electrodes must be in a certain ratio and 2) the avalanche front must emit a sufficient number of photons to maintain and develop the streamer.

At high power source, the spark discharge turns into an arc. Lightning also belongs to spark discharges. In this case, one electrode is the cloud and the other is the ground. The voltage in lightning reaches millions of volts, and the current reaches hundreds of kiloamperes. The charge transferred by lightning is usually 10-30 coulombs, and in some cases reaches 300 coulombs.

Spark discharge

Spark discharge(electric spark) - a non-stationary form of electrical discharge occurring in gases. Such a discharge usually occurs at pressures on the order of atmospheric pressure and is accompanied by a characteristic sound effect - the “crackling” of a spark. The temperature in the main channel of the spark discharge can reach 10,000. In nature, spark discharges often occur in the form of lightning. The distance “pierced” by a spark in the air depends on the voltage and is considered equal to 10 kV per 1 centimeter.

Conditions

Nature

A spark discharge is a bunch of bright, quickly disappearing or replacing each other thread-like, often highly branched stripes - spark channels. These channels are filled with plasma, which in a powerful spark discharge includes not only ions of the source gas, but also ions of the electrode substance, which intensively evaporates under the action of the discharge. The mechanism for the formation of spark channels (and, consequently, the occurrence of a spark discharge) is explained by the streamer theory of electrical breakdown of gases. According to this theory, from electron avalanches arising in the electric field of the discharge gap, under certain conditions, streamers are formed - dimly glowing thin branched channels that contain ionized gas atoms and free electrons split off from them. Among them we can highlight the so-called. leader - a weakly glowing discharge that “paves” the path for the main discharge. Moving from one electrode to another, it closes the discharge gap and connects the electrodes with a continuous conductive channel. Then the main discharge passes in the opposite direction along the laid path, accompanied by a sharp increase in the current strength and the amount of energy released in them. Each channel rapidly expands, resulting in a shock wave at its boundaries. The combination of shock waves from the expanding spark channels generates a sound perceived as the “crack” of a spark (in the case of lightning, thunder).

The ignition voltage of a spark discharge is usually quite high. The electric field strength in the spark decreases from several tens of kilovolts per centimeter (kV/cm) at the moment of breakdown to ~100 volts per centimeter (V/cm) after a few microseconds. The maximum current in a powerful spark discharge can reach values of the order of several hundred thousand amperes.

The behavior of a spark discharge can be seen very well in slow-motion footage of discharges (Fimp. = 500 Hz, U = 400 kV) obtained from a Tesla transformer. The average current and pulse duration are not sufficient to ignite an arc, but are quite suitable for the formation of a bright spark channel.

Notes

Sources

A. A. Vorobyov, High voltage technology. - Moscow-Leningrad, GosEnergoIzdat, 1945.
Physical Encyclopedia, vol. 2 - M.: Great Russian Encyclopedia p. 218.
Raiser Yu. P. Physics of gas discharge. - 2nd ed. - M.: Nauka, 1992. - 536 p. - ISBN 5-02014615-3

Spark, one of the forms of electrical discharge in gases; usually occurs at pressures on the order of atmospheric pressure and is accompanied by a characteristic sound effect: the “crackle” of a spark. Under natural conditions, I. r. most often observed in the form of lightning... ... Great Soviet Encyclopedia

An electric spark is a non-stationary electric discharge in a gas that occurs in an electric current. field at gas pressure up to several. hundreds of kPa. It is distinguished by a sinuous, branched shape and rapid development (approx. 10 7 s), accompanied by a characteristic sound... ... Big Encyclopedic Polytechnic Dictionary

- (electric spark), non-stationary electric. discharge in a gas that occurs in an electrical field at gas pressure up to several. atm. It is distinguished by its sinuous, branched shape and rapid development (approx. 10 7s). Tempo pa in ch. channel I. r. reaches 10,000 K... Natural science. encyclopedic Dictionary

An electric spark has the appearance of a thin, whimsically curved and brightly luminous strip, which is usually highly branched (Fig. 174). This luminous channel of the spark, however, is never at all similar to those acute-angled zigzags with which it is customary to conventionally depict lightning.

Rice. 174. Characteristic appearance of a spark.

A strip of spark with enormous speed penetrates the discharge gap, goes out and appears again. Photographing a spark using a camera with a fast-moving lens (Base camera) or with fast-moving film shows that several discharges run along the same channel of the spark, which is sometimes deformed. To study individual stages of spark development, photogates controlled by high-frequency current and based on the use of the Kerr phenomenon are used (§ 95). One of the first studies of the structure of the spark was carried out by Prof. Rozhansky in 1911 Rozhansky photographed a spark, deflecting the spark by the action of a magnetic field.

Gas breakdown, resulting in a spark discharge, occurs at a certain field strength, which should be greater, the higher the density of the gas and the lower its initial ionization.

Below are numerical data characterizing the size of the spark gap in room air. The electric field strength near the electrodes strongly depends on the curvature

surface of the electrode, therefore the minimum voltages at which an avalanche discharge begins for a given distance between the electrodes are not the same for the electrodes various shapes; between the tips, the spark discharge begins at a lower voltage than between the balls or plr electrodes.

The spark gap in room air

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Room air usually contains only a very small number of ions, approximately a few thousand per cubic centimeter(under normal electrical state of the atmosphere at the surface of the earth - on average about 700 pairs of ions per 1 cm

Rice. 175. Scheme of development of a negative streamer

When a sufficiently high voltage is applied to the electrodes, the growth of electron avalanches begins, but due to the small initial number of ions, it takes time for the process to end with the formation of a spark. If you connect the electrodes to a high voltage current source at extremely a short time, then the development of electronic labs will not have time to end with a spark discharge. Measuring the time during which channels of increased electrical conductivity are formed in the gas due to the development of avalanches showed that in in this case Photon ionization plays an important role.

In Fig. 175 presents a diagram explaining why the growth of an electrically conductive channel, or, as they say, the spread

streamer, occurs faster than the advancement of an electronic avalanche. In this figure, avalanches are conventionally shown as shaded cones, and the paths of photons are depicted as wavy lines. One must imagine that inside each cone representing a developing avalanche, the gas is ionized by electron impacts; newly detached electrons, accelerated by the field, ionize the gas particles they encounter, and thus the number of electrons moving to the anode and the number of positive ions drifting to the cathode increase exponentially. The left ends of the wavy lines show atoms that were “excited” by the impact of an electron and subsequently emitted a photon. Moving at speed, photons overtake the avalanche and in some place, which is depicted by the end of the wavy line, ionize a gas particle. The electron split off here, rushing towards the anode, generates a new avalanche far ahead of the first avalanche. Thus, while the first avalanche grows, say, by the amount of the small arrow shown in Fig. 175, the emerging channel of increased electrical conductivity of the gas, i.e., a streamer, extends to the size of the large arrow shown in the same figure. In the next stage, individual avalanches in the negative streamer, overtaking each other, merge, forming an integral channel of ionized gas (in the figure, the first avalanche has already overtaken the second, and the fourth has overtaken the fifth).

The physical and mathematical conditions under which streamer development can occur were theoretically studied by Meek and Loeb in 1940). As already explained above, a negative streamer is, in essence, the advancement of electron avalanches accelerated by photoionization and their merging into a common electrically conducting channel.

The positive streamer has a completely different structure and significantly different properties. A common feature Its only relationship with a negative streamer is photoionization, which in both cases plays a dominant role.

A positive streamer is a gas-discharge plasma channel that rapidly grows from the anode to the cathode. In Fig. 176 schematically explains how such a channel develops. The appearance of a positive streamer is preceded by the passage of electron avalanches across the gas-discharge gap. They leave in their wake big number newly formed positive ions, the concentration of which is especially high where avalanches are most developed, that is, near the anode (Fig. 176, top left). If the concentration of positive ions here reaches a certain value (close to ions in ), then, firstly, intense photoionization is detected, secondly, electrons released by gas particles that have absorbed photons are attracted by a positive space charge to the head part of the positive streamer, and, thirdly, due to photoionization, the concentration of positive ions on the path of the streamer to the cathode increases. In Fig. 176 photon paths are shown as wavy lines; photons are ejected in different directions from the region of positive space charge (short arrows indicate the direction of movement of the split-off electrons); It can be seen that many electrons are drawn into the region of the highest concentration of positive ions in the head part of the positive streamer. The saturation of space filled with positive charges with electrons turns this area into a gas-discharge plasma.

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This creates a channel in the gas that has high electrical conductivity. The formation of this channel with gas-discharge plasma is the development of a positive streamer (Fig. 176). If along the path of growth of this channel towards the cathode in the head part of the streamer there is a sufficient concentration of positive ions, then the streamer moves at enormous speed. Otherwise it breaks off.

The streamer development diagrams explained above only give an approximate idea of preparatory stage spark discharge. The actual picture of streamer development is more complex, since the resulting space charges sharply distort electric field, which caused the streamer to appear.

In long gas-discharge gaps, field unevenness and insufficient photoionization in the direction of the shortest distance from the head of the streamer to the electrode lead to curvature of the channel and the appearance of numerous branches.

The development of positive streamers begins at the positive electrode in places of highest field strength: near sharp protrusions, sharp edges and other irregularities in the anode surface. Therefore, during a discharge between the tip and the disk, sparks are often observed connecting the positive tip with the center of the negative disk, and sparks connecting the edges of the positively charged disk with the negative tip (Fig. 177); in the first case, breakdown occurs at a lower voltage.

Rice. 177. Characteristic appearance of a spark discharge between the tip and the disk with a large discharge gap.

Rice. 178. Photograph of a spark on moving film.

Field deformations by charges formed in the streamer and a combination of complex processes occurring in the streamer lead to the fact that the spark discharge often develops in jerks. Wherein

a new streamer retraces the path paved by the previous faded streamer. In Fig. 178 shows a photograph of a single spark discharge on. fast moving photographic film. Here you can see the jerky development of the spark and it is clear that the negative and positive streamers are growing towards each other. When the heads of the streamers meet, a conductive channel is formed, through which the discharge occurs.

A similar, but even more complex picture is revealed during the development of lightning. The initial stage is the development of a pilot lightning streamer, the glow of which is almost imperceptible. Typically, the pilot streamer propagates from a negatively charged cloud. Along the still narrow channel of increased ionization formed by the piloting lightning streamer, powerful electron avalanches rush at a speed of about thousands of kilometers per second, creating a rather bright glow. In this case, the electrical conductivity of the channel increases enormously and the cross section of the channel expands. This stage is called lightning leader development. When the initial ionization of the air is low, the development of the leader occurs spasmodically - with stops of tens of milliseconds after each of its propagations (such leaders are called “stepped” in contrast to the so-called “lancet”, which propagate with continuous rapidity).

Rice. 179. Photograph of lightning on moving film. Here the pauses between the first beats and the last pause are four times longer.

As the leader approaches the ground, charges of the opposite sign are induced in the ground, and from tall buildings, lightning rods, trees, a counter leader grows. At the moment of its merger with the leader descending from the cloud, i.e., when the discharge gap between the cloud and the ground turns out to be a closed electrically conductive channel, the main lightning discharge runs through this channel at a speed of the order of tens of thousands of kilometers per second. If the channel had branches (and this usually happens), then the main discharge spreads over all branches Diameter of the main channel

Lightning usually has a size of 10-20 cm and the brightest glow is in the lower part. Created in the channel high blood pressure, which after a lightning strike causes a rupture of the channel, which gives rise to the phenomenon of thunder. The charge carried by lightning is usually several coulombs and often several tens of coulombs. The instantaneous value of the lightning current is often tens and sometimes hundreds of thousands of amperes.

A lightning discharge usually carries away charges only from some part of the cloud. Charges from other parts of the cloud rush to this place. Therefore, most often, after the first lightning strike, after hundredths of a second, repeated lightning strikes (two, three or more) occur along the same, but sometimes somewhat deformed or otherwise branched channel; each of them is preceded by a leader that restores the electrical conductivity of the channel.

Rice. 180. Diagram of a thunderstorm (cumulonimbus) cloud.

Rice. 179 reproduces a picture of five lightning strikes on one channel, filmed on moving film. In some cases, a strong wind shifts the lightning channel so much that even when photographing with a conventional camera, individual strokes can be distinguished.

In Fig. 180 shows a diagram of the most common charge distribution in a thundercloud. Negative charges are usually distributed at the leading edge of the cloud and along its lower part. There is also a region of positive charges here; The entire upper part of the cloud is also positively charged. The direction of the wind (indicated by arrows in the figure) blowing the cloud away is usually opposite to the ground wind. At the beginning heavy rain carries away a positive charge from the cloud, later moderate negatively charged rain falls.

In the absence of a thunderstorm, the electric field in the atmosphere is directed from top to bottom, since the earth is negatively charged, and the positive charge is scattered in the atmosphere.

When there are no disturbing influences created, in particular, by thunderclouds, the electric field strength in the atmosphere decreases with height. Near the earth, the electric field strength is of the order of magnitude. At an altitude it is equal to, and at an altitude approximately The field strength at an altitude of 20 km is 100 times less than that of the earth.

This rapid decrease in electric field strength with height shows that, compared to uniform field The electric field in the atmosphere is highly complicated by the charges distributed in the atmospheric air.

During thunderstorms, the field strength in the atmosphere can be 100 and 1000 times higher than normal.

Under a thundercloud, the field direction most often reverses, from the ground to the negatively charged lower edge of the cloud, and the field strength near the ground before a lightning discharge can reach 200-300 thousand volts per meter. The potential difference between cloud and ground before a lightning strike is often hundreds of millions and sometimes billions of volts. Most lightning strikes come from negatively charged clouds. Lightning bolts are often several kilometers long. Lightning strikes often occur between individual clouds. Thunderstorms were observed, during which there were 4-7 thousand lightning strikes per hour. On average, about 44 thousand thunderstorms occur on the globe per day (an average of about 1800 thunderstorms at a time) and several thousand lightning strikes occur every minute.

Rice. 181. Photograph of ball lightning

In rare cases, lightning discharges of a completely different type are observed. In Fig. 181 one of the photographs of ball lightning is reproduced. According to observers' description ball lightning usually have the appearance of luminous balls with a diameter of about 10-20 cm, and sometimes several meters. Ball lightning moves smoothly, at low speed and in some cases abruptly. There have been cases when ball lightning, touching the ground or any objects, exploded and caused severe destruction.

Numerous attempts to reproduce this type of discharge in the laboratory did not give satisfactory results, despite the fact that some researchers (Plante in Gezehusu in 1900, Cawood et al.)

managed to get discharges ball type. In Fig. 182 Plante's experience is explained. If, using a high-voltage constant voltage source, the anode is immersed in the electrolyte and the cathode is brought to the surface of the electrolyte, an arc discharge is ignited. But when the cathode is immersed in the electrolyte and the anode is brought to the surface of the electrolyte, an arc cannot form, since the possibility of incandescence and thermionic emission from the dathode is excluded. Plante discovered that in this case, subject to certain conditions A luminous and rapidly rotating ball is formed between the anode and the surface of the electrolyte, which after some time slides along the surface of the electrolyte to the cathode.

Rice. 182. Scheme of Plante's experiment.

Rice. 183. Photo of beaded lightning.

One of the many hypotheses proposed to explain ball lightning (Meissner's hypothesis) interprets this type of discharge as a vortex of gas-discharge plasma occurring in the bend of linear lightning. According to another hypothesis (Mathias), it is assumed that in ball lightning the energy of the discharge is chemically accumulated, and unstable higher compounds of nitrogen and oxygen are formed, capable of decomposing with an explosion.

Sometimes lightning turns out to consist of several dozen small luminous balls (less than 10 cm in diameter), separated from each other by a distance of less than a meter. This type of discharge is called imprecise lightning (Fig. 183). There is not yet an acceptable, sufficiently substantiated theory of ball and bead lightning.

If, when using a high direct voltage, a plate made of a solid dielectric (glass, ebonite, etc.) is placed between the electrodes and this plate has such a thickness that a spark does not penetrate it, and the width is not too large, then a sliding spark discharge is observed, which passes along the surface of the plate and bends around it. To study this discharge, it is created on a photographic plate and then developed (Fig. 184). The discharge images obtained in this way are called Lichtenberg figures. Their radius is proportional to the voltage of the discharge pulse. This is used (using special devices for photographing a sliding discharge - klidonographs) in mass, statistical research lightning"

In the USSR, a systematic study of lightning and lightning protection methods is being conducted. The leading role in this area belongs to the high-voltage laboratory of the Energy Institute of the USSR Academy of Sciences.

When the voltage is not high enough to breakdown the gas-discharge gap, special type discharge-corona.

Rice. 184. The sliding will discharge the positive electrode.

Corona discharge on high-voltage networks causes power leaks.

A study of the corona showed that on the positive electrode the corona discharge at relatively low voltages consists of a series of electron avalanche pulses, lasting every ten-thousandths of a second. At higher voltages, the intermittency of the phenomena is less noticeable and the main role is played by streamers, breaking off where the field strength is too low for their propagation. The structure and character of the glow of the corona discharge on the negative electrode are to some extent similar to the near-cathode zone of the glow discharge.