Types of corona discharges. Applications of gas discharge

Corona discharge is the process of ionization of air along a wire under the influence of strong electromagnetic fields.

Air ionization theory

The ionization of air was noticed long ago, but they failed to correctly interpret it. With the advent of the first electrostatic generators in the middle of the 18th century, the discharge became commonplace. Even had time to try on a brutal action. True experiments with electricity began after Volta's invention of the galvanic energy source.

The world's first arc was received in 1802 by a Russian scientist with a memorable surname Petrov. He predicted the possibility of using this for the purposes of illumination. The fact that the entire scientific world has paid attention to the phenomenon is a strong annoyance. And it turned out to be clear where the electric current actually flows. After all, the negative carbon electrode was sharpened under the action of the arc, and a small hole formed on the anode. The scientific world saw Benjamin Franklin right in this: the charges increase the negative carbon rod, being positive. And only by the beginning of the 20th century, when experiments with cathode rays gave the first results, it became clear that a big mistake had been made 100 years ago.

When the arc burns, five-sixths of the luminous flux gives the anode. Its temperature in standard physical experiments is 4000 degrees Celsius. This is 1000 more than the cathode, which gives 10% of the luminous flux. The rest is taken from the arc directly, due to the flickering of the ionized gas. At such high temperatures, even ceramics and tungsten begin to melt. Welding was invented much later, since the 80s (XIX century) a carbon electrode, later N.G. Slavyanov suggest using metal.

Pavlov's experiment was repeated by Davy, the others have not yet been engaged in the arc. With his submission, the study of a discharge in a gas medium began. The first line spectra were discovered. Faraday and Wheatstone studied discharge in rarefied gases in the 1930s. Seeing the zeal of the British, a foreign engineer who accepted Russian citizenship, Jacobi tried to use a coal rod to illuminate the streets of St. Petersburg (1846). But the anode quickly burned out, increasing the spark gap, and the lamp went out. The situation was decided by Yablochkov, this already happened 30 years later, when the age of coal spark gaps was coming to an end. They found use in narrow areas for a long time, for example, when lighting the sky during the Second World War and repelling enemy raids.

The Ruhmkorff coil (approximately 1846) finally convinced people that high voltage could create a spark, and Nikola Tesla showed that with the help of a Faraday screen, even a mere mortal would be able to direct lightning in the right direction. Flames in the night sky over Wardenclyffe Tower are called the most incredible corona discharge in the history of mankind, with the exception of the one arranged later by the great inventor on the roofs of New York.

Scheme of the occurrence of a corona discharge

There is no exact definition of corona discharge in the literature. For the simple reason of the reluctance of the authors to deal with the topic and the abundance of duplicate information that misses the point of the content. The definition of a corona discharge given at the beginning cannot be called physically accurate either. The correct interpretation will not be accepted by most readers due to the presence of specific features. In physics, it is customary to divide the passage of current through air into three sections, visible on the graph:

1. The first is subordinate and direct. Here, the current flow is possible due to external ionization: flame, ultraviolet, radioactive or high-frequency radiation. The first two factors were already known to Volta (before the discovery of "animal electricity" by Galvani), who proposed to remove the static charge from the rubber of the electrophorus with the rays of the Sun or a candle.
2. The second section is in the saturation region. Scientists say that the current remains relatively constant, the charges actively recombine as they move between the electrodes. And with a growing potential difference, nothing changes. Until the voltage reaches the third section.
3. At a high potential difference, an avalanche-like process of impact ionization begins. The electrons acquire such a high speed that they knock electrons out of the gas molecules. In this section, the current increases rapidly with an increase in the potential difference, an electric arc may occur.

The visually observed discharge is called a spark discharge and occurs after the beginning of the second growth of the curve. At first, there is a quiet discharge, not noticeable to the eye. It is often called non-self-sustaining, requiring an external ionizing factor to keep the carriers moving. Lowering the voltage causes immediate recombination of all carriers.

A spark discharge is noted at voltages where an avalanche-like ionization is possible. Sparks jump at a frequency of 400 Hz and above, which is accompanied by audible noise. The voltage after each discharge drops, which is due to the presence of a free interval. Visually, the sparks merge into one. Subtypes of this type of ionization are related discharges:

• The brush discharge is similar to the palm of a fairy tale skeleton. Formed between the tip and the charged surface. Noticeably on neutralizers, power line insulators. Ionization begins from the side of the tip, in this place the field strength is increased, the charges flow into space, which generates an avalanche-like process.
• The corona discharge flashes between several sections of the same wire. Caused by impact ionization of air. Peculiar broken teeth are like lightning. Scientists explain their bizarre trajectory by the fact that the ionization process propagates along the path of least resistance; due to the isotropy of the gas, it is impossible to predict the exact path. The crown is sometimes smooth and can be positive or negative.

The corona discharge leads to a loss of energy on the power line and occurs continuously, which is audible as a low-frequency rumble and crackle. In rainy weather, the resistance of the wire drops, and tongues of ionized air may appear in the form of small lightnings along the wire or balls. Corona discharge is used in air purification filters (ionizers, Chizhevsky chandeliers), trapping smoke and dust particles, causing them to settle.

Electric arc

The above does not allow an accurate understanding of the electric arc. At a certain voltage value, impact ionization of the air begins. If the potential difference drops, the current does not change or increases (see and). This is the so-called section with negative differential resistance. The process between the electrodes is called an arc. The discharge is ignited by high voltage and the convergence of the rods, and then goes on its own.

It is known that the welder taps the electrode on the part to start the impact ionization. Then the electrode is removed, and the arc remains, does not go out. The voltage is also low. This is the peculiarity of the arc. This explains why open power lines do not carry voltages above 2 MV. And then the corona discharge begins, an arc appears, in order to put out, you have to make a lot of effort.

Tesla built the Wardenclyffe tower to achieve the transfer of energy by means of a corona discharge. The created arc was instructed to fly to the receiver, and from there to radiate further, around the entire globe. According to Tesla's plan, it was required to build transmitters that caught lightning tongues. Safety was ensured by a high voltage frequency (radio band).

Summing up, it should be noted that the electric arc is otherwise called self-discharge, the process can be maintained.

Ionization mechanisms

The corona discharge is formed at geometric breaks due to the increased field strength in this area. Neutralizers and drains work on this principle. The phenomena observed during a gas discharge are quantitatively described by two Townsend coefficients:

• Alpha: volume ionization coefficient. Numerically, this is the number of ionizations produced by an electron at a distance of 1 cm.
• Gamma: describes the ionization process at the cathode-gas interface. Here the electrons leave the surface and start marching along the field lines. It is equal to the ratio of electrons leaving the cathode to the number of ions falling here per unit of time.

Both coefficients grow together with the potential difference. After a non-self-sustained discharge, an avalanche-like ionization is noted with the formation of a cloud of positive charge between the electrodes. This moment corresponds to the appearance of the corona. A further increase in voltage leads to a violation of the stationarity of the positive cloud, and the current begins to oscillate around a specific value.

The foregoing is called the Rogowski theory and explains where the corona occurs, how sparking is formed. Everything is determined by the flight of electrons and the spatial distribution of charge. The main feature is that the circuit does not short-circuit during a corona discharge, as occurs during sparking (short-term) or arcing (permanently).

The alpha coefficient determines the distance of the glow from the electrode. Gamma rather characterizes the geometric shape of the surface and the potential difference that led to the appearance of the discharge.

Features of corona discharge

A corona discharge usually occurs at a location with the smallest radius of curvature. If it is a line, the maximum probability of formation appears on the mechanical defect. The area of ​​the most frequent occurrence of a charge is called corona, or corona electrode. The conductor is under positive or negative potential. Accordingly, crowns of a similar kind are also distinguished (see above).

Positive and negative discharge differ in appearance. In the first case, the glow is uniform; in the second, there are epicenters along the surface of the wire. The mechanism of the process between the electrodes:

1. At the beginning, a non-self-sustained discharge occurs. This happens due to a random action: raindrops, a gust of wind, etc.
2. If the potential difference continues to grow, a faint glow is formed in the area of ​​\u200b\u200bthe wire, accompanied by a barely audible crackle. The causing voltage is called critical or initial.
3. With a further increase in the potential difference (spark breakdown voltage), the current increases according to a quadratic law, the glow becomes stronger. Sparks begin to jump with ever-increasing frequency.
4. A total increase in the potential difference causes an arc discharge, which manifests itself as a short circuit in the circuit. Its burning is difficult to stop.

Important! The critical and spark voltages are different for positive and negative corona.

So, the corona discharge in the laboratory setup is the precursor of the spark discharge, and the spark discharge is the precursor of the arc discharge. In practice, at nominal mains voltage, electricians do not worry too much about protection. It is possible to increase the voltage by 10% without much damage, if the specified area does not experience frequent bad weather, mainly sandstorms.

If the distance between the electrodes is too small, the corona discharge is not formed: after the non-self-sustained one, a spark immediately follows. Wires in power lines are trying to spread over a distance, ceramic insulators are used. A corona discharge is often replaced by a brush discharge if a pronounced point is present. Both are just a formal designation of an identical phenomenon.

A spark discharge occurs when the electric field strength reaches a breakdown value for a given gas. The value depends on the gas pressure; for air at atmospheric pressure, it is about . Increases with increasing pressure. According to Paschen's experimental law, the ratio of the breakdown field strength to pressure is approximately constant:

A spark discharge is accompanied by the formation of a brightly glowing tortuous, branched channel, through which a short-term current pulse of high strength passes. An example is lightning; its length is up to 10 km, the channel diameter is up to 40 cm, the current strength can reach 100,000 or more amperes, the pulse duration is about.

Each lightning consists of several (up to 50) pulses following the same channel; their total duration (together with the intervals between pulses) can reach several seconds. The temperature of the gas in the spark channel can be up to 10,000 K. Rapid strong heating of the gas leads to a sharp increase in pressure and the appearance of shock and sound waves. Therefore, a spark discharge is accompanied by sound phenomena - from a weak crackling with a low-power spark to thunder that accompanies lightning.

The appearance of a spark is preceded by the formation of a highly ionized channel in the gas, called a streamer. This channel is obtained by overlapping individual electron avalanches that occur in the path of the spark. The ancestor of each avalanche is an electron formed by photoionization. The scheme of streamer development is shown in fig. 87.1. Let the field strength be such that an electron escaping from the cathode due to some process acquires energy sufficient for ionization over the mean free path.

Therefore, the multiplication of electrons occurs - an avalanche occurs (the positive ions formed in this case do not play a significant role due to the much lower mobility; they only determine the space charge, which causes a redistribution of the potential). The short-wavelength radiation emitted by an atom, in which one of the internal electrons was torn out during ionization (this radiation is shown in the diagram by wavy lines), causes the photoionization of molecules, and the formed electrons generate more and more new avalanches. After the avalanches overlap, a well-conducting channel is formed - a streamer, along which a powerful flow of electrons rushes from the cathode to the anode - a breakdown occurs.

If the electrodes have a shape in which the field in the interelectrode space is approximately uniform (for example, it is balls of a sufficiently large diameter), then breakdown occurs at a well-defined voltage, the value of which depends on the distance between the balls. The spark voltmeter is based on this, with which high voltage is measured. When measuring, the largest distance at which a spark occurs is determined. Multiplying then by get the value of the measured voltage.

If one of the electrodes (or both) has a very large curvature (for example, a thin wire or a point serves as an electrode), then at a not too high voltage, a so-called corona discharge occurs. With increasing voltage, this discharge turns into a spark or arc.

During a corona discharge, ionization and excitation of molecules do not occur in the entire interelectrode space, but only near the electrode with a small radius of curvature, where the field strength reaches values ​​equal to or greater than . In this part of the discharge, the gas glows. The glow has the appearance of a corona surrounding the electrode, which is the reason for the name of this type of discharge. The corona discharge from the tip looks like a luminous brush, which is why it is sometimes called a brush discharge. Depending on the sign of the corona electrode, one speaks of positive or negative corona. Between the corona layer and the non-corona electrode is the outer region of the corona. The breakdown regime exists only within the corona layer. Therefore, we can say that the corona discharge is an incomplete breakdown of the gas gap.

In the case of a negative corona, the phenomena at the cathode are similar to those at the glow discharge cathode. Positive ions accelerated by the field knock out electrons from the cathode, which cause ionization and excitation of molecules in the corona layer. In the outer region of the corona, the field is insufficient to provide the electrons with the energy needed to ionize or excite the molecules.

Therefore, the electrons that have penetrated into this region drift under the action of zero to the anode. Some of the electrons are captured by molecules, resulting in the formation of negative ions. Thus, the current in the outer region is determined only by negative carriers - electrons and negative ions. In this region, the discharge has a non-self-sustaining character.

In the positive corona, electron avalanches originate at the outer boundary of the corona and rush to the corona electrode - the anode. The appearance of electrons that generate avalanches is due to photoionization caused by the radiation of the corona layer. The current carriers in the outer region of the corona are positive ions, which drift under the action of the field towards the cathode.

If both electrodes have a large curvature (two corona electrodes), processes inherent in the corona electrode of this sign proceed near each of them. Both corona layers are separated by an outer region in which counter flows of positive and negative current carriers move. Such a corona is called bipolar.

The independent gas discharge mentioned in § 82 when considering meters is a corona discharge.

The thickness of the corona layer and the strength of the discharge current increase with increasing voltage. At a low voltage, the size of the corona is small and its glow is imperceptible. Such a microscopic corona arises near the point from which the electric wind flows (see § 24).

The crown, which appears under the action of atmospheric electricity on the tops of ship masts, trees, etc., was in the old days called the fires of St. Elmo.

In high voltage applications, in particular in high voltage transmission lines, corona leads to harmful current leakage. Therefore, measures must be taken to prevent it. For this purpose, for example, the wires of high-voltage lines take a sufficiently large diameter, the larger, the higher the line voltage.

Useful application in technology corona discharge found in electrostatic precipitators. The gas to be purified moves in a pipe along the axis of which a negative corona electrode is located. Negative ions, which are present in large quantities in the outer region of the corona, are deposited on particles or droplets polluting the gas and are carried along with them to the external non-corona electrode. Upon reaching this electrode, the particles are neutralized and settle on it. Subsequently, when hitting the pipe, the sediment formed by the trapped particles crumbles into the collection.

electrical discharge- the process of the flow of electric current associated with a significant increase in the electrical conductivity of the medium relative to its normal state.
The increase in electrical conductivity is provided by the presence of additional free charge carriers. Electric discharges are non-self-sustained, flowing due to an external source of free charge carriers, and independent, continuing to burn even after the external source of free charge carriers is turned off.
There are the following types of electrical discharges: spark, corona, arc (electric arc) and glow.

Let's attach the ball electrodes to the capacitor bank and start charging the capacitors with the help of an electric machine. As the capacitors are charged, the potential difference between the electrodes will increase, and, consequently, the field strength in the gas will increase. As long as the field strength is low, no changes can be seen in the gas. However, with a sufficient field strength (about 30,000 V / cm), an electric spark appears between the electrodes, which has the form of a brightly glowing tortuous channel connecting both electrodes. The gas near the spark is heated to a high temperature and suddenly expands, which causes sound waves and we hear a characteristic crackle. The capacitors in this setup are added to make the spark more powerful and therefore more effective.
The described form of gas discharge is called spark discharge, or spark breakdown of the gas. When a spark discharge occurs, the gas suddenly, abruptly, loses its insulating properties and becomes a good conductor. The field strength at which a spark breakdown of a gas occurs has a different value for different gases and depends on their state (pressure, temperature). At a given voltage between the electrodes, the field strength is the smaller, the farther the electrodes are from each other. Therefore, the greater the distance between the electrodes, the greater the voltage between them is necessary for the onset of a spark breakdown of the gas. This voltage is called breakdown voltage.
The occurrence of breakdown is explained as follows. There is always a certain amount of ions and electrons in a gas, arising from random causes. Usually, however, their number is so small that the gas practically does not conduct electricity. At relatively small values ​​of the field strength, which we encounter in the study of non-self-sustained conductivity of gases, collisions of ions moving in an electric field with neutral gas molecules occur in the same way as collisions of elastic balls. With each collision, the moving particle transfers part of its kinetic energy to the resting particle, and both particles fly apart after the impact, but no internal changes occur in them. However, with sufficient field strength, the kinetic energy accumulated by the ion between two collisions can become sufficient to ionize a neutral molecule upon collision. As a result, a new negative electron and a positively charged residue, an ion, are formed. Such an ionization process is called impact ionization, and the work that must be expended to produce an electron detachment from an atom is called ionization work. The value of the work of ionization depends on the structure of the atom and therefore is different for different gases.
The electrons and ions formed under the influence of impact ionization increase the number of charges in the gas, and in turn they are set in motion under the action of an electric field and can produce impact ionization of new atoms. Thus, this process "reinforces itself", and the ionization in the gas quickly reaches a very large value. All phenomena are quite analogous to an avalanche in the mountains, for the origin of which an insignificant lump of snow is enough. Therefore, the described process was called an ion avalanche. The formation of an ion avalanche is the process of spark breakdown, and the minimum voltage at which an ion avalanche occurs is the breakdown voltage. We see that in the case of a spark breakdown, the cause of gas ionization is the destruction of atoms and molecules in collisions with ions.
One of the natural representatives of the spark discharge is lightning - beautiful and not safe.

The occurrence of an ion avalanche does not always lead to a spark, but can also cause a different type of discharge - a corona discharge.
Let us stretch on two high insulating supports a metal wire AB with a diameter of a few tenths of a millimeter and connect it to the negative pole of a generator giving a voltage of several thousand volts, for example, to a good electric machine. We will take the second pole of the generator to the Earth. We will get a kind of capacitor, the plates of which are our wire and the walls of the room, which, of course, communicate with the Earth. The field in this capacitor is very non-uniform, and its intensity is very high near a thin wire. By gradually increasing the voltage and observing the wire in the dark, one can notice that at a known voltage, a weak glow (“crown”) appears near the wire, covering the wire from all sides; it is accompanied by a hissing sound and a slight crackle. If a sensitive galvanometer is connected between the wire and the source, then with the appearance of a glow, the galvanometer shows a noticeable current going from the generator along the wires to the wire and from it through the air of the room to the walls connected to the other pole of the generator. The current in the air between the AB wire and the walls is carried by ions formed in the air due to impact ionization. Thus, the glow of the air and the appearance of a current indicate a strong ionization of the air under the action of an electric field.
corona discharge can occur not only at the wire, but also at the tip and in general at all electrodes, near which a very strong inhomogeneous field is formed.

Application of corona discharge.
1) Electric gas cleaning (electric filters). A vessel filled with smoke suddenly becomes completely transparent when sharp metal electrodes are introduced into it, connected to an electrical machine. Inside the glass tube there are two electrodes: a metal cylinder and a thin metal wire hanging along its axis. The electrodes are connected to an electric machine. If a stream of smoke (or dust) is blown through the tube and the machine is set in motion, as soon as the voltage is sufficient to form a corona, the outgoing stream of air will become completely clean and transparent, and all solid and liquid particles contained in the gas will be deposited on electrodes. The explanation for the experience is as follows. As soon as the corona is ignited near the wire, the air inside the tube is strongly ionized. Gas ions, colliding with dust particles, "stick" to the latter and charge them. Since a strong electric field acts inside the tube, the charged particles move under the action of the field to the electrodes, where they settle. The described phenomenon finds itself at the present time a technical application for the purification of industrial gases in large volumes from solid and liquid impurities.
2) Counters of elementary particles. Corona discharge underlies the operation of extremely important physical devices: the so-called counters of elementary particles (electrons, as well as other elementary particles that are formed during radioactive transformations), the Geiger-Muller counter. It consists of a small metal cylinder A, provided with a window, and a thin metal wire stretched about the axis of the cylinder and insulated from it. The counter is connected to a circuit containing a voltage source V of several thousand volts. The voltage is chosen such that it is only slightly less than the "critical", i.e., necessary to ignite the corona discharge inside the meter. When a fast moving electron enters the counter, the latter ionizes the gas molecules inside the counter, which causes the voltage required to ignite the corona to decrease somewhat. A discharge occurs in the counter, and a weak short-term current appears in the circuit.
The current arising in the meter is so weak that it is difficult to detect it with an ordinary galvanometer. However, it can be made quite noticeable if a very large resistance R is introduced into the circuit and a sensitive electrometer E is connected in parallel with it. When a current I occurs in the circuit, a voltage U is created at the ends of the resistance, equal to Ohm's law U = IxR. If we choose a resistance value R very large (many millions of ohms), but much smaller than the resistance of the electrometer itself, then even a very small current will cause a noticeable voltage. Therefore, with each hit of a fast electron inside the counter, the leaflet of the electrometer will give a rejection.
Such counters make it possible to register not only fast electrons, but in general any charged, rapidly moving particles capable of producing gas ionization by means of collisions. Modern counters easily detect even a single particle hitting them and, therefore, make it possible to make sure with complete certainty and very great clarity that elementary particles really exist in nature.

In 1802, V.V. Petrov established that if two pieces of charcoal are attached to the poles of a large electrolytic battery and, bringing the coals into contact, slightly separate them, then a bright flame forms between the ends of the coals, and the ends of the coals themselves become white hot. emitting blinding light electric arc). This phenomenon was independently observed seven years later by the English chemist Davy, who proposed to name this arc "voltaic" after Volta.
Typically, the lighting network is powered by an alternating current. The arc, however, burns more steadily if a constant current is passed through it, so that one of its electrodes is always positive (anode) and the other negative (cathode). Between the electrodes is a column of hot gas, a good conductor of electricity. In ordinary arcs, this pillar emits much less light than hot coals. Positive coal, having a higher temperature, burns faster than negative coal. Due to the strong sublimation of coal, a depression forms on it - a positive crater, which is the hottest part of the electrodes. The temperature of the crater in air at atmospheric pressure reaches 4000 °C. The arc can also burn between metal electrodes (iron, copper, etc.). In this case, the electrodes melt and quickly evaporate, which consumes a lot of heat. Therefore, the temperature of the crater of a metal electrode is usually lower than that of a carbon electrode (2000-2500 °C).
By causing an arc to burn between the carbon electrodes in a compressed gas (about 20 atm), it was possible to bring the temperature of the positive crater to 5900 °C, i.e., to the temperature of the surface of the Sun. Under this condition, coal melting was observed.
An even higher temperature is possessed by a column of gases and vapors, through which an electric discharge occurs. The vigorous bombardment of these gases and vapors by electrons and ions driven by the electric field of the arc brings the temperature of the gases in the column to 6000-7000°. Therefore, in the arc column, almost all known substances are melted and turned into vapor, and many chemical reactions are made possible that do not take place at lower temperatures. It is not difficult, for example, to melt refractory porcelain sticks in an arc flame. To maintain an arc discharge, a small voltage is needed: the arc burns well when the voltage on its electrodes is 40-45 V. The current in the arc is quite significant. So, for example, even in a small arc, a current of about 5 A flows, and in large arcs used in industry, the current reaches hundreds of amperes. This shows that the resistance of the arc is small; consequently, the luminous gas column also conducts electricity well.
Such a strong ionization of the gas is only possible due to the fact that the arc cathode emits a lot of electrons, which ionize the gas in the discharge space with their impacts. Strong electron emission from the cathode is ensured by the fact that the arc cathode itself is heated to a very high temperature (from 2200° to 3500°C depending on the material). When we first bring the coals into contact to ignite the arc, then at the contact point, which has a very high resistance, almost all the Joule heat of the current passing through the coals is released. Therefore, the ends of the coals are very hot, and this is enough for an arc to break out between them when they are moved apart. In the future, the cathode of the arc is maintained in a heated state by the current itself, passing through the arc. The main role in this is played by the bombardment of the cathode by positive ions falling on it.

Application of an arc discharge.
Due to the high temperature, the arc electrodes emit dazzling light, and therefore the electric arc is one of the best light sources. It consumes only about 0.3 watts per candle and is significantly more economical. Than the best incandescent lamps. The electric arc was first used for lighting by P. N. Yablochkov in 1875 and was called the “Russian Light”, or “Northern Light”.
The electric arc is also used for welding metal parts (electric arc welding). Currently, the electric arc is very widely used in industrial electric furnaces. In world industry, about 90% of tool steel and almost all special steels are smelted in electric furnaces.
Of great interest is a mercury arc burning in a quartz tube, the so-called quartz lamp. In this lamp, the arc discharge does not occur in air, but in an atmosphere of mercury vapor, for which a small amount of mercury is introduced into the lamp, and the air is pumped out. The light of the mercury arc is extremely rich in invisible ultraviolet rays, which have strong chemical and physiological effects. Mercury lamps are widely used in the treatment of various diseases ("artificial mountain sun"), as well as in scientific research as a strong source of ultraviolet rays.

In addition to the spark, corona and arc, there is another form of self-discharge in gases - the so-called glow discharge. To obtain this type of discharge, it is convenient to use a glass tube about half a meter long, containing two metal electrodes. We will connect the electrodes to a direct current source with a voltage of several thousand volts (an electric machine is suitable) and we will gradually pump air out of the tube. At atmospheric pressure, the gas inside the tube remains dark, since the applied voltage of several thousand volts is not enough to break through a long gas gap. However, when the gas pressure drops sufficiently, a luminous discharge flashes in the tube. It has the form of a thin cord (crimson in air, other colors in other gases) connecting both electrodes. In this state, the gas column conducts electricity well.
With further evacuation, the luminous cord blurs and expands, and the glow fills almost the entire tube. Distinguish the following two parts of the discharge: 1) non-luminous part adjacent to the cathode, called the dark cathode space; 2) a luminous column of gas that fills the rest of the tube, up to the anode itself. This part of the discharge is called the positive column.
In a glow discharge, the gas conducts electricity well, which means that strong ionization is maintained in the gas all the time. In this case, unlike the arc discharge, the cathode remains cold all the time. Why does the formation of ions occur in this case?
The drop in potential or voltage per centimeter of the length of the gas column in a glow discharge is very different in different parts of the discharge. It turns out that almost the entire potential drop falls on dark space. The potential difference that exists between the cathode and the boundary of space closest to it is called the cathode potential drop. It is measured in hundreds, and in some cases thousands of volts. The entire discharge appears to exist due to this cathode fall. The significance of the cathode fall is that positive ions, running through this large potential difference, acquire a greater speed. Since the cathode fall is concentrated in a thin layer of gas, there are almost no collisions of ions with gas atoms, and therefore, passing through the cathode fall region, the ions acquire a very large kinetic energy. As a result, when they collide with the cathode, they knock out a certain amount of electrons from it, which begin to move towards the anode. Passing through the dark space, the electrons, in turn, are accelerated by the cathodic potential drop and, upon collision with gas atoms in the more distant part of the discharge, produce impact ionization. The positive ions that arise in this case are again accelerated by the cathode fall and knock out new electrons from the cathode, etc. Thus, everything is repeated until there is voltage on the electrodes.
This means that the causes of gas ionization in a glow discharge are impact ionization and the knocking out of electrons from the cathode by positive ions.

The use of a glow discharge.
This discharge is mainly used for lighting. Used in fluorescent lamps.

Depending on the gas pressure, the voltage applied to the electrodes, the shape and nature of the location of the electrodes, the following types of independent discharge are distinguished: glow, corona, arc and spark.

glow discharge observed at low gas pressures (about 0.1 mm Hg). If a constant voltage of several hundred volts is applied to the electrodes soldered into a glass tube and then the air is gradually pumped out of the tube, then the following phenomenon is observed: when the gas pressure decreases, at some point a discharge appears in the tube, which has the form of a luminous cord connecting the anode and cathode tubes (Fig. 1). With a further decrease in pressure, this filament expands and fills the entire cross section of the tube, and the glow near the cathode weakens. The first dark space is formed near the cathode 1 , which is adjacent to the ion glowing layer 2 (smoldering glow), which has a sharp boundary on the cathode side and gradually disappears on the anode side. Behind the smoldering glow, there is again a dark gap 3 , called faraday or second dark space. Behind him lies a glowing area 4 extending to the anode, or a positive column.

Of particular importance in a glow discharge are only two of its parts - the cathode dark space and the glow glow, in which the main processes that maintain the discharge occur. The electrons that ionize the gas are produced by photoemission from the cathode and collisions of positive ions with the cathode of the tube.

At present, the glow discharge is widely used as a light source in various gas tubes. In daylight sources, the discharge usually occurs in mercury vapor. Gas pipes are also used for advertising and decorative purposes.

A glow discharge is used for cathode sputtering of metals, since the cathode substance in the glow discharge gradually passes into a gaseous state and settles in the form of metal dust on the tube walls. By placing various objects in a glow discharge, they cover them with uniform and durable layers of metal. This method is used for the manufacture of high quality metal mirrors.

spark discharge, often observed in nature, is lightning. Lightning is a discharge between two charged clouds or between a cloud and the earth. Charge carriers in clouds are charged water droplets or snowflakes.

Under laboratory conditions, a spark discharge can be obtained by gradually increasing the voltage between two electrodes located in atmospheric air and having such a shape that the electric field between them differs little from a uniform one. At a certain voltage, an electric spark occurs. In this case, the spark discharge penetrates the discharge gap with great speed, goes out and reappears. A brightly glowing curved spark channel connects both electrodes and has a complex branching (Fig. 2). The glow in the spark is the result of intense ionization processes. The sound effects accompanying the spark are generated by an increase in pressure (up to hundreds of atmospheres) due to heating of the gas (up to 10 5 °C) in the places where the discharge passes. A spark occurs when the electric field strength in a gas reaches a certain specific value, which depends on the type of gas and its state.

If, leaving the voltage constant, reduce the distance between the electrodes, then the field strength in the gas gap will increase. At a certain value, a spark discharge will occur. The higher the applied voltage, the greater the distance between the electrodes at which a spark discharge will occur. The principle of operation of a spark voltmeter - a device for measuring very high voltages - is based precisely on this phenomenon.

arc discharge can be observed under the following conditions: if, after ignition of the spark discharge, the resistance of the circuit is gradually reduced, then the current in the spark will increase. When the resistance of the circuit becomes small enough, a new form of gas discharge, called an arc, will occur. In this case, the current strength increases sharply, reaching tens and hundreds of amperes, and the voltage across the discharge gap decreases to several tens of volts. This shows that new processes arise in the discharge, giving the gas a very high electrical conductivity.

At present, an electric arc burning at atmospheric pressure is most often obtained between special carbon electrodes. The hottest point of the arc is the depression that forms on the positive electrode and is called the arc crater. Its temperature at atmospheric pressure is about 4000 °C.

The electric arc is a powerful light source and is widely used in projection, spotlight and other lighting installations. Due to the high temperature, the arc is widely used for welding and cutting metals. The high temperature of the arc is also used in the construction of electric arc furnaces, which play an important role in modern electrometallurgy.

corona discharge observed at relatively high gas pressures (for example, at atmospheric pressure) in a sharply inhomogeneous electric field. To obtain a significant inhomogeneity of the field, the electrodes must have sharply different surfaces, i.e. one electrode - a very large surface, and the other - a very small one. So, for example, a corona discharge can be easily obtained by placing a thin wire inside a metal cylinder, the radius of which is much larger than the radius of the wire.

The field strength near the wire is of the greatest importance. When the field strength reaches the value Ε ≈ 3 MV/m, a discharge is ignited between the wire and the cylinder, and a current appears in the circuit. At the same time, a glow is observed near the wire, which has the form of a shell or crown surrounding the wire, from which the name of the discharge originated.

Corona discharge occurs both at a negative potential on the wire (negative corona) and at a positive one (positive corona), as well as at an alternating voltage between the wire and the cylinder.

Corona discharge is used in engineering for the construction of electrostatic precipitators designed to purify industrial gases from solid and liquid impurities.

In nature, a corona discharge sometimes occurs under the action of an atmospheric electric field on tree branches, tops of masts (the so-called St. Elmo's fires). Corona discharge can occur on thin live wires. The occurrence of a corona discharge on the tips of the conductors explains the action of a lightning rod that protects buildings and transmission lines from lightning strikes.

Literature

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsy i vykhavanne, 2004. - C. 289-291.

The appearance of streamers in the volume between the electrodes does not always lead to a spark, but can also cause a discharge of another type of corona discharge. The figure shows a diagram of a device that can be used to reproduce a corona discharge. In this device, a thin wire is placed along the axis of a hollow metal cylinder.

At a voltage between the wire and the cylinder, an inhomogeneous electric field arises in the space between them with a maximum intensity near the wire. When the field strength near the wire approaches the breakdown value of the air strength (about U p \u003d 30,000 V / m), a corona discharge is ignited between the wire and the cylinder and current flows in the circuit, i.e. around the wire there is a glow - a corona. The appearance of the corona at a negative wire potential (negative corona) is somewhat different from the positive corona.

At a negative potential of the wire, electron avalanches start at the wire, propagate towards the anode, and at some distance the streamers break off due to a decrease in the field strength. In the case of a positive corona, electron avalanches originate at the outer boundary (surface) of the corona and move towards the wire. In contrast to a spark discharge, an incomplete breakdown of the gas gap occurs in a corona discharge, since electron avalanches in it do not penetrate through the entire gas layer E = .

Inside the coronas there are both positive and negative ions. Outside the corona there will be ions of only one sign: negative with a negative corona; positive ions with a positive corona.

Corona discharge can occur not only at the wire, but also at the tip and in general at all electrodes, near which a very strong inhomogeneous field is formed. The corona discharge is accompanied by a hissing sound and slight crackling. Corona discharge occurs on high-voltage power lines and causes leakage of electronic charges, i.e. electricity.

Application of corona discharge.

1. Electric cleaning of gases (electrostatic precipitators). Such an experience is known - a vessel filled with smoke instantly becomes completely transparent if sharp metal electrodes that are under high voltage are introduced into it.

This effect is used to purify gases. The contained solid and liquid particles in the gas in the corona discharge interact with the ions and become charged particles (the ions “stick” to the dust particles) and are then directed to the electrodes and deposited. In addition, such electrostatic precipitators make it possible to extract many tons of valuable products from gases in the production of sulfuric acid and non-ferrous metals in linear production.

2. Electron particle counters.

The voltage U is chosen so that it is somewhat less than the "critical", i.e. required to ignite the corona discharge inside the counter. When a fast moving electron enters the counter, it ionizes the gas molecules inside the volume, which causes the corona ignition voltage to decrease. A discharge occurs in the counter, and a weak short-term current pulse appears in the circuit. To register the signal, a sensitive electrometer E is used, each time a particle (even one electron) enters the volume of the counter, the leaves of the electrometer give a rejection.

§7. Classification of electrical discharges.

Electric discharges in gases proceed in different ways, i.e. in the discharge, certain fundamental (elementary) processes are realized, which are for a given type of discharge and determine its form; its characteristic features.

As we already know, there is a limited number of elementary processes that can be realized in the volume of a gas discharge; we list these processes again:

1) Collisions of gas particles result: energy exchange, momentum, excitation of atoms, ionization.

2) Attachment of electrons result: a negative ion appears, the concentration of electrons decreases.

3) Recombination result: radiation (photon) is born.

4) Receipt and emission of radiation in the volume of the discharge.

5) Diffusion of charged particles.

6) Electrode effects: thermionic emission; external photoelectric effect, emission upon electron impact, emission upon impact of positive ions: emission upon impact of neutral atoms; field emission.

At the same time, all these elementary - fundamental processes in the discharges are not realized. Depending on the conditions, only some processes are realized, and this set of elementary processes determines the basic properties of the discharge, i.e. this type of discharge differs from the other by a set of elementary processes. This set or type of discharge itself is determined by the following parameters of the system: the magnitude of the current, the voltage between the electrodes; gas pressure, the geometry of the discharge chamber, the material of the electrodes and the state of their surface, the temperature of the electrodes, etc.

The type of discharge is mainly determined by the voltage on the electrodes, the magnitude of the discharge current, and the pressure in the discharge chamber. In this case, voltage and current are independent parameters of the system.

Thus, the dependence of voltage on current becomes the most important integral characteristic of the electric discharge U = f(I) is also called the current-voltage characteristic of the discharge. It is formed depending on internal processes, therefore, it can be used to determine the type of discharge.

So, let's consider how one type of discharge is transferred to another type using the current-voltage characteristic.

The OB section is a non-self-sustaining dark discharge, the formation of current carriers occurs only due to an external ionizer, recombination occurs in the OA section, and all charges reach the electrodes in the AB section, and charge recombination can be neglected.

Beyond point B, the ionization of neutral particles by electron impact begins, and avalanches of electrons and ions appear. However, if you remove the external ionizer, the discharge stops. This is a non-self-sustaining Townsend discharge - this is the BC section.

Secondary electrons knocked out of the cathode by positive ions, light quanta, and excited molecules play a significant role in the CD region. The need to maintain ionization due to the energy of external sources disappears - the discharge becomes independent, it is also called an independent Townsend discharge (this is the CE section).

In section EF, the Townsend discharge transforms into a normal glow discharge, which corresponds to section FH. On the NK section, the voltage also increases with growth. The discharge corresponding to the NK section is called an anomalous glow discharge.

Further, with increasing current, the temperature of the cathode increases, the role of thermionic emission increases, the discharge is contracted and an arc discharge is formed. The arc discharge is maintained by thermionic emission from the cathode.

Stationary glow discharge at low pressure.

As the current increases, an independent Townsell discharge (section CEF) can develop in different ways and have several forms. If at a pressure of about 1 mm. rt. Art. the discharge occurs between the electrodes connected to a direct current source, then a normal discharge is realized.

Section FH of the current-voltage characteristic corresponds to a glow discharge. A distinctive feature of a glow discharge is a peculiar potential distribution along the length of the interelectrode gap. The distribution of the potential leads to the fact that the glow discharge has a characteristic inhomogeneous appearance, and therefore, an inhomogeneous structure, the discharge seems to be divided into parts. A glow discharge consists of a cathode region and a positive column.

Let's consider the different parts of the discharge. Starting from the cathode to the anode.

The cathode region of the discharge.

The electrons needed to maintain the discharge are mainly emitted when the cathode is bombarded with positive ions. Secondary electrons leave the cathode with low velocities, as a result of which they (near the surface they form a negative space charge) do not yet have sufficient energy to excite gas molecules, therefore the molecules do not radiate, and a dark space is formed directly at the cathode surface, filled with slow electrons. This very thin, non-luminous layer of gas is called Aston's dark space. The current in this region is mainly generated by positive ions.

Further, the electron is accelerated by the field, the kinetic energy of the electrons becomes sufficient to excite the gas molecules, and this causes the appearance of a thin luminous layer of gas, called the first cathode glow. In this region, electrons partially or completely lose speed during collisions. Therefore, behind the first cathode glow, the next dark cathode space is formed. In this region, weak recombination of electrons with positive ions occurs, so very weak radiation occurs here. In the dark cathode space, electrons are strongly accelerated to velocities at which they intensely ionize gas molecules and, consequently, multiply.

At the end of the second dark cathode space, the number of electrons is already so large that the current is almost completely carried by electrons, and they noticeably reduce the positive space charge, even form a region of negative space charge. In this region, further acceleration of electrons stops, and the energy accumulated in the region of the second cathode dark space is spent mainly on intense excitation and ionization of molecules. This occurs in the region of the second cathode glow (negative cathode glow). As a result, the electron energy decreases, gradually the intensity of excitation and ionization also decreases, therefore, the number of electrons decreases (and due to recombination and diffusion), so that the negative space charge vanishes. Accordingly, the electric field strength changes and at the point of disappearance of the negative charge E takes a constant value (about 1 V/cm) and does not change up to the anode region of the charge. In this place, the positive column of the glow discharge begins.

The space occupied by Aston's dark space by the first cathode glow and the second dark space is called the cathode drop region. As can be seen from the figure, the potential drop between the electrodes is almost completely realized in a small area near the cathode. The length of this section varies inversely with the gas pressure. At P = 1 mm Hg. dc is about 10mm and U=100-250V.

In a normal glow discharge, the current density remains constant as the discharge current increases or decreases. But it depends on the pressure Р and changes according to the law P 2 . For example, at P = 1 mm Hg. average density j\u003d 0.1 mA / cm 2 \u003d 1 10 4 A / cm 2. But j also depends on the nature of the gas and on the material of the cathode. From I=jS it follows that at a low current part of the area takes part in the discharge.

Under these conditions, the cathode potential drop U k remains constant. For pressure range from 1-10 mmHg. the value of U k does not depend on pressure and is uniquely determined by the nature of the gas and cathode material. Examples

With an increase in the discharge current, there comes a moment when the entire area of ​​the cathode takes part in the discharge, from this moment, with a further increase in the current, an increase in the cathode potential drop begins. The field strength E increases until the necessary ionization is provided to sustain the increase in current. Under these conditions, a normal glow discharge transforms into an anomalous glow discharge.

where, k is a constant depending on the type of gas and cathode material.

Positive post.

The positive column is made up of plasma, and the plasma is a neutral electrically conductive medium. Therefore, the positive glow column plays the role of an ordinary conductor connecting the near-cathode region with the near-anode part of the discharge. Unlike other parts of the glow discharge, which have specific dimensions and structure depending on the type of gas, its pressure, and discharge current density, the length of the positive column is determined by the dimensions of the discharge chamber, and the structure of the column is an ionized gas ( n e ≈ n i), i.e. it can be of any length. The field strength is about 1 V/cm and tends to increase with increasing pressure. The intensity also changes with a change in the radius of the chamber (tube) - compression of the discharge increases the field: E always takes a value just sufficient to maintain the degree of ionization in the column that is needed for stationary burning of the discharge. The energy in the column is sufficient for ionization. And the ionization process compensates for the loss of electrons and ions due to recombination and diffusion, followed by neutralization on the electrodes and on the chamber walls, the glow of the positive column is associated with all these processes. Unlike other parts, in the positive column of a glow discharge, the chaotic motion of charged particles prevails over the directed one.

anode region.

The anode attracts electrons from the positive column and a negative space charge and an increase in the field strength are formed near the binding site, as a result of which the discharge current is transferred to the anode surface. The anode drop region is the passive part of the discharge. The anode does not emit charges. A glow discharge can exist without an anode region, as well as without a positive column. The positive column of the discharge does not depend on near-electrode processes. The difference between the cathode parts is the predominantly directed movement of electrons and ions.

The use of a glow discharge.

Glow discharge in rarefied gases finds a variety of applications in gas-filled rectifiers, converters, indicators, voltage stabilizers, gas-light fluorescent lamps. For example, in neon lamps (for signaling purposes) a glow discharge is used in neon, the electrodes are coated with a layer of barium and they have a cathode potential drop of the order of 70 V and ignite when turned on in the lighting network.

In fluorescent lamps, the glow discharge occurs in mercury vapor. Mercury vapor radiation is absorbed by the phosphor layer, which covers the inner surface of the gas-light tube.

Glow discharge is also used for cathode sputtering of metals. The surface of the cathode during a glow discharge, due to bombardment by positive gas ions, is strongly heated in separate small areas and therefore gradually passes into a vapor state. By placing objects near the discharge cathode, they can be covered with a uniform layer of metal.

In recent years, a glow discharge has found application in plasma chemistry and laser technology. In them, a glow discharge is used in an abnormal mode at elevated pressure.

1. p = 6.7 kPa ≈ 50 mm. rt. Art.

v= 15.7 m/s

2. p = 8 kPa ≈ 60 mm. rt. Art.

v= 21m/s

Typical volt-ampere characteristics of a glow discharge in a transverse air flow.

1 mm. rt. Art. = 133 Pa. 1kPa=1000/133=8mmHg