Independent arc discharge (low, medium and high pressures). Abstract: Arc discharge in gases

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Development of a lesson with a presentation in physics on the topic: "Electric current in gases"

Prepared the development of a lesson in physics: Semenchenko Galina Vasilievna, Barnaul KGOUNPO PU -13, teacher of physics, astronomy and electrical engineering, email: [email protected]

Epigraph:

“The day before yesterday we did not know anything about electricity, yesterday we did not know anything about the huge reserves of energy contained in the atomic nucleus, which we do not know today?”

/Louis de Broglie/

Electric current in a gas is a directed movement of positive ions to the cathode, and negative ions and electrons to the anode.

When a positive and negative ion collides, the negative ion can donate its excess electron to the positive ion and both ions will turn into neutral atoms.

The process of mutual neutralization of ions is called ion recombination.

When a positive ion and an electron or two ions recombine, a certain energy is released, equal to the energy spent on ionization.

Partially, it is emitted in the form of light, and therefore the recombination of ions is accompanied by luminescence (luminescence of recombination).

The process of passage of electric current in gases is called gas discharge.

Grades are of two types:

Independent - a discharge that occurs without anyone's help in gases.

Non-self-sustained - a discharge that occurs in gases with the help of an ionizer.

Ionizers are factors that cause gas ionization.

Factors include:

heating the gas to a high temperature;
x-rays;
rays resulting from radioactive decay;
cosmic rays;
bombardment of gas molecules by fast moving electrons or ions.

Non-self discharge

The electrical conductivity of the gas is created by external ionizers;

With the termination of the action of external ionizers, the non-self-sustained discharge ceases;

A non-self-sustaining gas discharge is not accompanied by gas glow.

self-discharge

For its implementation, it is necessary that as a result of the discharge itself, free charges are continuously formed in the gas. The main source of free charges is the impact ionization of gas molecules.

Positive ions formed during the collision of electrons with neutral atoms, when moving towards the cathode, acquire a large kinetic energy under the action of the field. When such fast ions hit the cathode, electrons are knocked out from the cathode surface.

In addition, the cathode can emit electrons when heated to a high temperature. This process is called thermionic emission. It can be considered as the evaporation of electrons from the metal. In many solid substances, thermionic emission occurs at temperatures at which the evaporation of the substance itself is still small. Such substances are used for the manufacture of cathodes.

Types of independent discharges.

Depending on the properties and state of the gas, the nature and location of the electrodes, as well as the voltage applied to the electrodes, various types of self-discharge occur.

Smoldering discharge.

A glow discharge is observed in gases at low pressures of the order of several tens of millimeters of mercury and less.

The main parts of a glow discharge are the cathode dark space, a negative or glow glow sharply distant from it, which gradually passes into the region of the Faraday dark space. These three regions form the cathode part of the discharge, followed by the main luminous part of the discharge, which determines its optical properties and is called the positive column.

At sufficiently low pressures, electrons knocked out of the cathode by positive ions pass through the gas almost without collisions with its molecules, forming electron or cathode rays.

Type of glow discharge

Glow discharge generated by a generator

Application of glow discharge

Glow discharge is used in gas-light tubes, fluorescent lamps, voltage stabilizers, to obtain electron and ion beams.

If a slit is made in the cathode, then narrow ion beams, often called channel beams, pass through it into the space behind the cathode.

The phenomenon of cathode sputtering is widely used, i.e. destruction of the cathode surface under the action of positive ions hitting it. Ultramicroscopic fragments of the cathode material fly in all directions along straight lines and cover the surface of bodies (especially dielectrics) placed in a tube with a thin layer.

In this way, mirrors are made for a number of devices, a thin layer of metal is applied to selenium photocells.

Glow discharge in production

Corona treatment of surfaces

corona discharge

A corona discharge occurs at normal pressure in a gas in a highly inhomogeneous electric field (for example, near spikes or wires of high voltage lines).

In a corona discharge, gas ionization and its glow occur only near the corona electrodes. In the case of cathode corona (negative corona), electrons that cause impact ionization of gas molecules are knocked out of the cathode when it is bombarded with positive ions.

If the anode is corona (positive corona), then the birth of electrons occurs due to the photoionization of the gas near the anode.

Corona is a harmful phenomenon, accompanied by current leakage and loss of electrical energy. To reduce corona, the radius of curvature of the conductors is increased, and their surface is made smoother.

Type of corona discharge

slide number 13

A special case of corona discharge - carpal

At an increased voltage, the corona discharge on the tip takes the form of light lines emanating from the tip and alternating in time. These lines, which have a number of kinks and bends, form a kind of brush, as a result of which such a discharge is called a brush discharge.

Corona discharge has to be considered when dealing with high voltage. If there are protruding parts or very thin wires, corona discharge can start. This results in power leakage. The higher the voltage of the high-voltage line, the thicker the wires should be.

Saint Elmo's fire

A charged thundercloud induces electric charges of the opposite sign on the Earth's surface under it. A particularly large charge accumulates on the tips. Therefore, before a thunderstorm or during a thunderstorm, cones of light like brushes often flare up on the points and sharp corners of highly raised objects. Since ancient times, this glow has been called the fires of St. Elmo.

Especially often climbers become witnesses of this phenomenon. Sometimes even not only metal objects, but also the ends of the hair on the head are decorated with small luminous tassels.

Saint Elmo's fires before a thunderstorm in the ocean

slide number 17

spark discharge

The spark discharge has the appearance of bright zigzag branching filaments-channels that penetrate the discharge gap and disappear, being replaced by new ones.

The spark discharge channels begin to grow sometimes from the positive electrode, sometimes from the negative, and sometimes from some point between the electrodes.

A spark discharge is accompanied by the release of a large amount of heat, a bright glow of gas, crackling or thunder.

All these phenomena are caused by electron and ion avalanches that occur in spark channels and lead to a huge increase in pressure, reaching 107 108 Pa, and an increase in temperature up to 10,000 C.

Application of spark discharge

With a small length of the discharge gap, the spark discharge causes a specific destruction of the anode, called erosion. This phenomenon was used in the electrospark method of cutting, drilling and other types of precision metal processing.

The spark gap is used as a surge protector in electrical transmission lines (eg telephone lines).

An electric spark is used to measure large potential differences using a spherical spark gap, the electrodes of which are two metal balls with a polished surface.

Electric spark machine

slide number 21

A typical example of a spark discharge is lightning.

The main lightning channel has a diameter of 10 to 25 cm, and the lightning length can reach several kilometers. The maximum current of a lightning pulse reaches tens and hundreds of thousands of amperes.

Lightning is linear and ball.

Ball lightning is a single brightly luminous, relatively stable, small mass that is observed in the atmosphere, floating in the air and moving along with air currents, containing great energy in its body, disappearing quietly or with great noise like an explosion, and leaving no material after its disappearance. traces, except for the destruction that she managed to do.

Fireball

slide number 23

How to behave during a thunderstorm?

You can’t take shelter in a thunderstorm near lonely standing trees, poles and other high local objects, you need to move 15 meters away.
It is dangerous to be in or near water.
You can’t pitch a tent near the water, as lightning often strikes river banks.
Never underestimate the danger of lightning.
If a thunderstorm caught you in a car, do not get out of it. Close all doors and windows and wait out the bad weather inside.
During a thunderstorm in a country house, disconnect electrical appliances from the network, and the TV from an individual antenna.
Lightning rarely strikes shrubs, almost never hits maple and birch, most often it hits oak and poplar.

arc discharge

The arc discharge was discovered by V. V. Petrov in 1802. This discharge is one of the forms of gas discharge, which occurs at a high current density and a relatively low voltage between the electrodes (on the order of several tens of volts).

The main cause of the arc discharge is the intense emission of thermoelectrons by a hot cathode. These electrons are accelerated by an electric field and produce impact ionization of gas molecules, due to which the electrical resistance of the gas gap between the electrodes is relatively small.

In some cases, an arc discharge is also observed at a relatively low cathode temperature (mercury arc lamp).

The arc discharge has found application in a mercury rectifier, which converts an alternating electric current into a direct current.

Application of an arc discharge

In 1876, P. N. Yablochkov first used an electric arc as a light source.

The arc discharge is used as a light source in searchlights and projectors.

The high temperature of the arc discharge makes it possible to use it for the construction of an arc furnace. Arc furnaces, powered by a very high current, are used in a number of industries: for the smelting of steel, cast iron, ferroalloys, bronze, the production of calcium carbide, nitrogen oxide, etc.

In 1882, N. N. Benardos first used an arc discharge for cutting and welding metal.

In 1888, N. G. Slavyanov improved this welding method by replacing the carbon electrode with a metal one.

W eminent physicists who studied the arc discharge.

Plasma applications

Low-temperature plasma is used in gas-discharge light sources - in luminous tubes for advertising inscriptions, in fluorescent lamps. A gas discharge lamp is used in many devices, for example, in gas lasers - quantum light sources.

High-temperature plasma is used in magnetohydrodynamic generators.

A new device, the plasma torch, has recently been created. The plasmatron creates powerful jets of dense low-temperature plasma, which are widely used in various fields of technology: for cutting and welding metals, drilling wells in hard rocks, etc.

Introduction.

Properties of an arc discharge.

1. Formation of an arc.

2. Cathode spot. Appearance and individual parts

arc discharge.

3. Potential distribution and current-voltage

arc discharge characteristic.

4. Temperature and radiation of individual parts of the arc discharge.

5. Generation of undamped oscillations using electrical

tric arc.

6. Positive arc discharge column at high

and ultra-high pressure.

III. Application of an arc discharge.

1. Modern methods of electrical processing.

2. Electric arc welding.

3. Plasma technology.

4. Plasma welding.
IV. Conclusion.

An arc discharge in the form of a so-called electric (or voltaic) arc was first discovered in 1802 by a Russian scientist, professor of physics at the Military Medical-Surgical Academy in St. Petersburg, and later an academician of the St. Petersburg Academy of Sciences, Vasily Vladimirovich Petrov. Petrov describes in one of his published books his first observations on an electric arc in the following words:

“If two or three charcoals are placed on a glass tile or on a bench with glass legs ... and if metal insulated guides ... connected with both poles of a huge battery, bring them closer to each other at a distance of one to three lines, then between them is a very bright white light or flame, from which these coals ignite more quickly or more slowly and from which the dark peace can be quite clearly illuminated ... ".

The path to the electric arc began in ancient times. Even the Greek Thales of Miletus, who lived in the sixth century BC, knew the property of amber to attract light objects - feathers, straw, hair, and even create sparks when rubbed. Until the seventeenth century, this was the only method of electrifying bodies that had no practical application. Scientists have been looking for an explanation for this phenomenon.

The English physicist William Gilbert (1544-1603) found that other bodies (for example, rock crystal, glass), like amber, have the property of attracting light objects after rubbing. He called these properties electrical, for the first time introducing this term into use (in Greek amber-electron).

Magdeburg burgomaster Otto von Guericke (1602-1686) designed one of the first electric machines. It was an electrostatic machine, which was a sulfur ball mounted on an axle. One of the poles was ... the inventor himself. When the crank was turned, bluish sparks flew out of the palms of the contented burgomaster with a slight crackle. Later, Guericke's machine was improved by other inventors. The sulfur ball was replaced by a glass one, and instead of the researcher's palms, leather pads were used as one of the poles.

Of great importance was the invention in the eighteenth century of the Leyden condenser jar, which made it possible to accumulate electricity. It was a glass vessel filled with water wrapped in foil. A metal rod passed through a cork was immersed in water.

The American scientist Benjamin Franklin (1706-1790) proved that water does not play any role in the collection of electric charges, glass-dielectric has this property.

Electrostatic machines have become quite widespread, but only as funny gizmos. True, there were attempts to treat patients with electricity, but it is difficult to say what the physiotherapeutic effect of such treatment was.

The French physicist Charles Coulomb (1736-1806), the founder of electrostatics, established in 1785 that the force of interaction of electric charges is proportional to their magnitudes and inversely proportional to the square of the distance between them.

In the forties of the eighteenth century, Benjamin Franklin put forward the theory that there is only one kind of electricity - a special electrical matter, consisting of tiny particles that can penetrate into the substance. If there is an excess of electrical matter in the body, it is positively charged; if it is deficient, the body is negatively charged. Franklin introduced the plus and minus signs into practice, as well as the terms: capacitor, conductor, charge.

M. V. Lomonosov (1711-1765), Leonhard Euler (1707-1783), Franz Aepinus (1724-1802) and other scientists came up with original theories about the nature of electricity. By the end of the eighteenth century, the properties and behavior of fixed charges were sufficiently studied and explained to some extent. However, nothing was known about electric current-moving charges, since there was no device that could make a large number of charges move. The currents drawn from the electrostatic machine were too small to be measured.

1. If you increase the current strength in a glow discharge, reducing the external resistance, then at a high current strength, the voltage at the tube clamps begins to drop, the discharge quickly develops and turns into an arc. In most cases, the transition is made abruptly and almost often leads to a short circuit. When selecting the resistance of the external circuit, it is possible to stabilize the transitional form of the discharge and observe at certain pressures a continuous transition of a glow discharge into an arc. In parallel with the voltage drop between the electrodes of the tube, there is an increase in the cathode temperature and a gradual decrease in the cathode drop.

The question of the development of an arc when a circuit is broken is technically important not only from the point of view of obtaining "useful" arcs, but also from the point of view of combating "harmful" arcs, for example, with the formation of an arc when a knife switch is opened. Let L be the self-induction of the circuit, W be its resistance, and ع be the emf. current source, U(I) is a function of the current-voltage characteristic of the arc. Then we must have: ع= L dI/dt+WI+U(I) (1) or

LdI/dt=(ع-WI)-U(I)=∆ (2).

The difference (ع - WI) is nothing more than the ordinate of the direct resistance AB (Fig. 1), and U (I) is the ordinate of the arc characteristic for a given I. For dI / dt to be negative, i.e. For the current I to necessarily decreased with time and a stable arc did not form between the electrodes of the switch, it is necessary that

Fig.1. The relative position of the direct resistance and the curve of the current-voltage characteristic of a steady arc for the cases: a) when the arc can not occur when the circuit is broken; b) when the arc occurs at a break in the current strength interval corresponding to points P and Q.

∆ع-WI took place.

To do this, the characteristic with all its points must lie above the resistance line (Fig. 1, a). This simple conclusion does not take into account the capacitance in the circuit and applies only to direct current.

The point of intersection of the direct resistance with the curve of the current-voltage characteristic of a steady arc corresponds to the lowest limit of the direct current strength, at which an arc can occur when the circuit is broken (Fig. 1, b). In the case of opening an alternating current arc with a knife switch, which is extinguished at each voltage transition through zero, it is essential that the conditions existing in the discharge gap during opening do not allow a new ignition of the arc with a subsequent increase in the voltage of the current source. This requires that, as the voltage increases, the discharge gap be sufficiently deionized. In switches of strong alternating currents, enhanced deionization is artificially achieved by introducing special electrodes that suck out charged gas particles due to bipolar diffusion, as well as by using mechanical blowing or by exposing the discharge to a magnetic field. At high voltages, oil switches are used.

2. The cathode spot, fixed on the carbon cathode, on the surface of liquid mercury is in continuous rapid motion. The position of the cathode spot on the surface of liquid mercury can be fixed with a metal pin immersed in the mercury and protruding slightly from it.

In the case of a small distance between the anode and the cathode, the thermal radiation of the anode strongly affects the properties of the cathode spot. With a sufficiently large distance between the anode and the carbon cathode, the dimensions of the cathode spot tend to a certain constant limit value, and the area occupied by the cathode spot on the carbon electrode in air is proportional to the current strength and corresponds to an atmospheric pressure of 470 a / cm². For a mercury arc 4000 A/cm² found in vacuum.

With a decrease in pressure, the area occupied by the cathode spot on the carbon cathode increases at a constant current strength.

The sharpness of the visible boundary of the cathode spot is explained by the fact that a relatively slow decrease in temperature with distance from the center of the spot corresponds to a rapid drop in both light radiation and thermionic emission, and this is equivalent to a sharp "optical" and "electrical" spot boundaries.

When the arc burns in air, the carbon cathode sharpens, while on the carbon anode, if the discharge does not cover the entire front area of the anode, a round depression is formed - a positive arc crater.

The formation of the cathode spot is explained as follows. The distribution of space charges in a thin layer at the cathode is such that here the discharge requires for its maintenance the smaller potential difference, the smaller the cross section of the discharge channel. Therefore, the discharge at the cathode must contract.

Directly adjacent to the cathode spot is a part of the discharge called the negative cathode brush or negative flame. The length of the cathode brush in the arc at low pressure is determined by the distance over which fast primary electrons fly, having received their velocities in the region of the cathode potential drop.

Between the negative brush and the positive column there is an area similar to the Faraday dark space of a glow discharge. In the Petrov arc in the air, in addition to the negative brush, there is a positive flame and a number of halos. Spectral analysis indicates the presence in these flames and halos of a number of chemical compounds (cyanide and nitrogen oxides).

Intermittent (even when using direct current sources). It occurs in a gas usually at pressures of the order of atmospheric. Under natural conditions, a spark discharge is observed in the form of lightning. On the outside, the spark discharge is a beam of bright zigzag branching thin strips, instantly penetrating the discharge gap, quickly fading and constantly ...

Phenomena of the passage of electric current through gases called electrical (gas) discharges. There are various forms of electric discharge, differing from each other in the magnitude of the discharge current, voltage, temperature and gas pressure. Charges can be stable and unstable (for example, spark). There is no strict quantitative boundary between the discharges; one type of discharge can pass into another. The main types of discharges: dark, smoldering, arc, spark, corona. Arc discharge is the highest form of discharge, which differs from other forms of discharge in its physical properties. Thus, the glow discharge has the following parameters:

pressure - several torr (mm Hg);
current density at the cathode - (10 -3 -10 -2) A / mm 2;
voltage - (200-300) V;
cathode voltage drop ~ 100 V.

Physical properties of arc discharge:

pressure up to 1 atm. and higher;
current density at the cathode - up to 10 8 A / mm 2;
small arc length - up to 20-30 mm;
low arc voltage - (12¸50) V;
high temperature of the arc column - (from 5 to 30) 10 3 K;
dazzling brightness (due to the recombination of charged particles with the release of light energy);
high concentration of charged particles in the cathode region of the discharge.

It received the name "arc" for the shape of a brightly luminous cord (pillar) of the discharge, which in the first experiments with low-current discharges bent upwards with a crescent-shaped arch under the action of ascending convective flows of air heated by the discharge. Although in most cases, for example, between vertical electrodes in a limited enclosed space, a similar discharge does not have an arcuate shape, its original name has been preserved.

Arc discharges are widely used in engineering. They are sources of light for spotlights and film projection equipment, in special ultra-high pressure CBD lamps (up to 100 atm). The arc is used in gastrons, thyratrons, mercury rectifiers for rectifying current and controlling its strength, etc. The electric arc has been widely used in metallurgy and welding technology for heating and melting metals.

The term "arc" applies only to stable or quasi-stable types of discharges. An arc is considered to be the final form of a discharge that has developed under any circumstances, if a sufficiently large current passes through the gas. Such a discharge can be obtained in various ways: from any stable low-power discharge; from an unstable spark discharge or by pushing two current-carrying, pre-contact electrodes.

The priority in the discovery of the arc discharge belongs to academician Vasily Vladimirovich Petrov - 1802. He spoke about the possibility of using an arc discharge for melting metals. This phenomenon was called an arc by the Englishman Gamfy Davy, who, independently of Petrov V.V., investigated this phenomenon in 1808-1810.

The history of the development of technology in the second half of the 19th century is remarkable for the development of ways for the practical use of electricity, including for the purposes of heating and melting metal. In May 1981, the whole world, by decision of UNESCO, celebrated the most important memorable date - the 100th anniversary of the creation of an industrial method of electric arc welding of metals by the Russian inventor Nikolai Nikolaevich Benardos.

GOST 19521 includes 35 technological varieties of the arc discharge. As technological features of the arc, the standard defines: the type of electrode, the nature of the effect on the base metal, the type of current used, the presence of external influence on the formation of the weld, the number of electrodes with a common welding current supply, the presence and direction of the electrode oscillations relative to the axis of the weld, the number of arcs with separate power supply current, etc. Let us dwell on the most significant of them.

Welding can be carried out with both consumable and non-consumable electrode. As a non-consumable electrode, graphite or metals with a high melting point are most often used - molybdenum, tantalum, tungsten, etc. The arc can be powered by alternating or direct current, as well as a combined method. With alternating current, the frequency can be not only 50 Hz, but also increased. Welding can be an arc of direct and indirect action (Fig. 13). When welding with a direct arc, the parts to be welded are included in the welding circuit, their heating is carried out due to the energy of charged particles reaching the active spot. When welding with an indirect arc, the parts to be welded are not included in the welding circuit, their heating is carried out due to heat transfer (mainly radiant) from the arc column.

The degree of gas ionization in the arc is up to several percent. This is considered a high degree of ionization, because with an ionization degree of more than 0.01%, the gas is in the plasma state at a temperature of more than 3000 K. This is a low-temperature plasma.

In manual arc welding, the current density is (10-15) A / mm 2, when welding with a consumable electrode in shielding gases, up to 400 A / mm 2. These values are much less than the above value of the current density on the cathode up to 10 8 A / mm 2, because in practice the current density is determined by its ratio to the transverse area of the electrode, and when studying the physical properties of the discharge - by the ratio of the current to the area of the cathode cells of the end electrode. The area of these cells is much smaller than the area of the electrode and is determined from the results of high-speed filming of the process.

In physics, it is customary to call an electrode any object to which a conductor is connected from a current source. In welding, it is customary to call an electrode - a wire electrode, and a flat electrode - a product. When welding with direct current, a distinction is made between direct and reverse polarity. With direct polarity, the cathode is the electrode, with reverse polarity, the product. Direct polarity welding is used to a lesser extent, for example, when welding with a non-consumable electrode in inert gases of steels. Most often, DC welding is performed on reverse polarity.

The composition of the gas phase can be different - air, shielding gases, metal vapors and components of the flux or electrode coating. Gas pressure - from vacuum (not lower than 50 torr) to several atmospheres.

Electric discharges are independent and non-self-sustaining. With independent discharges, the charged particles necessary for the existence of the discharge are formed due to the processes occurring in the discharge itself. The arc is an independent discharge. Electric particles - electrons and ions are formed due to the processes of emission and ionization. The arc energy is not enough for the formation of other types of particles.

Types of gas discharge and their application. The concept of plasma.

Branch:

Accounting and law

Speciality:

Jurisprudence

Group:

Compiled by:

Evtikhevich A. A.

Teacher:

Orlovskaya G.V.

2011
Content:

Page 1: gas discharge

Application of gas discharge

Page 2: spark discharge

corona discharge

Page 3: Application of corona discharge

Page 4: arc discharge

Page 5: Application of an arc discharge

glow discharge

Page 6-7: Plasma

Page 8: Literature

Gas discharge- a set of processes that occur when an electric current flows through a substance in a gaseous state. Typically, the flow of current becomes possible only after sufficient ionization of the gas and the formation of a plasma. Ionization occurs due to collisions of electrons accelerated in an electromagnetic field with gas atoms. In this case, an avalanche increase in the number of charged particles occurs, since in the process of ionization new electrons are formed, which, after acceleration, also begin to participate in collisions with atoms, causing their ionization. The occurrence and maintenance of a gas discharge requires the existence of an electric field, since a plasma can exist only if electrons acquire energy in an external field sufficient to ionize atoms, and the number of formed ions exceeds the number of recombined ions.

If the existence of a gas discharge requires additional ionization due to external sources (for example, using ionizing radiation), then the gas discharge is called dependent(such discharges are used in Geiger counters).

For the implementation of the gas discharge, both time-constant and alternating electric fields are used.

Depending on the conditions under which the formation of charge carriers occurs (gas pressure, voltage applied to the electrodes, the shape and temperature of the electrodes), there are several types of independent discharges: smoldering, spark, corona, arc.

Applications of gas discharge

Arc discharge for welding and lighting.
Super high frequency discharge.
Glow discharge as a light source in fluorescent lamps and plasma screens.
Spark discharge for ignition of the working mixture in internal combustion engines.
Corona discharge for cleaning gases from dust and other contaminants, for diagnosing the state of structures.
Plasmatrons for cutting and welding.
Discharges for pumping lasers, such as helium-neon laser, nitrogen laser, excimer lasers, etc.

in a Geiger counter,
in ionization vacuum gauges,
in thyratrons,
in krytrons,
in a Geissler tube.

spark discharge. 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 a gas discharge is called a spark discharge, or a spark breakdown of a 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 low field strengths, which we encounter in the study of the 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.
corona discharge. 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). One type of counter (Geiger-Muller counter) is shown in Figure 1.
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.
arc discharge. 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. By emitting dazzling 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.
glow discharge. 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.
And here's how it works. 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 we see 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.
This discharge is mainly used for lighting. Applicable in fluorescent lamp.

The word "plasma" (from the Greek. "plasma" - "decorated") in the middle of the XIX century. they began to call the colorless part of the blood (without red and white bodies) and the liquid that fills living cells. In 1929, the American physicists Irving Langmuir (1881-1957) and Levi Tonko (1897-1971) named the ionized gas in a gas discharge tube a plasma. The English physicist William Crookes (1832-1919), who studied the electric discharge in tubes with rarefied air, wrote: "Phenomena in evacuated tubes open up a new world for physical science, in which matter can exist in the fourth state." Any substance changes its state depending on the temperature. So, water at negative (Celsius) temperatures is in a solid state, in the range from 0 to 100 "C - in a liquid state, above 100 ° C - in a gaseous state. If the temperature continues to rise, atoms and molecules begin to lose their electrons - they are ionized and gas turns into plasma.At temperatures above 1,000,000 ° C, plasma is absolutely ionized - it consists only of electrons and positive ions.Plasma is the most common state of matter in nature, it accounts for about 99% of the mass of the universe.Sun, most stars, nebulae - this is a completely ionized plasma. The outer part of the earth's atmosphere (ionosphere) is also plasma. Radiation belts containing plasma are located even higher. Auroras, lightning, including balls, are all different types of plasma that can be observed in natural conditions on Earth And only an insignificant part of the Universe is made up of matter in a solid state - planets, asteroids and dust nebulae.Plasma in physics is understood as a gas consisting of of electrically charged and neutral particles, in which the total electric charge is zero, t. the condition of quasi-neutrality is satisfied (therefore, for example, a beam of electrons flying in a vacuum is not a plasma: it carries a negative charge). PLASMA is a partially or fully ionized gas in which the densities of positive and negative charges are almost the same. Under laboratory conditions, plasma is formed in an electric discharge in a gas, in the processes of combustion and explosion. When the laser beam was focused by a lens, a spark flashed in the air at the focus area, and a plasma was formed there. This aroused great interest among physicists. The first seed electrons appear as a result of their ejection from the atoms of the medium after the simultaneous absorption of several photons of a light wave. The energy of each photon of a ruby laser is 1.78 eV. Further, the free electron, absorbing photons, reaches an energy of 10 eV, sufficient for ionization and the birth of a new electron in the process of collision with the atoms of the medium. The discharge can burn for a long time and glows with a dazzling white light, it is impossible to look at it without dark glasses. The unusually high temperature, a unique property of an optical charge, presents great opportunities for using it as a light source. The possibility of creating a plasma filament with laser light opens up possibilities for transmitting energy over a distance. Charge carriers in plasma are electrons and ions formed as a result of gas ionization. The ratio of the number of ionized atoms to their total number per unit volume of plasma is called the degree of plasma ionization (a). Depending on the value of a, one speaks of weakly ionized (a - fractions of a percent), partially ionized (a - a few percent) to fully ionized (a is close to 100%) plasma. The average kinetic energies of various types of particles that make up a plasma can be different. Therefore, in the general case, plasma is characterized not by one temperature value, but by several - they distinguish between the electron temperature Te, the ion temperature Ti and the temperature of neutral atoms Ta. Plasma with ion temperature Ti< 105 К называют низкотемпературной, а с Тi >106 K - high temperature. High-temperature plasma is the main object of research on CTF (controlled thermonuclear fusion). Low-temperature plasma is used in gas-discharge light sources, gas lasers, MHD generators, etc. Plasma is most widely used in lighting engineering - in gas-discharge lamps illuminating streets and fluorescent lamps used indoors. And besides, in a variety of gas-discharge devices: electric current rectifiers, voltage stabilizers, plasma amplifiers and microwave generators, counters of cosmic particles. All so-called gas lasers (helium-neon, krypton, carbon dioxide, etc.) are actually plasma: gas mixtures in them are ionized by an electric discharge. The properties characteristic of a plasma are possessed by conduction electrons in a metal (ions rigidly fixed in the crystal lattice neutralize their charges), a set of free electrons and mobile "holes" (vacancies) in semiconductors. Therefore, such systems are called plasma of solids. Gas plasma is usually divided into low-temperature - up to 100 thousand degrees and high-temperature - up to 100 million degrees. There are low-temperature plasma generators - plasma torches that use an electric arc. Using a plasma torch, you can heat almost any gas up to 7000-10000 degrees in hundredths and thousandths of a second. With the creation of the plasma torch, a new field of science arose - plasma chemistry: many chemical reactions are accelerated or proceed only in a plasma jet. Plasmatrons are used both in the mining industry and for cutting metals. Plasma engines and magnetohydrodynamic power plants have also been created. Various schemes of plasma acceleration of charged particles are being developed. The central task of plasma physics is the problem of controlled thermonuclear fusion. Thermonuclear reactions are called fusion reactions of heavier nuclei from the nuclei of light elements (primarily hydrogen isotopes - deuterium D and tritium T), occurring at very high temperatures (> 108 K and above) Under natural conditions, thermonuclear reactions occur in the Sun: hydrogen nuclei combine with each other with each other, forming helium nuclei, while a significant amount of energy is released. An artificial fusion reaction was carried out in a hydrogen bomb.

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National Research Tomsk Polytechnic University

Department of High Voltage Engineering and Electrophysics

course project

Subject "Applied Physics and Plasma Chemistry"

arc discharge

Completed by a student of the 4ТМ41 group

Ashirbaev M.E.

Checked by professor, d.f.-m.s. TEVN

Pushkarev A.I.

Tomsk, 2015

arc discharge cathodic current-voltage

1. General information

2. Properties of the arc discharge

2.1 Arc formation

2.2 Cathode spot. Appearance and separate parts of the arc discharge

2.3 Potential distribution and current-voltage characteristic during arc discharge

2.4 Temperature and radiation of individual parts of the arc discharge

2.5 Generation of continuous oscillations using an electric arc

3. Application of arc discharge

3.1 Modern methods of electrical processing

3.2 Arc welding

3.3 Plasma technology

3.4 Plasma welding

Conclusion

List of sources used

1. General information

An arc discharge in the form of a so-called electric arc was first discovered in 1802 by a Russian scientist, professor of physics at the Military Medical-Surgical Academy in St. Petersburg, and later an academician of the St. Petersburg Academy of Sciences, Vasily Vladimirovich Petrov. Petrov describes in one of his published books his first observations on an electric arc in the following words:

The path to the electric arc began in ancient times. Even the Greek Thales of Miletus, who lived in the sixth century BC, knew the property of amber to attract light objects when rubbed - feathers, straw, hair, and even create sparks. Until the seventeenth century, this was the only method of electrifying bodies that had no practical application. Scientists have been looking for an explanation for this phenomenon.

The English physicist William Gilbert (1544--1603) found that other bodies (for example, rock crystal, glass), like amber, have the property of attracting light objects after rubbing. He called these properties electrical, for the first time introducing this term into use (in Greek, amber is an electron).

Of great importance was the invention in the eighteenth century of the Leyden jar - a condenser, which made it possible to accumulate electricity. It was a glass vessel filled with water wrapped in foil. A metal rod passed through a cork was immersed in water.

The American scientist Benjamin Franklin (1706-1790) proved that water does not play any role in the collection of electric charges, glass-dielectric has this property.

The French physicist Charles Coulomb (1736-1806) - the founder of electrostatics - in 1785 found that the force of interaction of electric charges is proportional to their magnitudes and inversely proportional to the square of the distance between them.

M. V. Lomonosov (1711-1765), Leonard Euler (1707-1783), Franz Aepinus (1724-1802) and other scientists came up with original theories about the nature of electricity. By the end of the eighteenth century, the properties and behavior of fixed charges were sufficiently studied and explained to some extent. However, nothing was known about the electric current - moving charges, since there was no device that could make a large number of charges move. The currents drawn from the electrostatic machine were too small to be measured.

2. Properties of the arc discharge

2.1 Arc formation

If the current strength is increased in a glow discharge, reducing the external resistance, then at a high current strength, the voltage at the tube clamps begins to drop, the discharge quickly develops and turns into an arc discharge. In most cases, the transition is made abruptly and almost often leads to a short circuit. When selecting the resistance of the external circuit, it is possible to stabilize the transitional form of the discharge and observe at certain pressures a continuous transition of a glow discharge into an arc. In parallel with the voltage drop between the electrodes of the tube, there is an increase in the cathode temperature and a gradual decrease in the cathode drop.

The use of the usual method of igniting an arc by moving the electrodes apart is due to the fact that the arc burns at relatively low voltages of tens of volts, while a voltage of the order of tens of kilovolts is needed to ignite a glow discharge at atmospheric pressure. The ignition process when the electrodes are moved apart is explained by local heating of the electrodes due to the formation of poor contact between them at the moment of breaking the circuit. The question of the development of an arc when a circuit is broken is technically important not only from the point of view of obtaining "useful" arcs, but also from the point of view of combating "harmful" arcs, for example, with the formation of an arc when a knife switch is opened. Let L be the self-induction of the circuit, W be its resistance, b be the emf. current source U(I) is a function of the current-voltage characteristic of the arc. Then we should have:

b= L dI/dt+WI+U(I) (1)

LdI/dt=(b-WI)-U(I)=? (2)

The difference (b - WI) is nothing more than the ordinate of the direct resistance AB (Fig. 1), and U (I) is the ordinate of the arc characteristic for a given I. For dI / dt to be negative, i.e. In order for the current I to certainly decrease with time and a stable arc does not form between the electrodes of the switch, it is necessary that

Rice. 1. The relative position of the resistance line and the curve of the current-voltage characteristic of a steady arc for the cases: a) when the arc cannot occur when the circuit is broken; b) when the arc occurs at a break in the current strength interval corresponding to points P and Q.

took place?<0, т. е. надо, чтобы во всех точках характеристики соблюдалось неравенство U(I)>b-wi. To do this, the characteristic with all its points must lie above the resistance line (Fig. 1, a). This simple conclusion does not take into account the capacitance in the circuit and applies only to direct current.

The point of intersection of the direct resistance with the curve of the current-voltage characteristic of a steady arc corresponds to the lowest limit of the direct current strength, at which an arc can occur when the circuit is broken (Fig. 1, b). In the case of opening an alternating current arc with a knife switch, which is extinguished at each voltage transition through zero, it is essential that the conditions existing in the discharge gap during opening do not allow a new ignition of the arc with a subsequent increase in the voltage of the current source. This requires that the discharge gap be sufficiently deionized as the voltage increases. In switches of high alternating currents, enhanced deionization is artificially achieved by introducing special electrodes that suck out charged gas particles due to bipolar diffusion, as well as by using mechanical blowing or by exposing the discharge to a magnetic field. At high voltages, oil switches are used.

2.2 cathode spot. Appearance and separate parts of the arc discharge

The cathode spot, fixed on the carbon cathode, on the surface of liquid mercury is in continuous rapid motion. The position of the cathode spot on the surface of liquid mercury can be fixed with a metal pin immersed in the mercury and protruding slightly from it.

In the case of a small distance between the anode and the cathode, the thermal radiation of the anode strongly affects the properties of the cathode spot. At a sufficiently large distance between the anode and the carbon cathode, the dimensions of the cathode spot tend to a certain constant limit value, and the area occupied by the cathode spot on the carbon electrode in air is proportional to the current strength and corresponds to an atmospheric pressure of 470 A/cm². For a mercury arc in vacuum, 4000 a/cm.

With decreasing pressure, the area occupied by the cathode spot on the carbon cathode increases at a constant current strength.

When the arc burns in air, the carbon cathode sharpens, while on the carbon anode, if the discharge does not cover the entire front area of the anode, a round depression is formed - positive arc crater.

With a horizontal arrangement of electrodes and high gas pressure, the positive column of the arc discharge bends upward under the action of convection currents of the gas heated by the discharge. Hence the very name of the arc discharge.

2.3 Potential distribution and current-voltage characteristic during an arc discharge

In the Petrov arc, high temperature and high pressure make it impossible to use the probe method to measure the potential distribution.

The potential drop between the arc electrodes is the sum of the cathode drop and Uk, the anode drop Ua and the drop in the positive column. The sum of the cathode and anode potential drops can be determined by bringing the anode and cathode closer together until the positive column disappears and measuring the voltage between the electrodes. In the case of an arc at low pressure, one can determine the potential values at two points of the arc column using the probe characteristic method, calculate the longitudinal potential gradient from this, and then calculate both the anodic and cathodic potential drops.

It has been found that in an arc discharge at atmospheric pressure, the sum of the cathode and anode drops is approximately the same value as the ionization potential of the gas or vapor in which the discharge occurs.

In the technique of using the Petrov arc with carbon electrodes, Ayrton's empirical formula is usually used:

U=a+bl+(c+dl)/I (3)

Here U is the voltage between the electrodes, I is the current in the arc, l is the length of the arc, a, b, c and d are four constants. The characteristic formula (3) is set for an arc between carbon electrodes in air. By l is meant the distance between the cathode and the plane drawn through the edges of the positive crater.

Let us rewrite formula (4) in the form

U=a+c/I+l(b+d/I). (4)

In (4), the terms containing the factor l correspond to the drop in the potential in the positive column; the first two terms are the sum of the cathode and anode drop Uk+Ua. The constants in (3) depend on the air pressure and on the cooling conditions of the electrodes, and, consequently, on the size and shape of the coals.

In the case of an arc discharge in an evacuated vessel filled with metal vapor (for example, mercury), the vapor pressure depends on the temperature of the coldest parts of the vessel and, therefore, the behavior of the characteristic strongly depends on the cooling conditions of the entire tube.

The dynamic characteristic of the arc discharge is very different from the static one. The type of dynamic characteristic depends on the rate of change of the arc mode. Practically the most interesting characteristic of the arc when powered by alternating current. Simultaneous oscillography of current and voltage gives the picture shown in Fig.2. The characteristic of the arc drawn from these curves for the entire period has the form shown in Fig. 2. The dotted line shows the voltage behavior in the absence of a discharge.

Rice. 2. Oscillogram of the current and voltage of the arc discharge at low frequency alternating current. Points A, B, C, etc. correspond to the points indicated by the same letters

The cathode, which has not yet had time to cool down after the discharge that took place in the previous half-cycle of the current, from the very beginning of the half-cycle, when the external emf. passes through zero, emits electrons. From point O to point A, the characteristic corresponds to a non-self-sustained discharge, the source of which is the electrons emitted by the cathode. At point A, the arc is ignited. After point A, the discharge current rapidly increases. In the presence of resistance in the external circuit, the voltage between the arc electrodes drops, although the emf. current source (dashed line in Fig. 3), running through the sinusoid, increases even more. With a decrease in voltage and current supplied by an external source, the discharge current begins to decrease.

With a decrease in the current in the arc, the voltage between its electrodes may again increase depending on the external resistance, but part of the BC characteristic in Fig. 3 may also be horizontal or have an opposite slope. At point C, the arc is extinguished.

After point C, the non-self-sustained discharge current decreases to zero along with a decrease in the voltage between the electrodes.

After the voltage passes through zero, the former anode begins to play the role of the cathode, and the picture repeats itself with the opposite signs of current and voltage.

The type of dynamic characteristic is influenced by all the conditions that determine the arc mode: the distance between the electrodes, the value of external resistance, self-induction and capacitance of the external circuit, the frequency of the alternating current supplying the arc, etc.

If an alternating voltage of amplitude less than the voltage of the direct current supplying the arc is applied to the electrodes of the arc fed by direct current, then the characteristic has the form of a closed loop covering the static characteristic sun from two sides. With an increase in the frequency of the alternating current, the axis of this loop rotates, the loop itself flattens and, finally, tends to take the form of a straight line segment OA passing through the origin (Fig. 3).

Rice. 3. Change in dynamic response at increased frequency of alternating current superimposed on direct

At a very low frequency, the loop of the dynamic characteristic turns into a segment of the static characteristic of the VS, since all the internal parameters of the discharge, in particular the concentration of ions and electrons, have time at each point of the characteristic to take values corresponding to a stationary discharge for given U and I. Conversely, at a very fast change and the discharge parameters do not have time to change at all, therefore I turns out to be proportional to and, which corresponds to the straight line OA passing through the origin of coordinates. Thus, with an increase in the frequency of the alternating current, the characteristic loop (Fig. 3) becomes increasing at all its points.

In connection with the possibility of complete ionization of gas in an arc discharge, the question arises of breaking the arc at low gas pressure and very high currents. An important role in the phenomenon of arc breaking is played by a significant decrease in gas density due to electrophoresis and suction of ions to the walls, especially in places where the discharge gap is very narrow. In practice, this leads to the need to avoid excessive narrowing when building mercury rectifiers for very high currents.

Electricians, who first dealt with an electric arc, tried to apply Ohm's law in this case as well. To obtain the results of the calculation according to Ohm's law, consistent with reality, they had to introduce the concept of the reverse electromotive force of the arc. By analogy with the phenomena in galvanic cells, the expected appearance of this emf. called arc polarization. The question of the back emf. The works of Russian scientists D. A. Lachinov and V. F. Mitkevich are devoted to the arc. Further development of ideas about electric discharges in gases showed that such a statement of the question is purely formal and can be successfully replaced by the idea of a falling arc characteristic. The validity of this point of view is confirmed by the failure of all attempts to directly detect experimentally the back emf. electric arc.

2.4 Temperature and radiation of individual parts of the arc discharge

In the case of an arc in air between the carbon electrodes, the radiation from incandescent electrodes, mainly from the positive crater, predominates. Anode radiation, like solid body radiation, has a continuous spectrum. Its intensity is determined by the anode temperature. The latter is a characteristic value for an arc in atmospheric air with an anode made of any given material, since the anode temperature does not depend on the current strength and is determined solely by the melting or sublimation temperature of the anode material. The melting or sublimation temperature depends on the pressure under which the melting or sublimating body is located. Therefore, the temperature of the anode, and hence the intensity of the radiation of the positive crater, depend on the pressure at which the arc burns. In this regard, classical experiments with a carbon arc under pressure are known, which led to very high temperatures.

The change in temperature of a positive crater with pressure is given by the curve in Fig. 4. A straight line on which points for pressures from 1 atm are laid on this drawing. and higher confirms the assumption that the temperature of the positive crater is determined by the melting or sublimation temperature of the anode material, since in this case there should be a linear relationship between ln R and 1/T. The deviation from the linear dependence at lower pressures is explained by the fact that at pressures below 1 atm. the amount of heat released at the anode is not sufficient to heat the anode to its melting or sublimation temperature.

Rice. 4. Change in the temperature of the carbon anode of an electric arc in air with a change in pressure. The scale along the y-axis is logarithmic

The temperature of the cathode spot of the Petrov arc is always several hundred degrees lower than the temperature of the positive crater. The high temperatures of the arc cord cannot be determined with a thermopile or bolometer. Currently, spectral methods are used to determine the temperature in the arc. At high currents, the gas temperature in the Petrov arc can be higher than the anode temperature and reaches 6000°K. Such high gas temperatures are characteristic of all cases of arc discharge at atmospheric pressure. In the case of very high pressures (tens and hundreds of atmospheres), the temperature in the central parts of the corded positive arc column reaches 10,000°K. In an arc discharge at low pressures, the gas temperature in the positive column is of the same order as in the positive column of a glow discharge.

The temperature of the positive arc crater is higher than the temperature of the cathode because at the anode all the current is carried by electrons bombarding and heating the anode. The electrons donate to the anode not only all the kinetic energy acquired in the region of the anode fall, but also the work function (the latent heat of evaporation of the electrons). On the contrary, a small number of positive ions hit the cathode and bombard and heat it compared to the number of electrons hitting the anode at the same current strength. The rest of the current at the cathode is carried out by electrons, which, when released, in the case

thermionic arc, the thermal energy of the cathode is expended on the work function.

2.5 Generation of continuous oscillations using an electric arc

Due to the fact that the arc has a falling characteristic, it can be used as a generator of continuous oscillations. A diagram of such an arc generator is shown in fig. 5. The conditions for the generation of oscillations in this scheme can be derived from consideration of the conditions for the stability of a stationary discharge for given parameters of the external circuit. Let the electromotive force of the direct current source supplying the discharge (Fig. 5) be equal to b, the voltage between the electrodes of the tube U, the strength of the stationary current through the discharge tube in this mode is equal to I, the capacitance of the cathode-anode of the tube plus the capacitance of all the supply wires C, self-induction in circuit L, the resistance through which the current is supplied from the source, R.

Rice. 5. Schematic diagram of the arc generator.

In the steady state mode of direct current, we will have:

b= Uo+IR (5)

Let us assume that this stationary regime is violated. The discharge current at any given time is I+ i, where i- a small value, and the potential difference between the electrodes is equal to U. Let's introduce the designation U?=dU/dI (dU/d i)i=0 is equal to the tangent of the slope of the tangent to the current-voltage characteristic at the operating point corresponding to the initially chosen mode (current I). Let's see how it will change i. If a i will increase, then this discharge mode is unstable; if, on the contrary, i decreases infinitely, then the discharge mode is stable.

Let us turn to the current-voltage characteristic of the considered discharge gap U= f(I+i) - current flows through the tube I+i and capacity With charging (or discharging). Potential difference across the capacitance With is balanced in this case not only by the voltage across the discharge gap, but also by the emf. circuit self-induction. Let be I+i2--total current through the resistance R. Denote the current charging the capacitance C through i1; instantaneous value of the potential difference on the capacitance C-- through U1. The potential difference between the arc electrodes will be U0+ iU".

Kommersant=U1+(i+I2)R, (6)

U1-U0 \u003d U "i + Ldi / dt, (7)

i2= i1+ i. (8)

Additional charge Q on capacitance C compared to stationary mode:

Q=?i 1 dt=(U 1 -U 0)C. (nine)

Subtracting (5) from (6), we find:

U 1 - U 0 =- i 2 R (10)

Expressions (7), (8) and (10) give:

U "i + Ldi / dt \u003d -R (i + i 1 ) . (11)

Expressions (7) and (9) give:

1/C?i 1 dt= U"i+ ldi/ dt. (12)

Differentiating (12) with respect to t and inserting the result into (11), we find:

U "i + Ldi / dt = -iR-RCU" di / dt-RLCdІi / dtІ. (13)

dІi/dtІ +(1/CR+U"/L)di/dt + 1/LC(U"/R+1)i=0 (14)

Formula (14) is a differential equation, which obeys the additional current i.

As is known, the complete integral of equation (14) has the form:

i=A1e^r1t+A2e^r2t, (15)

where r1 and r2 are the roots of the characteristic equation, defined by the formula

r=-1/2(1/CR+ U"/ L)+ v 1/4(1/ CR+ U"/ L)І-1/LC(U"/ R+1) . (16)

If the radical value in (16) is greater than zero, then r1 and r2 are both real, i changes aperiodically according to the exponential law, and solution (15) corresponds to an aperiodic change in current. In order for current oscillations to occur in the circuit we are considering, it is necessary that r 1 and r 2 be complex quantities, i.e., that

1/LC(U"/R+1)>1/4(1/CR+U"/L)І (17)

In this case, (15) can be represented as

i=A 1 e -dt+jшt+ A 2 e -dt-jшt, (18)

d=1/2(1/CR+U"/L); i= v-1.

At d < 0 колебания, возникшие в рассматриваемой цепи, будут раскачиваться. При d> 0, they decay rapidly, and the DC discharge will be stable.

Thus, in order for undamped oscillations to finally be established in the scheme under consideration, it is necessary that

(1/ CR+ U"/ L)<0. (19)

Since Р, L and С are essentially positive values, then inequality (19) can be observed only under the condition:

dU/di=U"<0. (20)

From this we conclude that oscillations in the circuit under consideration can arise only with a falling current-voltage characteristic of the discharge.

The study of the conditions under which r1 and r2 are real and both are less than zero leads to the conditions for stability of the DC discharge: Conditions (21) and (22) are general conditions. The stability of a discharge powered by a constant voltage.

(1/ CR+ U"/ L)>0 and (21)

U"/ R+1>0 . (22)

From (21) it follows that with an increasing current-voltage characteristic, the discharge is always stable. Combining this requirement with condition (22), we find that, with a decreasing characteristic, the discharge can only be stable when

|U"|

When applying the formulas of this paragraph directly to the question of generating oscillations with the help of an arc, one has to take U" from the "average characteristic" constructed on the basis of the ascending and descending branches of the dynamic characteristic.

With a periodic change in the current strength in the Petrov arc, the temperature and density of the gas and the velocities of aerodynamic flows change. When selecting the appropriate mode, these changes lead to the appearance of acoustic oscillations in the ambient air. The result is a so-called singing arc that reproduces pure musical tones.

3 . Application of an arc discharge

3.1 Modern methods of electrical processing

Among modern technological processes, one of the most common is electric welding. Welding allows you to weld, solder, glue, spray not only metals, but also plastics, ceramics and even glass. The range of application of this method is truly immense - from the production of powerful cranes, building metal structures, equipment for nuclear and other power plants, the construction of large-tonnage ships, nuclear icebreakers to the manufacture of the finest microcircuits and various household products. In a number of industries, the introduction of welding has led to a fundamental change in technology. So, a real revolution in shipbuilding was the development of the in-line construction of ships from large welded sections. Many shipyards in the country are now building large-capacity all-welded tankers. Electric welding made it possible to solve the problems of creating gas pipelines designed to work in northern conditions at a pressure of 100-120 atmospheres. Employees of the Institute of Electric Welding. E. O. Paton proposed an original method for manufacturing pipes based on welding technology, intended for such gas pipelines.

From such pipes with walls up to 40 millimeters thick, highly reliable gas pipelines are assembled that cross the continents.

Soviet scientists and specialists made a great contribution to the development of electric welding. Continuing and creatively developing the legacy of his great predecessors - V. V. Petrov, N. N. Benardos, N. G. Slavyanov, they created the science of the theoretical foundations of welding technology, developed a number of new technological processes. The names of academicians E. O. Paton, V. P. Vologdin, K. K. Khrenov, N. N. Rykalin and others are known to the whole world.

Currently, electric arc, electroslag and plasma-arc welding are widely used.

3.2 Arc welding

Arc welding. The simplest way is manual arc welding. A holder is connected to one pole of the current source with a flexible wire, and the workpiece to be welded is connected to the other. A carbon or metal electrode is inserted into the holder. With a short touch of the electrode to the product, an arc is ignited, which melts the base metal and the electrode rod, forming a weld pool, which, when solidified, gives a weld.

Manual arc welding requires a highly skilled worker and is characterized by not the best working conditions, but it can be used to weld parts in any spatial position, which is especially important when installing metal structures. The productivity of manual welding is relatively low and depends to a large extent on such a simple part as an electrode holder. And now, like a hundred years ago, the search continues for the best design. A series of simple and reliable electrode holders was made by Leningrad innovators M.E. Vasiliev and V.S. Shumsky.

In arc welding, the protection of the weld metal from oxygen and nitrogen in the air is of great importance. Actively interacting with the molten metal, atmospheric oxygen and nitrogen form oxides and nitrides, which reduce the strength and ductility of the welded joint.

There are two ways to protect the welding site: introducing various substances into the electrode material and electrode coating (internal protection) and introducing inert gases and carbon monoxide into the welding zone, coating the welding site with fluxes (external protection).

In 1932, at the Moscow Electromechanical Institute of Railway Engineers, under the leadership of Academician K. K. Khrenov, electric arc welding under water was carried out for the first time in the world. However, back in 1856, L. I. Shpakovsky for the first time conducted an experiment on melting copper electrodes immersed in water with an arc. On the advice of D. A. Lachinov, who received an underwater arc, N. N. Benardos in 1887 made underwater cutting of metal. It took 45 years for the first experience to receive scientific justification and turn into a method.

And on October 16, 1969, an electric arc broke out into space for the first time. Here is how this outstanding event was reported in the Izvestia newspaper; “The crew of the Soyuz-6 spacecraft, consisting of Lieutenant Colonel G.S. Shonin and flight engineer V.N. Kubasov, carried out experiments on welding in space. The purpose of these experiments was to determine the features of welding of various metals in outer space. Several types of automatic welding were carried out one by one. And further: "The experiment carried out is unique and is of great importance for science and technology in the development of technology for welding and installation work in space."

3.3 Plasma technology

This technology is based on the use of a high temperature arc. It includes plasma welding, cutting, surfacing and plasma-machining.

How to improve arc performance? To do this, you need to get an arc with a higher concentration of energy, i.e., the arc must be focused. This was achieved in 1957-1958, when at the Institute of Metallurgy. A. A. Baikov created equipment for plasma-arc cutting.

How to increase arc temperature? Probably, in the same way as increasing the pressure of a water or air jet by passing it through a narrow channel.

Passing through the narrow channel of the burner nozzle, the arc is compressed by a gas jet (neutral, oxygen-containing) or a mixture of gases and drawn into a thin jet. At the same time, its properties change dramatically: the temperature of the arc discharge reaches 50,000 degrees, the specific power reaches 500 or more kilowatts per square centimeter. The ionization of the plasma in the gas column is so great that its electrical conductivity turns out to be almost the same as that of metals.

A compressed arc is called a plasma arc. With its help, plasma welding, cutting, guiding, spraying, etc. are carried out. To obtain a plasma arc, special generators have been created - plasma torches.

The plasma arc, like the usual one, can be of direct and indirect action. The arc of direct action closes on the product, indirect action - on the second electrode, which is the nozzle. In the second case, it is not an arc that escapes from the nozzle, but a plasma jet, which arises due to heating by the arc and subsequent ionization of the plasma-forming gas. The plasma jet is mainly used for plasma spraying and processing of non-conductive materials. The gas surrounding the arc also performs a heat-shielding function. The largest load in the plasma torch is carried by the nozzle. The higher its heat resistance, the greater the current can be obtained in an indirect plasma torch. The outer layer of plasma gas has a relatively low temperature, so it protects the nozzle from destruction.

A significant increase in the temperature of the plasma-forming gas in direct plasma torches can lead to electrical breakdown and the occurrence of a double arc - between the cathode and nozzle and between the nozzle and the product. In this case, the nozzle usually fails.

3.4 Plasma welding

There are two designs of plasma torches. In some designs, gas is supplied along the arc, and good compression is achieved. In other designs, the gas surrounds the arc in a spiral, due to which it is possible to obtain a stable arc in the nozzle channel and provide reliable protection of the nozzle with a near-wall gas layer.

In plasma torches of direct action, the arc is not immediately ignited, since the air gap between the cathode and the product is too large. First, the so-called duty, or auxiliary, arc is excited between the cathode and the nozzle. It develops from a spark discharge, which occurs under the action of a high-frequency voltage created by an oscillator. The gas flow blows out the duty arc, it touches the metal being processed, and then the main arc is ignited. After that, the oscillator is turned off, and the pilot arc goes out. If this does not happen, a double arc may occur. The weld zone in plasma welding, as in its other types, is protected from the action of ambient air. To do this, in addition to the plasma-forming gas, a protective gas is fed into a special nozzle: argon or the cheaper and more common carbon dioxide. Carbon dioxide is often used not only for protection, but also for the formation of plasma. Sometimes plasma welding is carried out under a layer of flux.

Plasma-arc welding can be performed both automatically and manually. At present, this method has become quite widespread. Many factories have introduced plasma welding of aluminum alloys and steels. Significant savings came from the use of single-pass plasma welding of aluminum instead of multi-pass argon-arc welding. Welding is carried out on an automatic installation using carbon dioxide as a plasma-forming and protective gas.

Conclusion

In modern life, the use of electrical energy is the most widespread. The achievements of electrical engineering are used in all spheres of human practical activity: in industry, agriculture, transport, medicine, everyday life, etc. Advances in electrical engineering have a significant impact on the development of radio engineering, electronics, telemechanics, automation, computer technology, cybernetics. All this became possible as a result of the construction of powerful power plants, electrical networks, the creation of new electrical power systems, and the improvement of electrical devices. The modern electrical industry produces machines and apparatus for the production, transmission, conversion, distribution and consumption of electricity, a variety of electrical equipment and technological equipment, electrical measuring instruments and telecommunications equipment, regulatory, monitoring and control equipment for automatic control systems, medical and scientific equipment, electrical appliances and machines and much more. In recent years, various methods of electrical processing have been further developed: electric welding, plasma cutting and surfacing of metals, plasma-mechanical and electroerosive processing. It can be seen from the foregoing that the study of a discharge in a gas is of great importance for general scientific and technical progress. Therefore, there is no need to stop there, but it is necessary to continue research, looking for the unknown, thereby stimulating the construction of new theories in the future.

List of sources used

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