LAB #2.5

"Study of a gas discharge using a thyratron"

Objective: to study the processes occurring in gases during non-self-sustained and self-sustained discharge in gases, to study the principle of operation of the thyratron, to build the current-voltage and starting characteristics of the thyratron.

THEORETICAL PART

Ionization of gases. Non-self-sustained and self-sustained gas discharge

Atoms and molecules of gases under normal everyday conditions are electrically neutral, i.e. do not contain free charge carriers, which means that, like a vacuum gap, they should not conduct electricity. In fact, gases always contain a certain amount of free electrons, positive and negative ions, and therefore, although poorly, they conduct electricity. current.

Free charge carriers in a gas are usually formed as a result of the ejection of electrons from the electron shell of gas atoms, i.e. as a result ionization gas. Gas ionization is the result of external energy impact: heating, particle bombardment (electrons, ions, etc.), electromagnetic radiation (ultraviolet, X-ray, radioactive, etc.). In this case, the gas located between the electrodes conducts an electric current, which is called gas discharge. Power ionizing factor ( ionizer) is the number of pairs of oppositely charged charge carriers resulting from ionization per unit volume of gas per unit time. Along with the ionization process, there is also a reverse process - recombination: the interaction of oppositely charged particles, as a result of which electrically neutral atoms or molecules appear and electromagnetic waves are emitted. If the electrical conductivity of the gas requires the presence of an external ionizer, then such a discharge is called dependent. If the applied electric field (EF) is sufficiently large, then the number of free charge carriers formed as a result of impact ionization due to the external field turns out to be sufficient to maintain the electric discharge. Such a discharge does not need an external ionizer and is called independent.

Let us consider the current-voltage characteristic (CVC) of a gas discharge in a gas located between the electrodes (Fig. 1).

With a non-self-sustained gas discharge in the region of weak electric fields (I), the number of charges formed as a result of ionization is equal to the number of charges recombining with each other. Due to this dynamic equilibrium, the concentration of free charge carriers in the gas remains practically constant and, as a result, Ohm's law (1):

where E is the electric field strength; n– concentration; j is the current density.

and ( ) are the mobility of positive and negative charge carriers, respectively;<υ > is the drift velocity of the directed motion of the charge.

In the region of high EC (II), saturation of the current in the gas (I) is observed, since all the carriers created by the ionizer participate in the directed drift, in the creation of the current.

With a further increase in the field (III), charge carriers (electrons and ions), moving at an accelerated rate, ionize neutral atoms and gas molecules ( impact ionization), resulting in the formation of additional charge carriers and the formation electronic avalanche(electrons are lighter than ions and are significantly accelerated in the EP) – the current density increases ( gas amplification). When the external ionizer is turned off, the gas discharge will stop due to recombination processes.

As a result of these processes, flows of electrons, ions and photons are formed, the number of particles grows like an avalanche, there is a sharp increase in current with practically no amplification of the electric field between the electrodes. Arises independent gas discharge. The transition from an inconsistent gas discharge to an independent one is called email breakdown, and the voltage between the electrodes , where d- the distance between the electrodes is called breakdown voltage.

For e-mail breakdown, it is necessary that the electrons, along their path, have time to gain kinetic energy that exceeds the ionization potential of gas molecules, and on the other hand, that positive ions, along their path, have time to acquire kinetic energy greater than the work function of the cathode material. Since the mean free path depends on the configuration of the electrodes, the distance between them d and the number of particles per unit volume (and, consequently, on the pressure), the ignition of a self-sustained discharge can be controlled by changing the distance between the electrodes d with their unchanged configuration, and changing the pressure P. If the work Pd turns out to be the same, other things being equal, then the nature of the observed breakdown should be the same. This conclusion was reflected in the experimental law e (1889) German. physics F. Pashen(1865–1947):

The ignition voltage of a gas discharge for a given value of the product of gas pressure and the distance between the electrodes Pd is a constant value characteristic of a given gas .

There are several types of self-discharge.

glow discharge occurs at low pressures. If a constant voltage of several hundred volts is applied to the electrodes soldered into a glass tube 30–50 cm long, gradually pumping air out of the tube, then at a pressure of 5.3–6.7 kPa a discharge occurs in the form of a luminous tortuous reddish cord coming from cathode to anode. With a further decrease in pressure, the filament thickens, and at a pressure of » 13 Pa, the discharge has the form shown schematically in Fig. 2.

A thin luminous layer is attached directly to the cathode 1 - cathode film , followed by 2 - cathode dark space , passing further into the luminous layer 3 – smoldering glow , which has a sharp boundary on the cathode side, gradually disappearing on the anode side. Layers 1-3 form the cathode part of the glow discharge. Follows the smoldering glow faraday dark space 4. The rest of the tube is filled with luminous gas - positive post - 5.

The potential varies unevenly along the tube (see Fig. 2). Almost the entire voltage drop occurs in the first sections of the discharge, including the dark cathode space.

The main processes necessary to maintain the discharge occur in its cathode part:

1) positive ions, accelerated by the cathodic potential drop, bombard the cathode and knock out electrons from it;

2) the electrons are accelerated in the cathode part and gain sufficient energy and ionize the gas molecules. Many electrons and positive ions are formed. In the smoldering region, intense recombination of electrons and ions takes place, energy is released, part of which goes to additional ionization. The electrons that have penetrated into the Faraday dark space gradually accumulate energy, so that the conditions necessary for the existence of plasma arise (a high degree of gas ionization). The positive column is a gas-discharge plasma. It acts as a conductor connecting the anode to the cathode parts. The glow of the positive column is caused mainly by transitions of excited molecules to the ground state. Molecules of different gases emit radiation of different wavelengths during such transitions. Therefore, the glow of the column has a color characteristic of each gas. This is used to make luminous tubes. Neon tubes give a red glow, argon tubes give a bluish-green.

arc discharge observed at normal and elevated pressures. In this case, the current reaches tens and hundreds of amperes, and the voltage across the gas gap drops to several tens of volts. Such a discharge can be obtained from a low voltage source if the electrodes are first brought together until they touch. At the point of contact, the electrodes are strongly heated due to the Joule heat, and after they are removed from each other, the cathode becomes a source of electrons due to thermionic emission. The main processes supporting the discharge are thermionic emission from the cathode and thermal ionization of molecules due to the high temperature of the gas in the interelectrode gap. Almost the entire interelectrode space is filled with high-temperature plasma. It serves as a conductor through which the electrons emitted by the cathode reach the anode. The plasma temperature is ~6000 K. The high temperature of the cathode is maintained by bombarding it with positive ions. In turn, the anode, under the action of fast electrons incident on it from the gas gap, heats up more strongly and can even melt, and a recess is formed on its surface - a crater - the brightest point of the arc. Electric arc was first received in 1802. Russian physicist V. Petrov (1761–1834), who used two pieces of coal as electrodes. Hot carbon electrodes gave a dazzling glow, and between them a bright column of luminous gas appeared - an electric arc. The arc discharge is used as a source of bright light in projector spotlights, as well as for cutting and welding metals. There is an arc discharge with a cold cathode. Electrons appear due to field emission from the cathode, the gas temperature is low. The ionization of molecules occurs due to electron impacts. A gas-discharge plasma appears between the cathode and anode.

spark discharge occurs between two electrodes at a high electric field strength between them . A spark jumps between the electrodes, having the form of a brightly luminous channel, connecting both electrodes. The gas near the spark is heated to a high temperature, a pressure difference occurs, which leads to the appearance of sound waves, a characteristic crack.

The appearance of a spark is preceded by the formation of electron avalanches in the gas. The ancestor of each avalanche is an electron accelerating in a strong electric field and producing the ionization of molecules. The resulting electrons, in turn, accelerate and produce the next ionization, an avalanche increase in the number of electrons occurs - avalanche.

The resulting positive ions do not play a significant role, because they are immobile. Electron avalanches intersect and form a conducting channel streamer, along which electrons rush from the cathode to the anode - there is breakdown.

Lightning is an example of a powerful spark discharge. Different parts of a thundercloud carry charges of different signs ("-" is facing the Earth). Therefore, if the clouds approach each other with oppositely charged parts, a spark breakdown occurs between them. The potential difference between the charged cloud and the Earth is ~10 8 V.

Spark discharge is used to initiate explosions and combustion processes (candles in internal combustion engines), to register charged particles in spark counters, to treat metal surfaces, etc.

Corona (coronary) discharge occurs between electrodes that have different curvature (one of the electrodes is a thin wire or a point). In a corona discharge, ionization and excitation of molecules occur not in the entire interelectrode space, but near the tip, where the intensity is high and exceeds E breakdown. In this part, the gas glows, the glow has the form of a corona surrounding the electrode.

Plasma and its properties

Plasma is called a strongly ionized gas, in which the concentration of positive and negative charges is almost the same. Distinguish high temperature plasma , which occurs at ultrahigh temperatures, and gas-discharge plasma arising from gas discharge.

Plasma has the following properties:

A high degree of ionization, in the limit - complete ionization (all electrons are separated from the nuclei);

The concentration of positive and negative particles in plasma is practically the same;

high electrical conductivity;

glow;

Strong interaction with electric and magnetic fields;

Oscillations of electrons in plasma with a high frequency (>10 8 Hz), causing a general vibration of the plasma;

Simultaneous interaction of a huge number of particles.

Non-self-sustaining gas discharge is called such a discharge, which, having arisen in the presence of an electric field, can exist only under the action of an external ionizer.

Let us consider the physical processes that take place in a non-self-sustaining gas discharge. Let us introduce a number of notations: denote by the number of gas molecules in the volume under study V. Concentration of molecules Part of the molecules is ionized. Let us denote the number of ions of the same sign through N; their concentration Next, denote by ∆ n i- the number of pairs of ions arising under the action of the ionizer in one second per unit volume of gas.

Along with the process of ionization in the gas, recombination of ions occurs. The probability of meeting two ions of different signs is proportional to both the number of positive and negative ions, and these numbers, in turn, are equal to n. Therefore, the number of pairs of ions recombining per second per unit volume is proportional to n 2:

Hence, for the equilibrium concentration of ions (the number of pairs of ions per unit volume), the following expression is obtained:

.

(8.2.3)

The scheme of the experiment with a gas discharge tube is shown in Figure 8.1.

Let us analyze further the action of the electric field on the processes in ionized gases. Apply a constant voltage to the electrodes. Positive ions will be directed towards the negative electrode, and negative charges towards the positive electrode. Thus, part of the carriers from the gas-discharge gap will go to the electrodes (an electric current will appear in the circuit). Let the unit volume go every second ∆nj pair of ions. Now the equilibrium condition can be represented as

(8.2.4)

1. Consider the case weak field: The circuit will flow weak current. The current density is proportional in magnitude to the carrier concentration n, charge q, carried by each carrier and the speed of the directed movement of positive and negative ions and :

.

(8.2.5)

The speed of the directed movement of ions is expressed through mobility and tension electric field:

In a weak field (), the equilibrium concentration is equal to:.

Substitute this expression in (8.2.7):

(8.2.8)

In the last expression, the factor at does not depend on the intensity. Denoting it by σ, we get Ohm's law in differential form :

(8.2.9)

where – specific electrical conductivity.

Conclusion : in the case of weak electric fields, the current with a non-self-sustained discharge obeys Ohm's law.

2. Consider strong field . In this case, i.e., all generated ions leave the gas-discharge gap under the action of an electric field. This is explained by the fact that during the time it takes for an ion to fly in a strong field from one electrode to another, the ions do not have time to recombine noticeably. Therefore, all the ions produced by the ionizer participate in the creation of current and go to the electrodes. And since the number of ions generated by the ionizer per unit time ∆n i, does not depend on the field strength, then the current density will be determined only by the value ∆n i and will not depend on . In other words, with a further increase in the applied voltage, the current stops increasing and remains constant.

The maximum value of the current at which all the formed ions go to the electrodes is called the saturation current.

A further increase in the field strength leads to the formation avalanches electrons, when the electrons that have arisen under the action of the ionizer acquire energy over the mean free path (from collision to collision) sufficient to ionize gas molecules (impact ionization). The secondary electrons that have arisen in this case, having accelerated, in turn, produce ionization, etc. - occurs avalanche-like multiplication of primary ions and electrons created by an external ionizer and discharge current amplification.

Figure 8.2 shows the process of avalanche formation.

The results obtained can be depicted graphically (Fig. 8.3) in the form of a current-voltage characteristic of a non-self-sustaining gas discharge.

Conclusion : for a non-self-sustained discharge at low current densities, i.e. when the main role in the disappearance of charges from the gas-discharge gap is played by the recombination process, Ohm's law takes place( ); for large fields()Ohm's law is not fulfilled - saturation occurs, and with fields exceeding - an avalanche of charges occurs, causing a significant increase in current density.

Unlike electrolyte solutions, gas under normal conditions consists of neutral molecules (or atoms) and is therefore an insulator. A gas becomes a conductor of electric current only when at least some of its molecules are ionized (turned into ions) under the influence of an external influence (ionizer). During ionization, usually one electron escapes from a gas molecule, as a result of which the molecule becomes a positive ion. The escaping electron either remains free for some time, or immediately attaches (“sticks”) to one of the neutral gas molecules, turning it into a negative ion. Thus, in an ionized gas there are positive and negative ions and free electrons.

In order to knock out one electron from a molecule (atom), the ionizer must perform a certain work, called the work of ionization; for most gases, it has values ​​ranging from 5 to 25 eV. X-rays (see § 125), radioactive radiation (see § 139), cosmic rays (see § 145), intense heating, ultraviolet rays (see § 120) and some other factors can serve as gas ionizers.

Along with ionization in the gas, there is a process of ion recombination. As a result, an equilibrium state is established, characterized by a certain concentration of ions, the value of which depends on the power of the ionizer.

In the presence of an external electric field in an ionized gas, a current arises due to the movement of opposite ions in mutually opposite directions and the movement of electrons.

Due to the low viscosity of the gas, the mobility of gas ions is thousands of times greater than that of electrolyte ions, and is approximately

When the action of the ionizer stops, the concentration of ions in the gas quickly drops to zero (due to recombination and the removal of ions to the electrodes of the current source) and the current stops. The current, the existence of which requires an external ionizer, is called a non-self-sustaining gas discharge.

With a sufficiently strong electric field in the gas, self-ionization processes begin, due to which the current can exist even in the absence of an external ionizer. This kind of current is called an independent gas discharge.

The processes of self-ionization in general terms are as follows. Under natural conditions, a gas always contains a small amount of free electrons and ions created by such natural ionizers as cosmic rays and radiation from radioactive substances contained in the atmosphere, soil and water. A sufficiently strong electric field can accelerate these particles to such speeds at which their kinetic energy exceeds the work of ionization. Then the electrons and ions, colliding (on the way to the electrodes) with neutral molecules, will ionize them. New (secondary) electrons and ions formed during collisions are also accelerated by the field and, in turn, ionize new neutral molecules, etc. The described self-ionization of a gas is called impact ionization.

Free electrons cause impact ionization already at a field strength of the order of magnitude. As for ions, they can cause impact ionization only at a field strength of the order of magnitude. This difference is due to a number of reasons, in particular, the fact that for electrons the mean free path in a gas is much longer than for ions. Therefore, electrons acquire the kinetic energy necessary for impact ionization at lower field strengths than ions. However, even at fields that are not too strong, positive ions play a very important role in the self-ionization of the gas. The fact is that the energy of these ions is sufficient to knock out electrons from the metal. Therefore, positive ions accelerated by the field, hitting the metal cathode of the field source, knock out electrons from it, which in turn are accelerated by the field and produce impact ionization of neutral molecules.

Ions and electrons, whose energy is insufficient for impact ionization, can, nevertheless, in collision with molecules, lead them to an excited state, i.e., cause some energy changes in their electron shells. The excited molecule (or atom) then goes into a normal state, while emitting a portion of electromagnetic energy - a photon (processes

excitation of atoms and emission and absorption of photons by them will be considered in § 132-136). The emission of photons is manifested in the glow of the gas. In addition, a photon absorbed by any of the gas molecules can ionize it; this kind of ionization is called photonic. Finally, a photon hitting the cathode can knock out an electron from it (external photoelectric effect), which then causes impact ionization of the neutral molecule.

As a result of impact and photon ionization and the knocking out of electrons from the cathode by positive ions and photons, the number of ions and electrons in the entire gas volume increases sharply (avalanche-like). An external ionizer is no longer needed for the existence of a current in a gas. The gas discharge becomes independent. The described process of gas self-ionization is shown schematically in Fig. 208, where neutral molecules are depicted as white circles, positive ions as circles with a plus sign, electrons as black circles, and photons as wavy lines.

On fig. 209 is an experimental graph of the dependence of the current in the gas on the field strength or on the voltage between the cathode and the anode of the field source, since

where is the distance between the electrodes. On the section of the curve, the current increases approximately in proportion to the field strength according to Ohm's law). This is explained by the fact that with an increase in tension, the speed of the ordered movement of ions and electrons increases, and, consequently, the amount of electricity passing to the electrodes (current) in 1 s. It is obvious that the increase in current will stop when the field strength reaches a value at which all the ions and electrons created by the external ionizer in 1 s will approach the electrodes in the same time.

Gases at not too high temperatures and at pressures close to atmospheric are good insulators. This is explained by the fact that gases under normal conditions consist of neutral atoms and molecules and do not contain free charges (electrons and ions). A gas becomes a conductor of electricity when some of its molecules ionized, To do this, the gas must be subjected to the action of some kind of ionizer (for example, using a candle flame, ultraviolet and x-ray radiation, g-quanta, flows of electrons, protons, a-particles, etc.). The ionization energy of atoms of various gases lies in the range of 4 - 25 eV. In an ionized gas, charged particles appear that can move under the influence of an electric field - positive and negative ions and free electrons.

The passage of electric current through gases is called gas discharge.

Simultaneously with the process ionization gas is always going on and the reverse process - recombination process: positive and negative ions, positive ions and electrons, meeting, combine with each other to form neutral atoms and molecules. The balance of their velocities determines the concentration of charged particles in the gas. The processes of ion recombination, as well as the excitation of ions that do not lead to ionization, lead to glow gas whose color is determined by the properties of the gas.

The nature of the gas discharge is determined by the composition of the gas, its temperature and pressure, dimensions, configuration and material of the electrodes, applied voltage, current density, etc.

Let us consider a circuit containing a gas gap subjected to continuous, constant in intensity action of an external ionizer.

As a result of gas ionization, a current will flow in the circuit, the dependence of which on the applied voltage is given in Fig.

On the curve OA the current increases in proportion to the voltage, i.e., Ohm's law is fulfilled. With a further increase in voltage, Ohm's law is violated: the increase in current strength slows down (section AB) and finally stops completely (section VS). Those. we obtain a saturation current, the value of which is determined by the power of the ionizer. This is achieved when all the ions and electrons created by the external ionizer per unit time reach the electrodes in the same time. If in mode OS stop the action of the ionizer, then the discharge also stops. Discharges that exist only under the action of external ionizers are called dependent. With a further increase in the voltage between the electrodes, the current strength is initially slowly (section CD), and then sharply (section DE) increases and the discharge becomes independent. The discharge in the gas that persists after the termination of the action of the external ionizer is called independent.

The mechanism for the occurrence of self-discharge is as follows. At high voltages, electrons arising under the action of an external ionizer, strongly accelerated by an electric field, collide with gas molecules, ionize them, resulting in the formation of secondary electrons and positive ions. Positive ions move towards the cathode and electrons move towards the anode. The secondary electrons again ionize the gas molecules, and, consequently, the total number of electrons and ions will increase as the electrons move towards the anode like an avalanche. This is the reason for the increase in electric current in the area CD. The described process is called impact ionization. Impact ionization by electrons alone is not sufficient to maintain the discharge when the external ionizer is removed. To maintain the discharge, it is necessary that the electron avalanches "reproduce", i.e., that new electrons arise in the gas under the influence of some processes. This occurs at significant voltages between the electrodes of the gas gap, when avalanches of positive ions rush to the cathode, which knock electrons out of it. At this moment, when, in addition to electron avalanches, there are also ion avalanches, the current increases almost without increasing the voltage (section DE in Fig.), i.e. an independent discharge occurs. The voltage at which self-discharge occurs is called breakdown voltage.

It should be noted that during a discharge in gases, a special state of matter, called plasma, is realized. Plasma A highly ionized gas is called a gas in which the densities of positive and negative charges are almost the same. A distinction is made between high-temperature plasma, which occurs at ultrahigh temperatures, and gas-discharge plasma, which occurs during a gas discharge. Plasma is characterized by the degree of ionization a - the ratio of the number of ionized particles to their total number per unit volume of the plasma. Depending on the value of a, one speaks of weakly (a is fractions of a percent), moderately (several percent), and fully (close to 100%) ionized plasma.

There are four types of self-discharge: glow, spark, arc and corona.

1. Glow discharge occurs at low pressures. If a constant voltage of several hundred volts is applied to the electrodes soldered into a glass tube 30 - 50 cm long, gradually pumping air out of the tube, then at a pressure of ~ 5.3 - 6.7 kPa (several mm Hg) a discharge occurs in the form a luminous reddish winding cord running from the cathode to the anode. With a further decrease in pressure (~13 Pa), the discharge has the following structure.

Directly adjacent to the cathode is a dark thin layer 1 - aston dark space, followed by a thin luminous layer 2 - first cathode glow or cathode film, followed by a dark layer 3 - cathode (crookes) dark space, which later passes into the luminous layer 4 - smoldering glow, which has a sharp boundary on the cathode side, gradually disappearing on the anode side. It arises from the recombination of electrons with positive ions. A dark gap borders on a smoldering glow 5- faraday dark space, followed by a column of ionized luminous gas 6 - positive post. The positive column has no significant role in maintaining the discharge. The applied voltage is distributed unevenly along the discharge. Almost all of the potential drop occurs in the first three layers and is called cathodic potential drop.

The mechanism of layer formation is as follows. Positive ions near the cathode, accelerated by the cathodic potential drop, bombard the cathode and knock electrons out of it. In dark aston space, electrons accelerate and excite molecules, which begin to emit light, forming a cathode film 2. Electrons flying through film 2 without collisions ionize gas molecules behind this film. Many positive and negative charges are formed. In this case, the intensity of the glow decreases. This area is a cathode (crookes) dark space 3. The electrons that have arisen in the cathode dark space penetrate into area 4 of the glow glow, which is due to their recombination with positive ions. Further, the remaining electrons and ions (there are few of them) penetrate by diffusion into region 5 - the Faraday dark space. It appears dark because the concentration of recombining charges is low. In area 5 there is an electric field that accelerates the electrons and in the area of ​​the positive column 6 they produce ionization, resulting in the formation of plasma. The glow of the positive column is mainly due to the transitions of excited molecules to the ground state. It has a characteristic color for each gas. In a glow discharge, only three of its parts are of particular importance for its maintenance - up to the glow glow. In the cathode dark space, there is a strong acceleration of electrons and positive ions, knocking out electrons from the cathode (secondary emission). In the smoldering region, however, impact ionization of gas molecules by electrons occurs. Positive ions formed during impact ionization rush to the cathode and knock out new electrons from it, which, in turn, again ionize the gas, etc. In this way, a glow discharge is continuously maintained.

Application in technology. The glow of the positive column, which has a color characteristic of each gas, is used in discharge tubes to create advertisements (neon discharge tubes give a red glow, argon ones - bluish-green) and in fluorescent lamps.

2. spark discharge arises at high electric field strengths (~3 10 b V/m) in a gas under atmospheric pressure. An explanation of the spark discharge is given on the basis of streamer theory, according to which the appearance of a brightly luminous spark channel is preceded by the appearance of faintly luminous accumulations of ionized gas - streamers. Streamers arise both as a result of the formation of electron avalanches through impact ionization and as a result of photon ionization of a gas. The avalanches, chasing each other, form conducting bridges of streamers, along which, at the next moments of time, powerful flows of electrons rush, forming spark discharge channels. Due to the release of a large amount of energy during the considered processes, the gas in the spark gap is heated to a very high temperature (about 10 4 o C), which leads to its glow. The rapid heating of the gas leads to an increase in pressure and the appearance of shock waves, which explain the sound effects during a spark discharge. For example, crackling in weak discharges and powerful peals of thunder in the case of lightning.

Application in technology. For igniting the combustible mixture in internal combustion engines and protecting electrical transmission lines from surges (spark gaps).

3. arc discharge. If, after ignition of a spark discharge from a powerful source, the distance between the electrodes is gradually reduced, then the discharge becomes continuous, i.e. arc discharge occurs. In this case, the current increases sharply, reaching hundreds of amperes, and the voltage across the discharge gap drops to several tens of volts. An arc discharge can be obtained from a low voltage source, bypassing the spark stage. To do this, the electrodes (for example, carbon ones) are brought together until they touch, they are very hot with an electric current, then they are parted and an electric arc is obtained. At atmospheric pressure, the arc discharge has a temperature of ~3500 o C. As the arc burns, a depression forms on the anode - a crater, which is the hottest place in the arc. the arc discharge is maintained due to intense thermionic emission from the cathode, as well as thermal ionization of molecules due to the high temperature of the gas.

Application - for welding and cutting metals, obtaining high-quality steels (arc furnace) and lighting (spotlights, projection equipment).

4. corona discharge- high-voltage electric discharge at high (for example, atmospheric) pressure in a sharply inhomogeneous field near electrodes with a large surface curvature (for example, points). When the field strength near the tip reaches 30 kV/m, a corona-like glow appears around it, which is the reason for the name of this type of discharge. This phenomenon was in ancient times called the fires of St. Elmo. Depending on the sign of the corona electrode, a negative or positive corona is distinguished.

Application - in electrostatic precipitators used to purify industrial gases from impurities, when applying powder and paint coatings.

The process of current penetrating through a gas is called a gas discharge.

The current in the gas that occurs in the presence of an external ionizer is called dependent .

Let a pair of electrons and ions be let into the tube for some time, with an increase in the voltage between the electrodes of the tube, the current strength will increase, positive ions begin to move towards the cathode, and electrons - towards the anode.

There comes a moment when all particles reach the electrodes and with a further increase in voltage, the current will not change, if the ionizer stops working, then the discharge will also stop, because. there are no other sources of ions, for this reason the discharge of ions is called non-self-sustaining.

The current reaches its saturation.

With a further increase in voltage, the current increases sharply, if the external ionizer is removed, the discharge will continue: the ions necessary to maintain the electrical conductivity of the gas are now created by the discharge itself. gas discharge that continues after the termination of the external ionizer is called independent .

The voltage at which self-discharge occurs is called breakdown voltage .

An independent gas discharge is maintained by electrons accelerated by an electric field, they have a kinetic energy that increases due to the electric field. fields.

Self discharge types:

1) smoldering

2) arc (electric arc) - for metal welding.

3) crown

4) spark (lightning)

Plasma. Plasma types.

Under plasma understand a strongly ionized gas in which the concentration of electrons is equal to the concentration of + ions.

The higher the gas temperature, the more ions and electrons in the plasma and the fewer neutral atoms.

Plasma types:

1) Partially ionized plasma

2) fully ionized plasma (all atoms decayed into ions and electrons).

3) High temperature plasma (T>100000 K)

4) low-temperature plasma (T<100000 К)

St-va plasma:

1) Plasma is electrically neutral

2) Plasma particles move easily under the action of the field

3) Have good electrical conductivity

4) Have good thermal conductivity

Practical use:

1) Conversion of thermal gas energy into electrical energy using a magnetohydrodynamic energy converter (MHD). Operating principle:

A jet of high-temperature plasma enters a strong magnetic field (the field is directed perpendicular to the drawing plane X), it is divided into + and - particles, which rush to different plates, creating some kind of potential difference.

2) They are used in plasmatrons (plasma generators), with their help they cut and weld metals.

3) All stars, including the Sun, stellar atmospheres, galactic nebulae are plasma.

Our Earth is surrounded by a plasma shell - ionosphere, outside of which there are radiation poles surrounding our Earth, in which there is also plasma.

The processes in the near-Earth plasma are caused by magnetic storms, auroras, and also in space there are plasma winds.

16. Electric current in semiconductors.

Semiconductors are ve-va, in which the resistance decreases with increasing t.

Semiconductors occupy 4 subgroups.

Example: Silicon is a 4-valence element - this means that in the outer shell of an atom, there are 4 electrons weakly bonded to the nucleus, each atom forms 4 bonds with its neighbors, when Si is heated, the velocity of valence e increases, and hence their kinematic energy (E k), the speed e becomes so great that the bonds do not withstand t break, e leave their paths and become free, in el. the field they move m-y nodes of the lattice, forming el. current. As t increases, the number of broken bonds increases, and hence the number of connected e increases, and this leads to a decrease in resistance: I \u003d U / R.

When the bond is broken, a vacancy is formed with the missing e; its crystal is not unchanged. The following process takes place continuously: one of the atoms providing the bond jumps to the place of the formed hole and the steam-electric bond is restored here, and where it jumped from, a new hole is formed. Thus, the hole can move throughout the crystal.

Conclusion: in semiconductors there are 2 types of charge carriers: e and holes (electron-hole conductivity)