Self-sustaining and non-self-sustaining gas discharges. Ionization of gases

The process of passing email. current through gas called gas discharge.

There are 2 types of discharges: independent and non-independent.

If the electrical conductivity of the gas is created. external ionizers, then el. the current in it is called. non-self gas discharge. V

Consider email diagram, comp. from a capacitor, galvanometer, voltmeter and current source.

Between the plates of a flat-plate capacitor there is air at atmospheric pressure and room t. If U equal to several hundred volts is applied to the capacitor, and the ionizer does not work, then the galvanometer does not register the current, however, as soon as the space between the plates begins to permeate. stream of UV rays, the galvanometer will begin to register. current. If the current source is turned off, the flow of current through the circuit will stop; this current represents a non-self-sustaining discharge.

j = γ*E – Ohm’s law for el. current in gases.

With a sufficiently strong electric field in the gas, the process of self-ionization begins, due to which the current can exist in the absence of an external ionizer. This kind of current is called a self-sustaining gas discharge. Self-ionization processes in general outline is as follows. In natural conventional always present in gas small quantity free electrons and ions. They are created by such natures. ionizers, like cosmic ones. rays, radiation of radioactive substances, soda in soil and water. Quite strong electricity. the field can accelerate these particles to such speeds at which their kinetic energy exceeds the ionization energy when electrons and ions collide with neuters on the way to the electrodes. molecules will ionize these molecules. Arr. upon collision, new secondary electrons and ions are also dispersed. field and in turn ionize new neutrons. molecules. The described self-ionization of gases is called impact polarization. Free electrons cause impact ionization already at E = 10 3 V/m. Ions can cause impact ionization only at E = 10 5 V/m. This difference is due to a number of reasons, in particular the fact that the mean free path for electrons is much longer than for ions. Therefore, ions acquire the energy necessary for impact ionization at a lower field strength than ions. However, even at not too strong “+” fields, ions play an important role in self-ionization. The fact is that the energy of these ions is approx. sufficient to knock electrons out of metals. Therefore, the ions accelerated by the “+” field, hitting the metal cathode of the field source, knock out the electrons from the cathode. These knocked out electrons are decomposed. field and produce impact ionization of molecules. Ions and electrons, the energy of which is insufficient for impact ionization, can nevertheless, when colliding with molecules, cause them to become excited. state, that is, cause some energy changes in the electrical system. Neutral shells atoms and molecules. Exc. the atom or molecule after some time returns to its normal state, and it emits a photon. The emission of photons manifests itself in the glow of gases. In addition, photon, absorption. any of the gas molecules can ionize it, this kind of ionization is called photon ionization. Some photons hit the cathode, they can knock electrons out of it, which then cause impact ionization of neutrons. molecules.

As a result of impact and photon ionization and knocking out electrons from the “+” code by photons, the number of photons and electrons in the entire volume of the gas increases sharply (avalanche-like) and for the existence of a current in the gas an external ionizer is not needed, and the discharge becomes independent. The current-voltage characteristic of a gas discharge looks as follows.

Let the gas located between the electrodes (Figure 81.1) be subjected to continuous, constant intensity exposure to some ionizing agent (for example, X-rays). The action of the ionizer leads to the fact that one or more electrons are split off from some gas molecules, as a result of which these molecules turn into positively charged ions.

When not very low pressures the detached electrons are usually captured by neutral molecules, which thus become negatively charged ions. The number of pairs of ions generated under the influence of an ionizer per second per unit volume will be denoted by .

Along with the ionization process, ion recombination occurs in the gas, i.e., the neutralization of unlike ions when they meet or the reunification of a positive ion and an electron into a neutral molecule. The probability of two ions of opposite signs meeting is proportional to both the number of positive and the number of negative ions. Therefore, the number of ion pairs recombining per second per unit volume is proportional to the square of the number of ion pairs present per unit volume:

( - proportionality coefficient).

In equilibrium states, the number of emerging ions is equal to the number of recombining ones, therefore,

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

Under the influence of cosmic radiation and traces of radioactive substances present in earth's crust, in 1 cm3 of atmospheric air, on average, several pairs of ions appear per second. The coefficient for Eair is equal to Substituting these numbers into formula (81.3) gives for the equilibrium concentration of ions in the air a value of the order of . This concentration is not sufficient to cause noticeable conductivity. Clean, dry air is a very good insulator.

If a voltage is applied to the electrodes, then the loss of ions will occur not only due to recombination, but also due to the suction of ions by the field to the electrodes. Let a pair of ions be sucked out of a unit volume every second. If the charge of each ion is , then the neutralization of one pair of ions on the electrodes is accompanied by the transfer of charge e along the chain. Every second, pairs of ions reach the electrodes (S is the area of ​​the electrodes, l is the distance between them; the product is equal to the volume of the interelectrode space). Therefore, the current in the circuit is equal to

where is the current density.

In the presence of current, the equilibrium condition is as follows:

Substituting here expressions (81.1) and (81.4) for , we arrive at the relation

The current density is determined by the expression

where are the mobilities of positive and negative ions (see formula (79.5)).

Let us consider two limiting cases - the case of weak fields and the case of strong fields.

In the case of weak fields, the current density will be very small, and the term in relation (81.5) can be neglected in comparison with (this means that the loss of ions from the interelectrode space occurs mainly due to recombination). Then (81.5) turns into (81.2), and expression (81.3) is obtained for the equilibrium ion concentration. Substituting this value into formula (81.6) gives

The factor for E in the resulting formula does not depend on the field strength. Consequently, in the case of weak fields, a non-self-sustaining gas discharge obeys Ohm's law.

The mobility of ions in gases matters. Therefore, at equilibrium concentration and field strength, the current density will be

(see formula (81.6); the ions are assumed to be singly charged).

In the case of strong fields, the term in formula (81.5) can be neglected in comparison with This means that almost all the resulting ions reach the electrodes without having time to recombine. Under this condition, relation (81.5) has the form

This current density is created by all the ions generated by the ionizer in a gas column with a unit cross section enclosed between the electrodes. Consequently, this current density is greatest at a given ionizer intensity and a given distance between the electrodes. It is called saturation current density

Let us calculate under the following conditions: (approximately this is the rate of formation of ions in atmospheric air under normal conditions). Substituting these data into formula (81.8) gives

This calculation shows that the conductivity of air under normal conditions is negligible.

At intermediate values ​​of E, there is a smooth transition from a linear dependence on E to saturation, upon reaching which it ceases to depend on E (see solid curve in Fig. 81.2). Beyond the saturation region lies a region of sharp increase in current (see the section of the curve shown by the dashed line). This increase is explained by the fact that, starting from a certain value of E, electrons generated by an external ionizer manage, during their free path, to acquire energy sufficient to collide with a molecule and cause its ionization. The free electrons generated during ionization, having accelerated, in turn cause ionization. Thus, an avalanche-like multiplication of primary ions created by an external ionizer occurs, and the discharge current increases. However, the process does not lose the nature of a non-self-sustaining discharge, since after the termination of the external ionizer, the discharge continues only until all electrons (primary and secondary) reach the anode (the rear boundary of the space in which there are ionizing particles - electrons, moves towards the anode) . In order for the discharge to become independent, the presence of two counter avalanches of ions is necessary, which is possible only if carriers of both signs are capable of causing ionization by impact.

It is very important that non-self-sustaining discharge currents, enhanced by carrier multiplication, are proportional to the number of primary ions created by the external ionizer. This digit property is used in proportional counters (see next paragraph).

Gases are good insulators at temperatures that are not too high and at pressures close to atmospheric. If placed in dry atmospheric air, a charged electrometer, its charge remains unchanged for a long time. 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 only when some of its molecules are ionized. To ionize, the gas must be exposed to some kind of ionizer: for example, an electric discharge, x-ray radiation, radiation or UV radiation, candle flame, etc. (in the latter case, the electrical conductivity of the gas is caused by heating).

When gases are ionized, they escape from the external electron shell an atom or molecule with one or more electrons, resulting in the formation of free electrons and positive ions. Electrons can attach to neutral molecules and atoms, turning them into negative ions. Therefore, an ionized gas contains positively and negatively charged ions and free electrons. E Electric current in gases is called gas discharge. Thus, the current in gases is created by ions of both signs and electrons. A gas discharge with such a mechanism will be accompanied by the transfer of matter, i.e. Ionized gases are classified as conductors of the second type.

In order to remove one electron from a molecule or atom, it is necessary to perform a certain amount of work A and, i.e. expend some energy. This energy is called ionization energy , whose values ​​for atoms various substances lie within 4÷25 eV. The ionization process is usually characterized quantitatively by a quantity called ionization potential :

Simultaneously with the process of ionization in a gas, the reverse process always occurs - the process of recombination: positive and negative ions or positive ions and electrons, meeting, reunite with each other to form neutral atoms and molecules. The more ions appear under the influence of the ionizer, the more intense the recombination process.

Strictly speaking, the electrical conductivity of a gas is never zero, since it always contains free charges formed as a result of the action of radiation from radioactive substances present on the surface of the Earth, as well as cosmic radiation. The intensity of ionization under the influence of these factors is low. This insignificant electrical conductivity of the air causes the leakage of charges from electrified bodies, even if they are well insulated.

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

Let us consider a circuit containing a gas gap (Fig.), subjected to continuous, constant-intensity exposure to an ionizer. As a result of the action of the ionizer, the gas acquires some electrical conductivity and current flows in the circuit. Figure shows the current-voltage characteristics (current versus applied voltage) for two ionizers. The productivity (the number of ion pairs produced by the ionizer in the gas gap in 1 second) of the second ionizer is greater than the first. We will assume that the productivity of the ionizer is constant and equal to n 0. At not very low pressure, almost all of the detached electrons are captured by neutral molecules, forming negatively charged ions. Taking into account recombination, we assume that the concentrations of ions of both signs are the same and equal to n. The average drift velocities of ions of different signs in an electric field are different: , . b - and b + – mobility of gas ions. Now for region I, taking into account (5), we can write:

As can be seen, in region I, with increasing voltage, the current increases, as the drift speed increases. The number of pairs of recombining ions will decrease as their speed increases.

Region II - the region of saturation current - all ions created by the ionizer reach the electrodes without having time to recombine. Saturation current density

j n = q n 0 d, (28)

where d is the width of the gas gap (the distance between the electrodes). As can be seen from (28), the saturation current is a measure of the ionizing effect of the ionizer.

At a voltage greater than U p p (region III), the speed of electrons reaches such a value that when they collide with neutral molecules they are capable of causing impact ionization. As a result, additional An 0 ion pairs are formed. The quantity A is called the gas gain coefficient . In region III, this coefficient does not depend on n 0, but depends on U. Thus. the charge reaching the electrodes at constant U is directly proportional to the performance of the ionizer - n 0 and the voltage U. For this reason, region III is called the proportionality region. U pr – proportionality threshold. The gas amplification factor A has values ​​from 1 to 10 4.

In region IV, the region of partial proportionality, the gas gain coefficient begins to depend on n 0. This dependence increases with increasing U. The current increases sharply.

In the voltage range 0 ÷ U g, current in the gas exists only when the ionizer is active. If the action of the ionizer is stopped, the discharge also stops. Discharges that exist only under the influence of external ionizers are called non-self-sustaining.

Voltage Ug is the threshold of the region, the Geiger region, which corresponds to the state when the process in the gas gap does not disappear even after the ionizer is turned off, i.e. the discharge acquires the character of an independent discharge. Primary ions only give impetus to the occurrence of a gas discharge. In this region, massive ions of both signs also acquire the ability to ionize. The current value does not depend on n 0.

In region VI, the voltage is so high that the discharge, once occurring, does not stop - the region of continuous discharge.

Non-self-discharge is called a discharge in which the current is maintained only due to the continuous formation of charged particles for some external reason and stops after the source of charge formation ceases. Charges can be created both on the surface of the electrodes and in the volume of the discharge tube. Independent discharges characterized by the fact that the charged particles necessary to maintain the discharge are created during the discharge itself, that is, their number at least does not decrease over time (at a constant applied voltage). You can remove the current-voltage characteristic of a self-discharge (see G.N. Rokhlin, Fig. 5.1, page 156).

The mechanism for the transition of a non-self-sustaining discharge into one of the forms of an independent one depends on many reasons, but the general criterion for the transition is the condition that, on average, each charged particle that disappears for one reason or another creates for itself at least one substituent during its existence.

Let us describe the processes occurring in the discharge tube during both types of discharges.

Non-self-sustaining discharge- is possible only in the presence of “artificial” emission of electrons from the cathode (heating, exposure to short-wave radiation).

Townsend avalanche. An electron, one way or another released from the cathode, under the influence electric field accelerates between the electrodes and acquires energy. There is a possibility of ionization of atoms and the creation of new electrons and ions. Thus, the “released” electrons under the influence of the field acquire some energy and also ionize the atoms. Thus, the number of free electrons increases in a power-law progression (we do not consider deionization mechanisms).

Independent discharge. The above process is not enough to describe the occurrence of a self-discharge: this mechanism does not explain the appearance of new electrons from the cathode. In general, for the discharge to become independent, each electron ejected from the cathode as a result of a chain of interactions must eject at least 1 more electron from the cathode. Let us remember that when an atom is ionized by an electron, in addition to a free electron, an ion also appears, which moves under the action of a field in the direction opposite to the electrons - towards the cathode. As a result of the collision of an ion with the cathode, an electron can be emitted from the latter (this process is called secondary electron emission ). The mechanism itself corresponds dark self-discharge. That is, under such conditions no generation of radiation occurs. The falling nature of this section (see Rokhlin G.N., Fig. 5.1, page 156) is explained by the fact that at higher currents lower electron energies are needed to maintain the independence of the discharge and, therefore, smaller accelerating fields.

Normal glow discharge- the current density at the cathode and the voltage drop are constant. When increasing total current the emitting area of ​​the electrode increases at a constant current density. At such currents, a glow of the positive column and near-electrode regions already occurs. The generation of electrons from the cathode still occurs due to secondary processes (bombardment by ions, fast atoms; photoemission). The near-electrode regions and the discharge column are formed during the transition from a dark independent discharge to a glowing one.

Anomalous glow discharge. The entire area of ​​the cathode emits electrons, so as the current increases, its density increases. In this case, the cathode voltage drop increases very sharply, since each time to increase the number of emitted electrons per unit area (i.e., current density), more and more energy is required. The mechanism of electron emission from the cathode remained unchanged.

At transition to arc discharge appears thermionic emission from the cathode- the current has a thermal effect on it. That is, the emission mechanism is already fundamentally different from previous cases. The cathode voltage drop decreases and becomes of the order of the filling gas potential (before this, the voltage drop arising in the process of secondary emission was added).

Arc discharge. Large currents, low voltage drop, large luminous flux of the discharge column.

With a heated cathode, the current-voltage characteristic will look different. It does not depend on the processes of secondary emission; everything is determined only by ionizations in the discharge gap (they are described by α). After the discharge is ignited, the cathode is also heated by ions coming from the discharge gap.

The form of self-discharge, which is established after the breakdown of the gas gap, depends on the conditions in the external circuit, processes on the electrodes and in the gas gap.

LABORATORY WORK No. 2.5

"Study of gas discharge using a thyratron"

Purpose of the work: study the processes occurring in gases during non-self-sustained and self-sustained discharge in gases, study the operating principle of the thyratron, construct the current-voltage and starting characteristics of the thyratron.

THEORETICAL PART

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

Atoms and molecules of gases under normal everyday conditions are electrically neutral, i.e. do not contain free charge carriers, which means, like a vacuum gap, they should not conduct electricity. In reality, gases always contain a certain amount of free electrons, positive and negative ions and therefore, although poorly, 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, bombardment by particles (electrons, ions, etc.), electromagnetic irradiation (ultraviolet, x-rays, radioactive, etc.). In this case, the gas located between the electrodes conducts electric current what is called gas discharge. Power ionizing factor ( ionizer) is the number of pairs of oppositely charged charge carriers resulting from ionization in a 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, resulting in the appearance of electrically neutral atoms or molecules and the emission of electromagnetic waves. If the electrical conductivity of a gas requires the presence of an external ionizer, then such a discharge is called dependent. If the attached 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 require 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).

In a non-self-sustaining gas discharge in the region of weak EF (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 consequence, Ohm's law (1):

Where E– electric field strength; n– concentration; j– current density.

And ( ) – respectively, the mobility of positive and negative charge carriers;<υ > – drift speed of directional movement of the charge.

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

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

As a result of these processes, flows of electrons, ions and photons are formed, the number of particles increases like an avalanche, and there is a sharp increase in current with virtually no increase in electron density between the electrodes. Arises independent gas discharge. The transition from an insolvent 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 email breakdown, it is necessary that electrons along their path length have time to gain kinetic energy exceeding the ionization potential of gas molecules, and on the other hand, that positive ions along their path length have time to acquire kinetic energy greater than the work function of the cathode material. Since the free path depends on the configuration of the electrodes, the distance between them d and the number of particles per unit volume (and, therefore, on pressure), the ignition of a self-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. Pashena(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 you apply to the electrodes soldered into a glass tube 30–50 cm long constant voltage at several hundred volts, gradually pumping out the air from the tube, then at a pressure of 5.3-6.7 kPa a discharge appears in the form of a luminous, winding reddish cord running from the cathode to the anode. With a further decrease in pressure, the cord thickens, and at a pressure of ≥ 13 Pa, the discharge has the form schematically shown in Fig. 2.

A thin luminous layer 1 is applied directly to the cathode cathode film , followed by 2 – cathode dark space , which later turns into 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. Behind the smoldering glow comes Faraday dark space - 4. The rest of the tube is filled with luminous gas - positive column - 5.

The potential varies unevenly along the tube (see Fig. 2). Almost the entire voltage drop occurs in the first areas 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 cathode potential drop, bombard the cathode and knock electrons out of it;

2) electrons are accelerated in the cathode part and gain sufficient energy and ionize gas molecules. Many electrons and positive ions are produced. In the region of the smoldering glow, intense recombination of electrons and ions occurs, energy is released, part of which is used for additional ionization. Electrons penetrating 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 represents 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 glow tubes. Neon tubes give a red glow, argon tubes give a bluish-green glow.

Arc discharge observed in normal and high blood pressure. 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 source low voltage, if you first bring the electrodes closer together until they touch. At the point of contact, the electrodes become very hot due to 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 caused by 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 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 influence of fast electrons attacking it from the gas gap, heats up more and can even melt and a depression is formed on its surface - a crater - the brightest place of the arc. Electric arc was first obtained in 1802. Russian physicist V. Petrov (1761–1834), who used two pieces of coal as electrodes. The red-hot carbon electrodes gave off 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 floodlights, as well as for cutting and welding metals. There is a cold cathode arc discharge. Electrons appear due to field emission from the cathode; the gas temperature is low. Ionization of molecules occurs due to electron impacts. A gas-discharge plasma appears between the cathode and anode.

Spark discharge occurs between two electrodes with a high EF voltage between them . A spark jumps between the electrodes, looking like a brightly glowing channel, connecting both electrodes. The gas near the spark heats up to high temperature, a pressure difference occurs, which leads to the appearance of sound waves, a characteristic crackling sound.

The occurrence of a spark is preceded by the formation of electron avalanches in the gas. The founder of each avalanche is an electron, which accelerates in a strong electron beam and produces 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 inactive. Electron avalanches intersect and a conducting channel is formed streamer, along which electrons flow from the cathode to the anode - occurs breakdown.

An example of a powerful spark discharge is lightning. Different parts of a thundercloud carry charges of different signs ("–" faces the Earth). Therefore, if clouds come together 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 (spark plugs in engines internal combustion), for recording charged particles in spark counters, for treating metal surfaces, etc.

Corona (coronary) discharge occurs between electrodes that have different curvatures (one of the electrodes is a thin wire or a point). During a corona discharge, ionization and excitation of molecules does not occur 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 appearance of a crown surrounding the electrode.

Plasma and its properties

Plasma is a highly ionized gas in which the concentration of positive and negative charges is almost the same. Distinguish high temperature plasma , which occurs at ultra-high temperatures, and gas discharge plasma , which occurs during a gas discharge.

Plasma has the following properties:

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

Concentration of positive and negative particles in plasma almost the same;

high electrical conductivity;

Glow;

Strong interaction with electrical and magnetic fields;

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

Simultaneous interaction of a huge number of particles.