The process of passing email. current through the 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. nesamost. gas discharge. V

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

There is air between the plates of a flat capacitor atmospheric pressure and room t. If a U equal to several hundred volts is applied to the capacitor, and the ionizer does not work, then the current galvanometer does not register, however, as soon as the space between the plates begins to penetrate. flow of UV rays, the galvanometer will start registering. current. If the current source is turned off, the flow of current through the circuit will stop, this current is a non-self-sustained discharge.

j = γ*E - Ohm's law for el. current in gases.

With a sufficiently strong e. 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 an independent gas discharge. The processes of self-ionization in general terms are as follows. In nature. conv. A gas always contains a small amount of free electrons and ions. They are created by such natures. ionizers, like space. rays, radiation radioactive substances, soda in soil and water. Fairly strong email. the field can accelerate these particles to such speeds at which their kinetic energy exceeds the ionization energy when electrons and ions collide on the way to the electrodes with neutrons. molecules will ionize those molecules. arr. upon collision, new secondary electrons and ions also accelerate. field and in turn ionize new neutrons. molecules. The described self-ionization of gases is called impact polishing. Free electrons cause impact ionization already at E=10 3 V/m. Ions, on the other hand, 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 for electrons the mean free path 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. enough to knock electrons out of metals. Therefore, the ions dispersed by the “+” field, hitting the metal cathode of the field source, knock out the electrons from the cathode. These knocked-out electrons field and produce impact ionization of molecules. Ions and electrons, the energy of which is insufficient for impact ionization, can nevertheless lead them into excitation when colliding with molecules. state, that is, to cause some energy changes in the email. shells of neutral atoms and molecules. Excit. an atom or molecule after some time goes into a normal state, while it emits a photon. The emission of photons is manifested in the glow of gases. In addition, a photon, absorb. any of the gas molecules can ionize it, this kind of ionization is called photonionization. Some of the photons hit the cathode, they can knock electrons out of it, which then cause impact ionization of the neutron. molecules.

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

Electric self-sustained and non-self-sustained discharge occurs in various gaseous media under certain conditions. A person uses, as a rule, an independent discharge. The article gives a description of these phenomena.

What is in gases?

Before considering the independent and non-self-sustaining gas discharge, let's define this phenomenon. Discharge is understood as the occurrence of an electric current in a gas. Since gaseous media are inherently insulators, this means that the current is due to the presence of free carriers in them. electric charge. In addition to them, an electric field must also exist in order for the charges to acquire a directed movement.

An electric field can be created by applying an external potential difference to the volume of gas (presence of electrodes: negative cathode and positive anode).

The following processes can be sources of charge carriers:

• Thermal ionization. It arises due to the mechanical collision of high-energy gas particles (atoms, molecules) and the knocking out of electrons from them. This process is activated when the temperature rises.
• Photoionization. Its essence lies in the absorption of a high-energy photon by an electron and its detachment from the atom.
• Cold emission of electrons. Occurs due to ion bombardment of the cathode surface.
• Thermionic emission. This process is due to the evaporation of high-energy electrons from the cathode and their participation in the subsequent plasma ionization.

These processes underlie the classification of types of discharges (independent and non-independent).

The concept of discharge independence

Consider the case of a cathode tube. It is a sealed container in which there is some gas under a certain pressure. At the ends of this tube are electrodes. If a small potential difference is applied to them, then practically no current will arise. This is due to the lack of a sufficient number of charge carriers.

If, however, the gas is heated or subjected to ultraviolet irradiation, then the voltmeter will immediately record the appearance of a current. This a prime example non-self-sustained category. It is so called because for its existence an external source of ionization (radiation, temperature) is necessary. It is worth removing this source, as the voltmeter readings will again become equal to zero.

If, in the absence external sources ionization to increase the voltage between the electrodes of the tube, then a current will begin to appear, which will go through several stages (saturation, increase, decrease). In this case, one speaks of an independent electric discharge. It no longer requires external sources, the necessary charge carriers are generated within the system itself. The processes of their formation remain the same as for a non-self-sustaining discharge. At high voltages and high current densities, thermal emission of cathode electrons is also added.

Current-voltage characteristic of the discharge

It is convenient to study a gas self-sustained and non-self-sustaining discharge using the dependence of voltage on current strength (or vice versa), which is commonly called the current-voltage characteristic. It allows you to judge not only the magnitude of the voltage and current in the system, but also the electrical processes occurring in it.

Below is the current-voltage characteristic, which reflects all the main phases of the development of the discharge.

As you can see, there are three of them: dark, smoldering and arc. We will describe these phases in more detail later in this article.

Dark Discharge

It is described by the interval AC. As the voltage U increases, the current I increases due to the increase in the speed of the ions. However, these velocities are not high, so a non-self-sustained discharge takes place. In the BC region, it saturates and becomes independent, since the speed of the ions becomes sufficient to knock electrons out of the cathode when bombarded. These electrons lead to additional ionization of the gas.

The dark charge got its name because its glow is almost zero: low plasma concentration, low currents (10 -8 A), no recombination of ions and electrons.

glow discharge

On the current-voltage characteristic, it corresponds to the zone between points C and F. The figure shows that the voltage changes (falls and rises), while the current constantly increases. Two subzones are of interest:

1. Points OE - normal glow discharge. The reason for the increase in current here is associated with an increase in the plasma area in the gas. That is, at first these are narrow small channels, then, due to cold electron emission, they expand until they reach the entire volume of the tube. From this point on, there is a transition to the next subzone.
2. EF points - anomalous discharge. The current of this self-sustained discharge in the gas begins to grow due to hot electron emission. The temperature of the cathode gradually rises, and it begins to emit negatively charged particles.

All neon and fluorescent lamps operate in the normal glow discharge region.

Spark and arc discharges

These types of self-sustained discharges cover the FG zone in the figure. This is where the most complex processes take place.

When the voltage between the electrodes rises to the maximum value (point F), and the thermal emission of electrons from the cathode is activated, then favorable conditions will be created for the formation of an unstable spark discharge. It represents short-term breakdowns (microseconds), which have a characteristic zigzag shape. A striking example in nature is lightning in the atmosphere.

The discharge occurs through narrow channels, which are called streamers. They are narrow broken lines of highly ionized plasma that connect the cathode surface with the anode one. The current strength in them reaches tens of thousands of amperes.

Stabilization of the spark charge leads to the formation of a stable arc (point G region). In this case, the entire volume of gas in the tube is a highly ionized plasma. The surface of the cathode is heated up to 5000-6000 K, and the anode - up to 3000 K. Such a strong heating of the cathode leads to the formation of so-called "hot spots" on it, which become powerful source thermoelectrons and are the cause of erosive wear of this electrode. The voltage during an arc discharge is not high (several tens of volts), but the current can reach 100 A or more. The welding arc is a prime example of this type of discharge.

Thus, the existence of independent and non-self-sustaining discharges in gases is due to the mechanisms of its ionization and plasma formation with increasing voltage and current in the system.

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 ejection of electrons from electron shell 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 radiate electromagnetic waves. 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 is sufficient to maintain an 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– tension electric field; 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 area of ​​smoldering glow, 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 the 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 place 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. Arc discharge is used as a source bright light in searchlights, projection installations, 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 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 generated 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:

High degree 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 electrical 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.

An electric current is a flow that is caused by the ordered movement of electrically charged particles. The movement of charges is taken as the direction of the electric current. Electricity may be short term or long term.

The concept of electric current

During a lightning discharge, an electric current can occur, which is called short-term. And to maintain the current for a long time, it is necessary to have an electric field and free electric charge carriers.

An electric field is created by bodies charged differently. The current strength is the ratio of the charge transferred through the cross section of the conductor in a time interval to this time interval. It is measured in amperes.

Rice. 1. Current formula

Electric current in gases

Gas molecules do not conduct electricity under normal conditions. They are insulators (dielectrics). However, if you change the conditions environment, then gases can become conductors of electricity. As a result of ionization (when heated or under the action of radioactive radiation) an electric current arises in gases, which is often replaced by the term "electric discharge".

Self-sustained and non-self-sustained gas discharges

Discharges in gas can be self-sustaining and non-self-sustaining. The current begins to exist when free charges appear. Non-self-sustaining discharges exist as long as an external force acts on it, that is, an external ionizer. That is, if the external ionizer ceases to operate, then the current stops.

An independent discharge of electric current in gases exists even after the termination of the external ionizer. Independent discharges in physics are divided into quiet, smoldering, arc, spark, corona.

• Quiet - the weakest of the independent discharges. The current strength in it is very small (no more than 1 mA). It is not accompanied by sound or light phenomena.
• Smoldering - if you increase the voltage in a quiet discharge, it goes to the next level - to a glow discharge. In this case, a glow appears, which is accompanied by recombination. Recombination - the reverse ionization process, the meeting of an electron and a positive ion. It is used in bactericidal and lighting lamps.

Rice. 2. Glow discharge

• Arc - the current strength ranges from 10 A to 100 A. In this case, ionization is almost 100%. This type of discharge occurs, for example, during the operation of a welding machine.

Rice. 3. Arc discharge

• sparkling - can be considered one of the types of arc discharge. During such a discharge for very a short time a certain amount of electricity flows.
• corona discharge – ionization of molecules occurs near electrodes with small radii of curvature. This type of charge occurs when the electric field strength changes dramatically.

What have we learned?

By themselves, the atoms and molecules of a gas are neutral. They are charged when exposed to the outside. Speaking briefly about the electric current in gases, it is a directed movement of particles (positive ions to the cathode and negative ions to the anode). It is also important that when the gas is ionized, its conductive properties improve.

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Gases at not too high temperatures and at pressures close to atmospheric are good insulators. If you place a charged electrometer in dry atmospheric air, then 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. For ionization, the gas must be exposed to some kind of ionizer: for example, an electric discharge, X-rays, radiation or UV radiation, a candle flame, etc. (in the latter case, the electrical conductivity of the gas is caused by heating).

When gases are ionized, one or more electrons are ejected from the outer electron shell of an atom or molecule, which leads to the formation of free electrons and positive ions. Electrons can attach to neutral molecules and atoms, turning them into negative ions. Therefore, in an ionized gas there are positively and negatively charged ions and free electrons. E electric current in gases is called a 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 conductors of the second kind.

In order to tear off one electron from a molecule or atom, it is necessary to perform a certain 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. Quantitatively, the ionization process is usually characterized by a quantity called ionization potential :

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

Strictly speaking, the electrical conductivity of a gas is never equal to zero, since it always contains free charges resulting from the action of radiation from radioactive substances present on the surface of the Earth, as well as from cosmic radiation. The intensity of ionization under the action of these factors is low. This slight electrical conductivity of the air is the cause of the leakage of charges of 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, dimensions, 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 in intensity action of an ionizer. As a result of the action of the ionizer, the gas acquires some electrical conductivity and current will flow in the circuit. Figure shows current-voltage characteristics (dependence of current on applied voltage) for two ionizers. Productivity (the number of pairs of ions 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 performance of the ionizer is constant and equal to n 0 . At a not very low pressure, almost all the split off electrons are captured by neutral molecules, forming negatively charged ions. Taking recombination into account, 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 + are the 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, since the drift velocity increases. The number of pairs of recombining ions will decrease as their speed increases.

Region II - saturation current region - 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 colliding with neutral molecules, they are able to cause impact ionization. As a result, additional An 0 pairs of ions are formed. The value A is called the gas amplification factor . 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 voltage U. For this reason, region III is called the proportional 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 begins to depend on n 0. This dependence increases with increasing U. The current increases sharply.

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

The voltage U g 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, I already acquire the ability to ionize massive ions of both signs. The magnitude of the current does not depend on n 0 .

In area VI, the voltage is so high that the discharge, once it has occurred, no longer stops - the area of ​​\u200b\u200bcontinuous discharge.