Lesson summary "Electron beams. Cathode-ray tube"

· Electronic beams.Electron beams are understood as directed electron flows, the transverse dimensions of which are much less than their length. Electron beams were first discovered in a gas discharge occurring at reduced pressure.

In a glow discharge, positive ions knock out a large number of electrons from the cathode. If the discharge occurs in the tube at very high rarefaction, then the mean free path of electrons increases and the cathode dark space expands. Electrons knocked out of the cathode by positive ions move almost without collision and form cathode rays. These rays propagate normally to the cathode surface. If a hole is made in the anode of an electron tube, then part of the electrons accelerated by the electric field , flies through the hole, forming an electron beam behind the anode.

· Properties and application of electron beams. Electron beams cause the glow (fluorescence) of certain substances. These include glass, zinc, cadmium sulfides, etc. These substances are called phosphors. This property of electron beams is used in vacuum electronics - the glow of TV screens, oscilloscopes, electron-optical converters, etc. Getting on bodies, electron beams cause them to heat up. This property is used for welding ultrapure metals in a vacuum.

Electron beams are deflected in electric and magnetic fields. The ability to control an electron beam using an electric and magnetic field and the glow of screens coated with a phosphor under the action of electron beams are used in cathode ray tubes.

· Cathode-ray tube. The device of a cathode ray tube is shown in fig. 12.4.1. It is a glass vacuum bottle. L , in which there is an "electron gun", consisting of a heated cathode To emitting electrons, and an anode with a diaphragm (usually several anodes located one after another) D1 , D2 . A potential difference is created between the cathode and anode U , allowing to accelerate the electrons to high speed and get a narrow beam. Where the electron beam hits the screen E , coated with a fluorescent composition, a bright luminous dot appears.

The electron beam is controlled by two pairs of plates From 1 and From 2 located perpendicular to each other. Field of plates From 1 shifts the beam in the horizontal direction, the field of plates From 2 - in vertical. On plates From 1 and From 2 You can apply either DC or AC voltage. Depending on this, the luminous spot on the screen will either remain in place or move, forming a straight line, a sinusoid, etc. The oscilloscope device is based on this property. In more complex cases, alternating dark and light spots can be obtained on the screen, which give an image of objects. We observe such a phenomenon in the cathode ray tube of the TV.

Review questions:

1. What is the ionization of a gas and the recombination of ions in a gas?

2. What is a gas discharge?

3. What is the difference between independent and non-self-sustaining gas discharges?

4. What are arc and glow discharges?

5. What is plasma? What properties does it have?

6. What is a diode, how does it work and why can it work as an AC rectifier?

7. What are electron beams, what properties do they have, where are they used?

8. Give examples of the use of a glow discharge in engineering.

9. Give examples of the practical application of plasma.

10. Describe the mechanism of formation of electron-ion avalanches.

Summary:

In the process of studying the topic, we got acquainted with the properties of gas discharges and the flow of electric current in gases and vacuum.

Application

Appendix N 1.

The distribution of electrons and holes is described by the Fermi–Dirac function.

,

where f F-D(E) is the probability that the energy state is occupied and can fluctuate from 0 before 1 ,

E F is the Fermi level, often called the Fermi energy or the electrochemical potential.

According to the Pauli principle, each quantum state can only be occupied by one electron. With a larger number of them, at absolute zero temperatures, all states are lower E F filled in:

f F-D ( E) = 1 , and above E F are free of electrons and f F-D(E) = 0 . Since at T \u003d 0ºK, conduction electrons have non-zero energy, but are distributed over all allowed states from 0 to E F (eV) then

.

The Fermi level in an intrinsic semiconductor is given by the equation:

Density of states g(E)

The number of states per unit energy interval per unit volume of a semiconductor as a function of energy.

In two phases adjacent to each other, electronic equilibrium is achieved when the Fermi levels are equal. -

Annex No. 2.

To determine the type of function φ(x) we used the Poisson equation known from electrostatics, which relates the field potential U(x) with bulk density ρ(x) stationary charges that create this field.

This equation looks like:

accept ρ(х) = qNd

Glossary

Amorphous substances	From a thermodynamic point of view, amorphous HP is in a metastable state and should crystallize with time. Amorphous substances behave like liquids with an abnormally high viscosity. These include glasses, plastics, and resins. As the temperature rises, they gradually soften and become capable of flowing like liquids [§1.1].
Anisotropy	The heterogeneity of the properties of a crystal in different directions, which is the result of its symmetry and internal structure[§1.1].
Acceptor levels	Impurities that capture electrons from the valence band of a semiconductor are called acceptor acceptor levels. Semiconductors containing such impurities are called hole semiconductors, or semiconductors p-type; often referred to as acceptor semiconductors. [§ 3.6.1].
Adsorption layer	See [§ 4.2.2].
barrier capacity	With reverse voltage applied to p-n transition, charge carriers of both signs are on both sides of the transition, and there are very few of them in the region of the transition itself. Thus, in reverse voltage mode p-n transition is a capacitance. This container is called barrier (C b). [§ 8.5].
Van der Waals connections	Interaction forces in such crystals are determined by the presence of natural or induced electrical moments in molecules [§ 1.3].
Valence band	When the atoms approach each other at a distance of about 10–8 cm, the wave functions of atomic electrons will overlap. Due to this, the energy level of valence electrons turns into a zone. This zone is called the valence zone [§ 2.1].
hydrogen bond	In hydrogen bonded crystals, each hydrogen atom is bound by attractive forces simultaneously with two other atoms. The hydrogen bond, together with the electrostatic attraction of the dipole moments of water molecules, determines the properties of water and ice[§1.1].
Current-voltage characteristic of p-n junction	See [§8.4].
Media lifetime	The average lifetime of charge carriers in a semiconductor is usually called carrier lifetime[ § 3.8].
degenerate gas	In a degenerate gas, not all free electrons can participate in the formation of electrical conductivity, but only those that are located directly at the Fermi level [§ 5.2.2].
Charge carrier generation	The generation of charge carriers (the formation of free electrons and holes) occurs under the influence of thermal chaotic effects of atoms of the crystal lattice (thermal generation), under the influence of light quanta absorbed by the semiconductor (light generation) and other energy factors [§ 3.4].
heterojunction	A heterojunction is a junction formed at the interface between two semiconductors with different band gaps. [§ 9.3].
Defects in a crystal	Violations of the periodicity of the lattice, which are not reduced to thermal motions, are called defects [§ 1.7].
Schottky defects	In real crystals, some nodes of the crystal lattice, in which atoms should be located, turn out to be unoccupied [§ 1.7].
Frenkel defects	They arise when an atom leaves its place in a node of the crystal lattice and is placed in an interstices surrounded by atoms located in their rightful places [§ 1.7].
Locations	This type of defect arises when an incomplete additional atomic plane is wedged between the atomic planes [§ 1.7].
Hole	A vacancy in a covalent bond is called a hole. An incomplete bond will have an excess positive charge equal in magnitude to the charge of an electron [§ 3.2].
Donor levels	Impurities that are the source of conduction electrons are called donors, and the energy levels of these impurities are donor levels. Semiconductors containing a donor impurity are called electronic semiconductors, or semiconductors n-type; often referred to as donor semiconductors[§3.6.1].
drift current	The current due to an external electric field is called drift current.[ § 3.8].
Diffusion current	The current resulting from the diffusion of carriers from an area where their concentration is increased towards an area with a lower concentration is called Diffusion drift-free current. [ § 3.8].
Diffusion length	The average distance that carriers travel during their lifetime is called diffusion length of charge carriers..
double electrical layer	The combination of positive ions at the surface of a metal and electrons appearing above the surface is called double electric layer..
Forbidden zone	The allowed energy zones are separated from each other by an interval called the forbidden zone or energy gap [§ 2.1].
Conduction band	If, however, in the uppermost occupied, but not complete, zone, there are free energy levels to which electrons can pass, then they form the so-called conduction band[§ 2.1].
Ionic crystals	Ionic crystals (NaCl, KC1, etc.) are characterized by the fact that the attractive forces acting between the ions are electrostatic. [§1.1].
Miller indices	In ristallography, it is customary to use special symbols to designate planes. Miller indexes.[§ 1.6].
injection laser	See [§10.6].
Population inversion	Population inversion - the ratio between the populations of different energy levels of atoms or molecules of a substance, in which the number of particles at the top of a given pair of levels is greater than at the bottom. [§10.5].
Crystal	A crystal is a collection of atoms ordered in space and held near the equilibrium position by interaction forces. Structural units of HP are atoms, molecules or ions. Thermodynamically stable HPs are crystalline, since they have a minimum internal energy, with an increase in temperature, upon reaching a certain temperature, called the melting point, they jump into a liquid state. The crystal has a discontinuous periodic structure. [§1.1].
covalent crystal	In covalent crystals (diamond, Ge, Si, etc.), the valence electrons of neighboring atoms are shared, so a covalent crystal can be considered as one huge molecule [§1.1].
Symmetry class	Crystallography shows that there are a total of 32 possible combinations of symmetry elements. Each of these possible combinations is called symmetry class. In nature, there are only crystals belonging to one of the 32 symmetry classes [§ 1.3].
Hall coefficient	See [§ 6.1.1].
Contact potential difference	See [§ 7.1.1].
coherence	Coherence is the coordinated flow of several oscillatory or wave processes in time. Those. if the phase difference of two oscillations remains constant in time, or if two ideal monochromatic oscillations have the same frequency, then such oscillations are called coherent. [§10.5].
lasers	Stimulated coherent radiation is called with stimulated or induced and the emitters of such waves are called lasers (from the English Light Amplification by Stimulated Emission of Radiation - amplification of light due to induced radiation). [§10.4].
metal connection	In metallic crystals, the bond (metallic bond) is due to the collective interaction of mobile electrons with the core of the crystal lattice. Transition metals are also characterized by a covalent bond carried out by electrons of unfilled inner shells [§1.1].
molecular crystals	In molecular crystals, molecules are interconnected by relatively weak electrostatic forces (van der Waals forces) due to the dynamic polarization of molecules [§1.1].
Non-equilibrium concentration	If, with the help of any external action, the dynamic equilibrium of the concentrations of electrons and holes in a semiconductor is disturbed, then an additional nonequilibrium charge carrier concentration. [§3.8].
non-degenerate gas	In the case of a non-degenerate gas, the filling density of the conduction band with electrons is so small that they almost never meet so close that their behavior could be limited by the Pauli principle [§ 5.2.1, § 5.2.2].
Non-self-sustained gas discharge	The process of current flowing through a gas is called a gas discharge. The current in the gas, which occurs in the presence of external ionizer, called non-self-sustaining gas discharge.
Axis of symmetry	If the crystal has an axis of symmetry (rotary axis), then it can be aligned with itself, i.e. brought to a position indistinguishable from the original, by turning through a certain angle around this axis. Depending on the symmetry of the crystal, the angle of rotation required to align the crystal with itself can be 360, 180, 120, 90, 60 degrees. ( 2p / p, where n= 1, 2, 3, 4 or 6) [§ 1.3].
Main carriers	Electrons constituting the vast majority of charge carriers in semiconductors P-type, called main charge carriers, and holes minor.. And vice versa, holes make up the vast majority of charge carriers in semiconductors. p-type, called main charge carriers, and electrons minor.[§ 3.6.2, § 3.6.3].
Ohmic transition	Contact, the electrical resistance of which is small and does not depend on the direction of the current in a given operating range of currents. [§9.3.3].
Broadcast period	Broadcast a represented by a vector having a specific direction and a numerical value equal to a, called broadcast period[§1.3].
Plane of symmetry	If one half of the crystal coincides with the other when reflected in a certain plane, as in a mirror, then such a plane is called the plane of symmetry [§ 1.3].
swivel axis	This element of symmetry results from the simultaneous application of two operations: rotation around an axis and mirror reflection in a plane perpendicular to the axis [§ 1.3].
Semiconductors	Semiconductors, a wide class of substances with an electronic mechanism of electrical conductivity, in terms of its specific value s occupying an intermediate position between metals (s ~ 10 4 -10 6 Ohm -1 cm -1) and good dielectrics (s ~ 10 -12 -10 -11 Ohm -1 cm -1) (ranges of s values ​​are given at room temperature) [§ 3.1].
Impurity semiconductor	A semiconductor that has impurities is called impurity, and its electrical conductivity due to the presence of impurities in the crystal is called impurity [§ 3.6.1].
n-type semiconductor	See Donor Levels. [§ 3.6.1].
p-type semiconductor	See Acceptor Levels [§ 3.6.1].[ § 3.6.3].
Impurity conductivity	Conductivity caused by the presence of impurities from atoms with a different valency in a semiconductor crystal is called impurity [§ 3.6.2].
Schottky transition	Rectifying metal-semiconductor contact P-type called Schottky transition. The most important feature of the Schottky transition compared to r-p transition is no minority carrier injection. [§9.1].
Surface phenomena in semiconductors	Physical phenomena that occur near the surface of a semiconductor crystal caused by a violation of the distribution of the potential of the crystal lattice of the semiconductor due to its breakage at the surface; the presence of uncompensated valence bonds in surface atoms; lattice potential distortion due to surface atoms; distortion of the lattice potential due to possible surface defects in the crystal structure. [§9.2].
Surface potential	If we take the potential in the volume of the semiconductor equal to zero, then the surface potential will be different from zero due to the presence of charges between the volume and the surface. The potential difference between the surface and the volume is called surface potential[§9.2].
Breakdown	Tunnel - based on the tunnel effect we studied - when electrons pass through a potential barrier r-p- transition without changing its energy.
Avalanche - The mechanism of avalanche breakdown is similar to the mechanism of impact ionization in gases. Under the influence of a strong electric field, electrons can be released from covalent bonds and receive energy sufficient to overcome the potential barrier in r-p- transition. Moving at high speed in the area r-p- transition they collide with neutral atoms and ionize them.
Thermal - Electrical and thermal breakdown in many cases occur simultaneously. During electrical breakdown, the semiconductor heats up and then thermal breakdown occurs. Thermal generation of electron-hole pairs leads to an increase in the concentration of minority charge carriers and to an increase in the reverse current, and an increase in current, in turn, leads to a further increase in temperature. The process grows like an avalanche. With excessive heating of the crystal, the pn junction irreversibly fails.
Work function	The work function is called the work to move an electron from a conductor to the surrounding space is equal to the product of the electron charge e on the passed potential difference φ 0 .[§ 4.2.1].
Recombination of charge carriers	The process of transformation of a free electron into a bound electron and the disappearance of a pair of charge carriers (electron-hole) is called recombination.
Interaction forces	The nature of the interaction forces between atoms in crystals is well known. These are the electric forces of repulsion and attraction of positively and negatively charged particles present in each atom. [§1.1].
Syngony	In crystallography, it is customary to combine 32 symmetry classes into 7 symmetry systems or 7 syngonies, which bear the following names in ascending order of symmetry: triclinic system, which includes two symmetry classes, trigonal system, which combines seven classes, monoclinic system, which includes three classes, hexagonal system - five classes, rhombic, also with three classes, tetragonal system with seven classes, cubic system [§ 1.3]. [§ 1.3].
Own semiconductor	A semiconductor will be intrinsic if the influence of impurities on its properties is negligible. In it, free charge carriers arise only due to the breaking of valence bonds [§ 3.2].
stimulated emission	A process may arise in which all excited atoms emit almost simultaneously, interconnected and so that the generated photons are absolutely indistinguishable from those that caused this generation. Such stimulated coherent emission is called stimulated or induced[§10.4.].
Thermocouple	See [§11.2.1].
thermoelement	See [§ 11.2.2].
Thermoelectric phenomena	See [§10.1.1].
Broadcast	The crystal has a discontinuous periodic structure. From a geometric point of view, such a structure can be created using a parallel displacement operation called broadcast[§1.3].
Solid	A solid state (SS) is such a state of aggregation of a substance, which is characterized by the constancy of the shape of the considered macro-system and the special nature of the thermal motion of the atoms that make up the macrosystem. There are crystalline and amorphous HPs. Thermodynamically stable CTs are crystalline, since they have a minimum internal energy [§1.1].
Broadcasting group	The position of any point in the spatial lattice is determined by a combination of displacements ma+nb+pc. Combination of three vectors a, b, c called broadcasting group[§1.3].
Thermal breakdown p-n junction	Thermal breakdown of the p-n junction occurs due to the pulling out of valence electrons from bonds in atoms during thermal vibrations of the crystal lattice. Thermal generation of electron-hole pairs leads to an increase in the concentration of non-main charge carriers and to an increase in the reverse current. [§8.4].
tunnel effect	The tunnel effect is that electrons pass through the potential barrier of the p-n junction without changing their energy. [§8.6].
Photoconductivity of semiconductors	The phenomenon of photoconductivity is the increase in the electrical conductivity of a semiconductor under the influence of electromagnetic radiation. [§ 10.1].
Photoresistive effect	The essence of this phenomenon is that when light quanta are absorbed with energy sufficient to ionize the semiconductor's own atoms or ionize impurities, the concentration of charge carriers increases. [§10.2].
Center of symmetry	If there is a point in the crystal that has the property that when the radius-vector is replaced r, any of the particles that make up the crystal to its inverse vector - r, the crystal goes into a state indistinguishable from the original, then this point is called the center of symmetry or the center of inversion [§ 1.3].
Extraction of charge carriers	For minor carriers (holes in n- region and electrons in R - region) there is no potential barrier in the electron-hole transition, and they will be drawn by the field into the region pn transition. This phenomenon is called extraction.[§ 8.2].
unit cell	A parallelepiped built on three elementary translations a, b, c is called an elementary parallelepiped or an elementary cell.[ §1.3].
Elements of symmetry	plane of symmetry, axis of symmetry, center of symmetry, mirror-rotary axis of symmetry[ §1.3].
Electrochemical potential	Energy electrochemical potential- the work that must be spent to change the number of particles in the system per unit, provided that the volume and temperature are constant [§ 3.3].
Electrical breakdown p-n junction	Electrical breakdown occurs as a result of internal electrostatic emission (Zener breakdown) and under the action of impact ionization of semiconductor atoms (avalanche breakdown). [§ 8.4].
Electronic emission	See [§ 4.2.2].
Electronic-hole transition (p-n transition).	The transition between materials with n- and p-type electrical conductivity is called the p-n junction. [§ 7.2].
Electrostatic domain	See Gunn Effect [§ 5.6].
Fermi energy	At a temperature equal to absolute zero T = 0 K the energy of the entire atomic system, including the electron gas, is minimal. However, a characteristic situation is observed in this case, when the electrons located at the upper energy levels still have a sufficiently large energy that they cannot lose and go to the lower levels due to the Pauli prohibition. The energy of the electrons occupying the highest of the occupied levels is denoted ε max and is called the Fermi energy [§ 2.1, § 3.3].
Effective mass	The effect on the motion of an electron in the field of the periodic crystal potential of ions and other electrons leads to the fact that the properties of current carriers in a crystal (conduction electrons and holes) differ in many respects from the properties of electrons in free space. And their mass (effective mass) can be very different from the mass of a free electron and depend on the direction of motion [§ 3.5].
Gann effect	See [§ 5.6].
Zinner effect	See [§ 5.6].
Seebeck effect	See [§ 10.1.1].
Peltier effect	See [§ 10.1.2].
Thomson effect	See [§ 10.1.3].
hall effect	The phenomenon of the appearance in a semiconductor with a current flowing through it of a transverse electric field under the influence of a magnetic field is called the Hall effect. [§ 6.1.1].
stark effect	See [§ 5.6].

>>Physics: Electron beams. Cathode-ray tube

If a hole is made in the anode of a vacuum tube, then part of the electrons accelerated by the electric field will fly into this hole, forming an electron beam behind the anode. The number of electrons in the beam can be controlled by placing an additional electrode between the cathode and anode and changing its potential.
Properties of electron beams and their application. The electron beam, falling on the body, causes them to heat up. In modern technology, this property is used for electronic vacuum melting of ultrapure metals.
When decelerating fast electrons falling on a substance, a x-rays. This phenomenon is used in x-ray tubes.
Some substances (glass, zinc and cadmium sulfides), bombarded by electrons, glow. At present, among the materials of this type (phosphors), those are used in which up to 25% of the energy of the electron beam is converted into light energy.
Electron beams are deflected by an electric field. For example, passing between the plates of a capacitor, the electrons deviate from the negatively charged plate to the positively charged ( fig.16.20).
The electron beam is also deflected in a magnetic field. Flying over the north pole of the magnet, the electrons deviate to the left, and flying over the south, deviate to the right ( fig.16.21). The deviation of electron streams coming from the Sun in the Earth's magnetic field leads to the fact that the glow of the gases of the upper layers of the atmosphere (aurora borealis) is observed only at the poles.

The ability to control the electron beam using an electric or magnetic field and the glow of a phosphor-coated screen under the action of the beam is used in a cathode ray tube.
A cathode ray tube is the main element of one of the types of televisions and an oscilloscope - a device for studying fast-changing processes in electrical circuits ( fig.16.22).

The device of the cathode ray tube is shown in Figure 16.23. This tube is a vacuum cylinder, one of the walls of which serves as a screen. A source of fast electrons is placed at the narrow end of the tube - electron gun (fig.16.24). It consists of a cathode, a control electrode and an anode (more often several anodes are located one after another). Electrons are emitted by a heated oxide layer from the end face of a cylindrical cathode FROM surrounded by a heat shield H. Then they pass through the hole in the cylindrical control electrode AT(it regulates the number of electrons in the beam). Each anode ( A 1 and A 2) consists of discs with small holes. These disks are inserted into metal cylinders. A potential difference of hundreds and even thousands of volts is created between the first anode and the cathode. A strong electric field accelerates the electrons, and they acquire a greater speed. The shape, location, and potentials of the anodes are chosen so that, along with the acceleration of electrons, the focusing of the electron beam is also carried out, i.e., the beam cross-sectional area on the screen is reduced to almost point sizes.

On its way to the screen, the beam sequentially passes between two pairs of control plates, similar to the plates of a flat capacitor (see Fig. 16.23). If there is no electric field between the plates, then the beam is not deflected and the luminous dot is located in the center of the screen. When the potential difference is imparted to the vertically located plates, the beam is shifted in the horizontal direction, and when the potential difference is imparted to the horizontal plates, it is shifted in the vertical direction.
The simultaneous use of two pairs of plates allows you to move the luminous dot on the screen in any direction. Since the mass of electrons is very small, they react almost instantly, that is, in a very short time, to a change in the potential difference of the control plates.
In a cathode ray tube used in a television (the so-called kinescope), the beam created by the electron gun is controlled using a magnetic field. This field is created by coils put on the neck of the tube ( fig.16.25).

A color kinescope contains three spaced apart electron guns and a screen of a mosaic structure, composed of three types of phosphors (red, blue and green). Each electron beam excites phosphors of the same type, the glow of which together creates a color image on the screen.
Cathode ray tubes are widely used in displays- devices connected to electronic computers (computers). The display screen, similar to the TV screen, receives information recorded and processed by the computer. You can directly see the text in any language, graphics of various processes, images of real objects, as well as imaginary objects that obey the laws written in the computer program.
Narrow electron beams controlled by electric and magnetic fields are formed in cathode ray tubes. These beams are used in oscilloscopes, TV kinescopes, and computer displays.

???
1. How is electron beam control carried out?
2. How does a cathode ray tube work?

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10

Lesson content lesson summary support frame lesson presentation accelerative methods interactive technologies Practice tasks and exercises self-examination workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures graphics, tables, schemes humor, anecdotes, jokes, comics parables, sayings, crossword puzzles, quotes Add-ons abstracts articles chips for inquisitive cheat sheets textbooks basic and additional glossary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in the textbook elements of innovation in the lesson replacing obsolete knowledge with new ones Only for teachers perfect lessons calendar plan for the year methodological recommendations of the discussion program Integrated Lessons

If you have corrections or suggestions for this lesson,

Page 1

Beams of electrons moving at high speeds can be used to produce X-rays, melt and cut metals. The ability of electron beams to be deflected by electric and magnetic fields and cause crystals to glow is used in cathode ray tubes.

Electron beams are obtained using an electron gun - a vacuum device, usually a diode, in which electrons fly out of the cathode due to Ch. Beam focusing is carried out by electronic lenses that create the necessary electric power.

Beta rays are beams of electrons. The zero index reflects the fact that the electron mass is negligibly small compared to the nucleon mass. Index - 1 indicates that the particle under consideration has a negative sign, equal in magnitude, but opposite in sign, to the proton charge.

UV irradiation or an electron beam (initiating agent) initiates a fast molecular-radical p-tion, releasing the energy stored in the mixture in the form of a short pulse of coherent radiation.

Therefore, electric fields with a continuous change in potential are used to influence electron beams.

It should be noted that electron beams strongly interact with matter. The maximum allowable sample thickness is only a few microns. This circumstance largely limits the possibilities of the method for studying liquid disperse systems. Usually fine-crystalline samples deposited on specially treated substrates are studied.

Therefore, it turns out to be possible to inform the beam of electrons flying along o: n sg. The beam of electrons, interacting with this field, can give the line part of its energy and thereby amplify the waves traveling in the line, or excite such waves.

In an ordinary, unpolarized beam of electrons or positrons, the spins of particles are directed randomly. Thus, after some time (relaxation time) an ordinary beam of electrons or positrons becomes polarized - the spins of the particles take on an ordered orientation.

Such waves can be excited by longitudinal beams of electrons or ions. As for the waves propagating in the direction of the electron drift (a 0), only the presence of a density gradient turns out to be sufficient for their growth in time.

Polymer chains are crosslinked directly by high energy electron beams. These electrons generate PE macroradicals by extracting hydrogen radicals. Typically, this method is used for the manufacture of 1-1 kV cables with XLPE insulation.

Electrostatic cathode electron lens. / - cathode. 2 - focusing electrode. 3-anode. Thin lines are equipotentials. O is one of the points of the cathode. Shaded space-section of the region occupied by the flow of electrons emitted by point O.| Electrostatic cylindrical electronic lenses. a diaphragm with a slit. b-immersion lens consisting of two plates. In the region where charged particles pass, the lens field does not change in the direction parallel to the diaphragm slits or the gaps between the plates of adjacent electrodes.| Cross-section of electrodes of electrostatic cylindrical lenses by a plane passing through the z-axis perpendicular to the mean plane. a-cylindrical (slit diaphragm. b-immersion cylindrical lens. - single cylindrical lens. d-cathode cylindrical lens. K, and K2 - potentials of the corresponding electrodes. | 2-field lines 3-magnetic pole 4-excitation winding Doublet of two quadrupole electrostatic lenses

The main means of microwave vacuum electronics, which serves to convert the energy of a DC source into the energy of an electromagnetic field of microwave oscillations, are electron beams - extended electron flows limited in cross section.

Electron beams are created using special electro-optical devices - the so-called electron guns, which emit accelerated electrons, the trajectories of which are approximately parallel to the axis of the gun.

Let us consider such basic characteristics of electron beams as the power, perveance, and intensity of the electron beam, as well as the relationship between the configuration of the electron beam and the SV of the device.

Beam power (the product of the current carried by it I for voltage U, by which the electrons were accelerated) determines the power of the microwave device: P=U I.

An important characteristic of the electron flow is the perveance, defined as  . Perveance is a measure of the intensity of a flow. In microwave devices, as a rule, intense electron flows are used, in which the force of mutual repulsion of electrons significantly affects the movement of electrons, so that their action cannot be neglected. Intensive, as calculations show, should be considered flows in which the perveance takes values ​​greater than 10 -8 -10 -7 A/B 3/2 . Due to the smallness of the numerical value of the perveance, a more convenient value is often used - the microperveance  m , defined by the equality

 =  m  10 -6 . (1.34)

The power of the electron flow through perveance can be expressed by the formula

P=U I=U 5/2 .

As can be seen from the formula, at a constant perveance, the power grows very quickly as U(so, with an increase in voltage by an order of magnitude, the power increases by more than 300 times).

However, in all devices, it is more profitable to increase the power not so much by increasing the voltage, but by increasing the beam current, since the higher the operating voltage, the more complex the design of the insulators in the device and the more complex the power sources and, as a result, the bulkiness and complexity of high-voltage equipment. Reducing the operating voltage at a given beam power not only reduces the complexity of the equipment, but also leads to a decrease in the dimensions of the device due to a reduction in the length of the active sections of the electrodynamic system (EMF). In a TWT, as the perveance increases, the gain and efficiency can increase.

In order for the formed beam to be successfully used in microwave electronic devices, it is necessary, while maintaining a good shape, to pass it through the entire space of interaction with high-frequency fields. Since significant Coulomb forces of mutual repulsion of charges act in high-current electron beams, leading to "swelling" of the beams, this problem often turns out to be no less complicated than the formation of the beam itself.

To combat the "swelling" of the beams, a constant magnetic field parallel to the beam axis is most often used. Due to the relatively large length of the devices, a sufficiently strong magnetic field must be created over a large area. Therefore, the mass of the magnetic focusing system (MFS) is very large. Lower power and mass costs of magnetic systems are realized by using periodic magnetic focusing, in which the electron beam is passed along an alternating magnetic field. Such a system is assembled from separate short magnetic rings separated by bushings made of a material with high magnetic permeability. A similar result is achieved with the help of periodic electrostatic focusing, which is carried out by a number of periodically located electrostatic lenses. Such a system has even less weight and power consumption.

In addition to magnetic confinement, there is another way to combat the "swelling" of beams, which consists in introducing a certain amount of positively charged ions into the volume of the electron beam, which, with their space charge, compensate for the negative space charge of electrons. In the simplest case, ions can be created by leaving a certain amount of “unpumped” gas in the volume of the device. The beam electrons on their way will ionize the molecules of this gas. The secondary electrons formed as a result of ionization are ejected outside the beam by Coulomb forces, while positive ions will be retained by these forces in its volume. As a result, even at very low pressures of the residual gas, such a quantity of positive ions can be formed that their concentration is comparable to the electron concentration in the beam. At this point, the accumulation of ions will stop and a stationary state will be established, in which a quasi-neutral medium resembling plasma is formed in the beam volume. The space charge of the electrons turns out to be compensated, and the beam does not "swell". The described phenomenon, called ion focusing, is observed at residual gas pressures exceeding 10 -6 mm Hg. Art.

Depending on the shape of the cross section, electron beams are divided into three main types: ribbon, axially symmetric and tubular.

The system of formation of an electronic flow is a set of electric and magnetic fields, as well as the electrodes and magnetic circuits that form them, necessary to create electronic flows of the desired configuration. It contains four areas:

1) the region of the electron gun, in which there is a source of electrons - a cathode and an anode, between which an accelerating voltage is applied U 0 ;

2) transition region - the region between the gun and the region of the regular part of the MFS, in which the strength of the electrostatic field created by the electrodes decreases sharply, the action of the space charge forces continues, which at the end of the region become the main defocusing force tending to expand the flow, the focusing forces of the magnetic fields directed to the beam axis; in the transition region, the formation of the electron flow ends and the parameters of the flow created by the gun are “matched” with the parameters of the regular part of the formation system;

3) the area of ​​the regular part of the formation system, in which the EMF of the device is located and the flow interacts with the microwave field;

4) the area of ​​the collector, in which the electrons of the "waste" flow, perceived by a special metal surface, complete their movement in the system; the greater the efficiency of the device, the less power dissipated in the collector; the shape of the collector surface is chosen in such a way that the thermal loads on this surface do not exceed the allowable specific value.

ELECTRON BEAM- a flow of electrons moving along close trajectories in one direction, having dimensions that are much larger in the direction of movement than in the transverse plane. Since E. p. is a set of charges of the same name. particles, inside it there is space charge electrons, creating own. electric field. On the other hand, electrons moving along close trajectories can be considered as linear currents that create their own. magn. field. Electrical field of spaces. creates a force tending to expand the beam ("Coulomb repulsion"), magn. the field of linear currents creates a Lorentz force that tends to compress the beam. The calculation shows that the action of spaces. charge begins to noticeably affect (at electron energies in several keV) at currents in several. tenths of mA, while the "pulling" action of own. magn. The field noticeably manifests itself only at electron velocities close to the speed of light - the energy of electrons is of the order of MeV. Therefore, when considering E. p., used in decomp. electronic devices, tech. installations, first of all, it is necessary to take into account the effect of own. spaces. charge, and the action of own. magn. fields should be taken into account only for relativistic beams.

E. p intensity. Main The criterion for the conditional division of E. p. into non-intensive and intense is the need to take into account the action of the field of own. spaces. beam electron charge. Obviously, the greater the beam current, the more spaces. charge, stronger repulsion. On the other hand, the greater the speed of electrons, the less will affect the nature of the motion of electrons own. electric beam field - the higher the electron energy, the "harder" the beam. Quantitative action of the field of spaces. charge is characterized by the coefficient. space charge - perveance, defined as

where I-beam current; U- accelerating voltage, which determines the energy beam electrons.

Noticeable influence of spaces. charge on the motion of electrons in the beam begins to appear at P>=P*=\u003d 10 -8 A / V 3/2 \u003d 10 -2 μA / V 3/2. Therefore, it is customary to refer to intense beams E. p. Р>P*.

Non-intense beams (with R<Р* ) of small cross section, often called electron beams, calculated according to the laws of geom. electron optics without taking into account the action of the field proper. spaces. charge, are formed using electronic searchlights and are used mainly in decomp. cathode ray devices.

In intense beams, the action of own. spaces. charge significantly affects the characteristics of E. p. First, intense E. p. in a space free from external. electric and magn. fields, due to the Coulomb repulsion expands indefinitely; secondly, at the expense of denial. electric charge of the beam electrons, the potential drops in the beam. If using ext. electric or magn. fields limit the expansion of an intense beam, then at a sufficiently large current, the potential inside the beam can drop to zero, the beam will "break off". Therefore, for intense beams there is a concept of limiting (maximum) perveance. Practically, when the beam expansion is limited, ext. fields, it is possible to form extended stable intense beams with P 5 . 10 µA/V 3/2 .

Complete math. the description of intense electromagnetic fields is difficult, since a real electron flow consists of many moving electrons, and it is practically impossible to take into account the interaction between them. With the introduction of certain simplifying assumptions, in particular, replacing the sum of forces acting on a selected electron from neighboring electrons by the force of action on this electron of a certain electrically charged medium with a continuously distributed density of spaces. charge and breaking the entire beam into a set of "current tubes", it is possible to calculate with the help of a computer with sufficient for practical. goals accuracy osn. intensive beam parameters: beam shape (envelope), distribution of current density and potential over the beam cross section.

Geometry E. p. In practice, beams of three configurations are used: tape (flat), having a rectangular shape in cross section with a “thickness”, a much smaller “width”, axisymmetric, having a circle shape in cross section, and tubular, having a ring shape in cross section. For the formation of E. items of these types, appropriate electron guns and restraint systems.

Influence of spaces. charge is not the same in beams decomp. configuration. Naib. influence on the nature of the movement of electrons at the boundary E. p. has a component of the electric strength. fields created spaces. charged, directed perpendicular to the axis of the axisymmetric beams and to the wide side of the ribbon beams.

The radial component of the electric strength. field at the boundary of an axisymmetric beam is directly proportional to the beam current and inversely proportional to the radius of its cross section and the velocity of beam electrons. This creates an off-axis force that tends to expand the beam. The repulsive force is the greater, the greater the current, the smaller the speed and the radius of the beam. Theoretically, in axisymmetric beams, the trajectories of electrons cannot cross the axis, and the beam cross section cannot be reduced to a point, because as the cross section decreases, the repulsive force increases indefinitely.

Envelopes of axisymmetric electron beams: g 0 is the angle of entry of the beam into the field-freeranstvo; r 0 - initial radius; 1 - divergent beam (g 0 >0); 2-cylindrical beam (g 0 =0); 3, 4, 5-converging sheaves (g 0<0). Пучок 4 - опти small, since the crossover (the smallest cross section) the beam is at the furthest distance (z/ l=0.5) from the original plane.

Envelope of an intense axisymmetric beam in a space free from electric. and magn. fields, is described by a dependence close to exponential. On fig. the envelopes of axisymmetric beams are shown, which, before entering the free space, are cylindrical (curve 2, g 0 = 0), divergent (curve 1, g 0 > 0) and converging (curves 3-4, g 0<0) формы (g 0 - угол наклона касательной к огибающей пучка, угол входа). Как видно на рис., пучки, первоначально сформированные как цилиндрические (g 0 = 0) и расходящиеся (g 0 >0), in a field-free space expand indefinitely; beams formed as converging are initially compressed ( r/r 0 <1), проходят плоскость наименьшего сечения (плоскость кроссовера), затем также начинают расширяться. Радиус мин. сечения пучка - радиус кроссовера-определяется выражением

where r 0 - radius of E. p. to the entrance to the free space.

The smaller the crossover radius, the smaller the perveance and the larger | g 0 |. With an increase (in absolute value) of the beam entry angle into the field-free space (g 0), the crossover plane first moves away from the original plane, beyond

then it starts to approach it (successively curves 3, 4, 5). For each perveance value, there is an optimal "angle of entry" g 0 , for which the crossover is max. remote from the original plane, that is, an EP with a given perveance can be drawn to the greatest distance with a radius not exceeding the original one.

Tape intense beams in a free from electric. and magn. fields in space also expand indefinitely (become "thicker"), the contour of the envelope of the beam is described parabolic. by law. In contrast to an axisymmetric beam, a ribbon beam can theoretically be brought into a line at an optimal entrance angle, i.e., a linear focus can be obtained. Bundles of other configurations in free space also expand indefinitely; tubular E. p. expands somewhat less than a solid axisymmetric.

Experiment. verification of the obtained calculated relations is difficult, since the very concept of the boundary (envelope) of an intense beam is arbitrary, since in real beams the current density at a distance from the axis of the axisymmetric or from the cf. the plane of the ribbon beams decreases gradually, and the boundary of the beam is conventionally considered to be a circle or a straight line, along which the current density is a certain small fraction (~0.1) of its max. axis values.

E. p potential. The potential drop inside an intense beam limits the possibility of forming an extended intense beam with high perveance. Theo-retich. studies show that in an intense unlimited flow that fills the space between two flat parallel conducting surfaces with the same potential, which determines the energy of the flow electrons, with an increase in current cf. plane, a potential minimum is formed. Upon reaching P= 18.64 μA / V 3/2 potential drops to zero, a virtual cathode, part of the electrons passes through the minimum plane, part is reflected to the original plane, the normal current flow is violated. Experiment. verification confirms this, precisely when approaching P to 18.64 µA/V 3/2, instabilities arise in the flow, electronic layers, the passage of current is disturbed.

In real E. p., limited ext. electric and magn. fields, a drop in potential also occurs, but since in most devices that use intense electron beams, an extended beam is passed through a tube with a positive beam. potential, it is possible to maintain a potential close to the potential of the tube on the beam surface. But even in the presence of a conductive tube, the potential on the axis of the axisymmetric or cf. plane of the strip beams noticeably decreases, and upon reaching a sufficiently large perveance (greater than in the case of an unbounded flow), instability arises, the beam breaks.

Formation of E. p. Since E. p. in free space expands indefinitely, with practical. the use of intense beams, in addition to the system that forms the beam, the electron gun, requires a system that limits the beam divergence. The expansion of E. p. is limited by external. electric and magn. fields. Classic an example of an extended intensive E. p. from about to to B r and l l l yuen and - tsilindrich. beam bounded by a longitudinal homogeneous magnet. field. When defining the ratio of four quantities - beg. radius r 0 , beam current I, U 0 , which determines the energy of electrons before entering the magnet. field, and magn. induction longitudinal homogeneous magn. fields B 0 - it is theoretically possible to obtain a stable cylindrical. E. p. At the optimal ratio r 0 , I, U 0 and B 0 max. the perveance of the Brillouin flux reaches 25.4 µA/V 3/2 . At max. the perveance potential on the beam axis is only 1/3 of the value at the boundary. With limited magnetic using the field of tubular beams, even higher values ​​of perveance can be obtained.

In practice, it is not possible to form extended E. p. with a perveance close to the theoretically maximum possible due to a number of reasons: the spread of the beginning. velocities of electrons emitted by the cathode, the difficulty of creating limiting fields of a strictly specified configuration, practical. inability to strictly fulfill the beginning. the conditions for introducing the beam into the restriction system, etc. Real EMs have wavy and pulsating boundaries, and the shape of the beam does not remain unchanged. Therefore, to prevent the beam electrons from settling on the surface of the passage channel, the radius of the conducting tube through which an intense beam is passed is chosen to be 20-30% larger than the beam radius.

Lit.: Alyamovsky I. V., Electron beams and electron guns, M., 1966; Molokovskii S. I., Sushkov A. D., Intensive electron and ion beams, 2nd ed., M., 1991.

A. A. Zhigarev.