Lesson summary "Electron beams. Cathode ray tube"

ELECTRON BEAM

- a flow of electrons moving along close trajectories in one direction, having dimensions significantly larger in the direction of movement than in the transverse plane. Since E. p. is a collection of charges of the same name. particles, inside it there is space charge electrons, creating their own. electric . On the other hand, electrons moving along similar trajectories can be considered as linear currents that create their own. mag. field. Electric field of spaces. charge creates a force tending to expand the beam (“Coulomb repulsion”), mag. the field of linear currents creates a Lorentz force, tending to compress the beam. The calculation shows that spaces. charge begins to have a noticeable effect (at electron energies of several keV) at currents of several. tenths of mA, while the “contracting” action of its own. mag. field is noticeably manifested only at electron velocities close to the speed of light - electron energy of the order of MeV. Therefore, when considering E. items used in dep. electronic devices, technical installations, first of all it is necessary to take into account the effect of its own. spaces. charge, and the action of its own. mag. fields are taken into account only for relativistic beams.

Intensity of E. p. Basic The criterion for the conditional division of electrical energy into non-intensive and intensive ones is the need to take into account the action of the field of its own. spaces. charge of the beam electrons. Obviously, the larger the beam, the more spaces. charge, stronger repulsion. On the other hand, the more electrons, the less will affect the nature of the movement of the electrons' own. electric beam field - the higher the electrons, the “harder” the beam. Quantitative action of the field of spaces. charge is characterized by a coefficient. space charge - perv ean s o m, defined as

Where I-beam current; U- accelerating, which determines the energy of the beam electrons.

Noticeable influence of spaces. charge on electrons in the beam begins to appear when P>=P* == 10 -8 A/V 3/2 = 10 -2 µA/V 3/2. Therefore, it is customary to refer to intense beams as electron beams with P>P*.

Low-intensity beams (with R<Р* ) small cross-section, often called electron beams, calculated according to the laws of geom. electronic optics without taking into account the action of the intrinsic field. spaces. charge, are formed using electronic spotlights and are used mainly in various. electron beam devices.

In intense beams the action of intrinsic spaces. charge significantly affects the characteristics of electrical energy. Firstly, intense electrical energy in a space free from external influences. electric and mag. fields, due to Coulomb repulsion it expands indefinitely; secondly, due to denial. electric As the electron charge in the beam increases, the potential in the beam drops. If using external electric or mag. fields to limit the expansion of an intense beam, then with a sufficiently large current inside the beam it can drop to zero, the beam will “break off”. Therefore, for intensive beams there is a concept of limiting (maximum) perveance. Practically, when limiting beam expansion, ext. fields, it is possible to form extended stable intense beams with P 5 . 10 µA/V 3/2.

Complete math. It is difficult to describe intense electron beams, since a real electron flow consists of many moving electrons, and it is almost impossible to take them into account. By introducing certain simplifying assumptions, in particular, replacing the sum of forces acting on the chosen one from neighboring electrons with 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 sufficiently practical one. goals accuracy main. parameters of the intense beam: beam shape (envelope), current density and potential across the beam cross section.

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

The influence of spaces. charge is not the same in different beams. configurations. Naib. The nature of the movement of electrons at the boundary of the electron field is influenced by the component of the electrical intensity. fields created by spaces. charge, directed perpendicular to the axis of the axisymmetric beams and the wide side of the tape beams.

Radial component of electric voltage. 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 speed of the beam electrons. This creates a force directed away from the axis, tending to expand the beam. The larger the current, the smaller the speed and radius of the beam, the greater the repulsion. Theoretically, in axisymmetric beams, electron trajectories cannot cross the axis, and the beam cannot be reduced to a point, since as the cross section decreases, the repulsive force increases indefinitely.

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

Envelope of an intense axisymmetric beam in a space free from electricity. and mag. fields, is described by a dependence close to exponential. In Fig. the envelopes of axisymmetric beams are shown, having before entering the free cylindrical (curve 2, g 0 = 0), divergent (curve 1, g 0 >0) and convergent (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), expand indefinitely in field-free space; bundles formed as converging ones 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 the EP before entering the free one.

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

thus begins to approach it (sequentially curves 3, 4, 5). For each value of the perveance, there is an optimal “angle of approach” g 0, at which the crossover is at its maximum. is removed from the original plane, that is, an electron beam with a given perveance can be drawn to the greatest distance with a radius not exceeding the original one.

Intensive tape beams in a free-from-electricity environment. and mag. The fields in space also expand indefinitely (become “thicker”), and the contour of the beam envelope is described by a parabolic. by law. Unlike an axisymmetric beam, a ribbon beam at an optimal entrance angle can theoretically be reduced to a line, i.e., a linear beam can be obtained. Bundles of other configurations in free space also expand without limit; Tubular EP expands somewhat less than solid axisymmetric one.

Let's experiment. verification of the obtained calculated relationships is difficult, since the very concept of the boundary (envelope) of an intense beam is conditional, since in real beams the current density when moving away from the axisymmetric axis or from the sr. 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 maximum. values on the axis.

Potential of E. p. The potential drop inside the intense beam limits the possibility of forming an extended intense beam with high perveance. Theoretical Research shows 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 increasing current in avg. plane, a minimum potential is formed. Upon reaching P= 18.64 µA/V 3/2 potential drops to zero, a virtual, Some electrons pass through the minimum plane, some are reflected to the original plane, and current flow is disrupted. Let's experiment. the check confirms this, precisely when approaching P to 18.64 μA/V 3/2, instabilities appear in the flow of the electronic layers, and the passage of current is disrupted.

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

Formation of E. p. Since the electronic space in free space expands without limit, for practical purposes. When using intense beams, in addition to the system that forms the beam—an electron gun—a system is required that limits the beam divergence. The expansion of E. p. is limited with the help of external. electric and mag. fields. Classic an example of an extended intensive e.p.-t.n. FLOW OF BRILL LUEN - cylindrical. a beam limited by a longitudinal homogeneous magnetic field. field. When defined the ratio of four quantities - beginning. radius r 0 , beam current I, voltage U 0 , determining the energy of electrons before entering the magnet. field, and magnetic induction of longitudinal homogeneous magnetic field. 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 at the beam axis is only 1/3 of the value at the boundary. With limited magnetic With the field of tubular beams, even larger perveance values can be obtained.

Feedback circuits for the cases of TWT with external feedback (a and TWT with internal feedback (b.

The electron beam must transfer a certain minimum energy to the field, above the level of the system’s own losses. Hence, in any specific system there arises the need to provide a certain, as they say, starting value of the electronic current.

Schematic representation of a multi-beam electron gun with a cylindrical electrode system..| Schematic representation of a multi-beam electron gun with a small emitting cathode area.

After passing the focusing point, the electron beam diverges at a large angle. An electron lens with a large aperture deflects electron beams so that they fall perpendicularly onto the plane of the raster lens. Each microlens in the raster lens generates its own electron beam. If we assume that the current density in the main electron beam is distributed according to the Gaussian law, then.

The electron beam, discharging all the elementary capacitances in turn, creates current pulses in the signal plate circuit - a video signal.

An electron beam, consisting along its length of separate groups of electrons - electron bunches, can be considered as a current containing higher harmonic components. Such an electron beam is called bunched or modulated.

The electron beam is characterized by the geometric shape of the cross section. In the vast majority of cases, the beams have a circular cross-section and are called cylindrical. To significantly increase the beam current, tubular beams with a ring-shaped cross-section, as well as ribbon beams with a rectangular cross-section, can be used.

The electron beam is used for welding metals, welding metal with ceramics, etc. A distinctive feature of the weld when welding two metals is the large depth of the seam with a small width (the so-called dagger weld) and high uniformity of the seam. The required beam diameters are varied and range from 0 01 to 5 - 10 mm. Since a sharply defined beam diameter is usually not needed, spectral width tolerances are less stringent than for beam processing processes.

The electron beam is focused by the positive space charge of a straight ion beam with a circular cross-section.

An electron beam accelerated from the anode to the cathode will not propagate into the region behind the anode if its current is greater than the limiting one; accumulation of spaces, electron charge behind the anode, blocking the beam (virtual cathode), creates a potential. The depth of the hole reaches values greater than 1 MB. Ions can be created due to the ionization of residual gas atoms by electrons or introduced by specially formed gas jets. When ions are formed, the electron charge is partially neutralized, the blocking effect of the accumulated electron charge is weakened, and the electron beam propagates further beyond the anode.

ELECTRON BEAM- a flow of electrons moving along close trajectories in one direction, having dimensions significantly larger in the direction of movement than in the transverse plane. Since E. p. is a collection of charges of the same name. particles, inside it there is space charge electrons, creating their own. electric field. On the other hand, electrons moving along similar trajectories can be considered as linear currents that create their own. mag. field. Electric field of spaces. creates a force tending to expand the beam (“Coulomb repulsion”), mag. the field of linear currents creates a Lorentz force, tending to compress the beam. The calculation shows that the action of spaces. charge begins to have a noticeable effect (at electron energies of several keV) at currents of several. tenths of mA, while the “contracting” action of its own. mag. field is noticeably manifested only at electron velocities close to the speed of light - electron energy of the order of MeV. Therefore, when considering E. items used in dep. electronic devices, technical installations, first of all it is necessary to take into account the effect of its own. spaces. charge, and the action of its own. mag. fields are taken into account only for relativistic beams.

E. p intensity. Basic The criterion for the conditional division of electrical energy into non-intensive and intensive ones is the need to take into account the action of the field of its own. spaces. charge of the beam electrons. Obviously, the greater the beam current, the more spaces there are. charge, stronger repulsion. On the other hand, the higher the speed of the electrons, the less it will affect the nature of the movement of the electrons. electric beam field - the higher the electron energy, the “harder” the beam. Quantitative action of the field of spaces. charge is characterized by a coefficient. space charge - perv ean s o m, defined as

Where I-beam current; U-accelerating voltage that determines energy electron beam.

Noticeable influence of spaces. charge on the movement of electrons in the beam begins to appear when P>=P* == 10 -8 A/V 3/2 = 10 -2 µA/V 3/2. Therefore, it is customary to refer to intense beams as electron beams with P>P*.

In intense beams the action of intrinsic spaces. charge significantly affects the characteristics of electrical energy. Firstly, intense electrical energy in a space free from external influences. electric and mag. fields, due to Coulomb repulsion it expands indefinitely; secondly, due to denial. electric As the electron charge in the beam increases, the potential in the beam drops. If using external electric or mag. fields to limit the expansion of an intense beam, then with a sufficiently large current, the potential inside the beam can drop to zero, and the beam will “break off”. Therefore, for intensive beams there is a concept of limiting (maximum) perveance. Practically, when limiting beam expansion, ext. fields, it is possible to form extended stable intense beams with P 5 . 10 µA/V 3/2.

Complete math. Description of intense electron beams is difficult, since a real electron flow consists of many moving electrons, and it is almost impossible to take into account the interaction between them. By introducing certain simplifying assumptions, in particular, replacing the sum of forces acting on a selected electron from neighboring electrons with the force of action on this electron by a certain electrically charged medium with a continuously distributed spatial density. 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 purposes. goals accuracy main. parameters of an intense beam: 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 the shape of a rectangle in cross section with a “thickness” much smaller than the “width”, axisymmetric, having the shape of a circle in cross section, and tubular, having the shape of a ring in cross section. For the formation of electrical energy of such types, appropriate electron guns and restriction systems.

Radial component of electric voltage. 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 speed of the beam electrons. This creates a force directed away from the axis, tending to expand the beam. The larger the current, the smaller the speed and radius of the beam, the greater the pushing force. Theoretically, in axisymmetric beams, electron trajectories cannot cross the axis, and the beam cross-section cannot be reduced to a point, since as the cross-section decreases, the repulsive force increases indefinitely.

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

Envelope of an intense axisymmetric beam in a space free from electricity. and mag. fields, is described by a dependence close to exponential. In Fig. shows the envelopes of axisymmetric beams that have a cylindrical (curve 2, g 0 = 0), divergent (curve 1, g 0 >0) and convergent (curves 3-4, g 0) before entering free space<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), expand indefinitely in field-free space; bundles formed as converging ones 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 is the radius of the EP before entering the free space.

Intensive tape beams in a free-from-electricity environment. and mag. The fields in space also expand indefinitely (become “thicker”), and the contour of the beam envelope is described by a parabolic. by law. Unlike an axisymmetric beam, a ribbon beam at an optimal entrance angle can theoretically be brought into a line, i.e., a linear focus can be obtained. Bundles of other configurations in free space also expand without limit; Tubular EP expands somewhat less than solid axisymmetric one.

Potential E. p. The potential drop inside the intense beam limits the possibility of forming an extended intense beam with high perveance. Theoretical Research shows 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 increasing current in avg. plane, a minimum potential is formed. Upon reaching P= 18.64 µA/V 3/2 potential drops to zero, a virtual cathode ,Some of the electrons pass through the minimum plane, some are reflected to the original plane, and the normal current flow is disrupted. Let's experiment. the check confirms this, precisely when approaching P to 18.64 μA/V 3/2, instabilities appear in the flow of the electronic layers, and the passage of current is disrupted.

Formation of E. p. Since the electronic space in free space expands without limit, for practical purposes. When using intense beams, in addition to the system that forms the beam—an electron gun—a system is required that limits the beam divergence. The expansion of E. p. is limited with the help of external. electric and mag. fields. Classic an example of an extended intensive e.p.-t.n. FLOW OF BRILL LUEN - cylindrical. a beam limited by a longitudinal homogeneous magnetic field. field. When defined the ratio of four quantities - beginning. radius r 0 , beam current I, U 0, which determines the energy of electrons before entering the magnet. field, and magnetic induction of longitudinal homogeneous magnetic field. 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 at the beam axis is only 1/3 of the value at the boundary. With limited magnetic With the field of tubular beams, even larger perveance values can be obtained.

In practice, it is not possible to form extended EPs with a perveance close to the theoretically maximum possible due to a number of reasons: the scatter of the beginning. speeds of electrons emitted by the cathode, difficulties in creating limiting fields of a strictly specified configuration, practical. the inability to strictly fulfill the beginning. conditions for introducing the beam into the limiting system, etc. Real electron beams 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 flight channel, the radius of the conductive tube through which an intense beam is passed is selected to be 20-30% larger than the beam radius.

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

A. A. Zhigarev.

Electron beams represent a stream of rapidly flying electrons. Electron beams are formed in an electron tube and various gas-discharge devices.

Electron beams have the following properties:

cause the glow of some solids and liquids (glass, zinc and cadmium sulfides). Currently, phosphors are used in which up to 25% of the energy of the electron beam is converted into light:
When fast electron beams are decelerated, X-rays appear in matter. This is used in X-ray tubes;
electron beams are deflected in electric fields, for example, in the field of a flat capacitor, the electron beam is shifted towards a positively charged plate;
electron beams are deflected in magnetic fields due to the Lorentz force acting on electrons. Flying over the north pole of a magnet, electrons are deflected in one direction, and flying over the south pole - in the opposite direction. The deviation of electron flows coming from the Sun in the Earth's magnetic field leads to the fact that they bend around the Earth's surface and only in the polar regions a small part of these particles invades the upper layers of the atmosphere and causes the glow of atmospheric gases at the poles (northern lights);
When electron beams hit a substance, they heat it and have a mechanical effect. The heat produced by an electron beam hitting a body is used to melt ultrapure metals in a vacuum;
When an electron beam hits a photographic film, it causes it to darken.

Due to 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, it is used in a cathode ray tube.

Literature

Aksenovich L. A. Physics in secondary school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyakhavanne, 2004. - P. 298.