Toward the Thirtieth Anniversary of Soviet Physics

Soviet Electronics over 30 Years

S. Yu. Lukyanov

The inseparable connection that exists among the various parts of modern physics greatly complicates a precise definition of the boundaries and content of any one of its branches. By electronics one should, naturally, understand that field of science in which the properties, processes of formation, motion, and absorption of free charged particles (above all electrons) are studied. If, however, we do not wish to regard gas discharge, the physics of metals, semiconductors, and dielectrics as parts of electronics, then we must restrict ourselves to the study of the motion of charged particles only in vacuum or in very rarefied gases, and not in gases under higher pressure, liquids, or solids. Further, in speaking of electronics it is expedient to leave aside those phenomena which are observed, for example, in cosmic rays when charged particles move with very great velocities, close to the speed of light, since these questions belong to the field of nuclear physics.

In accordance with what has been said, in setting forth the achievements of Soviet physicists in the field of electronics, we shall consider work on the following problems.

1) Electron and ion emission (photoelectric effect, thermionic and autoelectronic emission, secondary emission, surface ionization, and other types of ion emission).
2) Electron and ion optics.
3) Investigation of the basic properties of electrons and ions.

Alongside purely physical work, the article will, of course, also briefly characterize certain interesting and important works relating to technical electronics.

1. Introduction

Electronic physics belonged among the leading branches of natural science during the first two decades of our century, when the construction of classical atomic physics was being completed, when the electronic theory had been created and was experiencing a period of flourishing.

metals and took the first steps in Bohr’s quantum theory. The most important experiments on the determination of the fundamental electronic constants, experiments that demonstrated the real existence of free electrons appearing in the processes of thermionic and photoelectric emission, the establishment of the quantum equation for the photoelectric effect, the proof of the existence of discrete levels in the atom*), and, finally, mass-spectrographic proof of the presence of isotopes in stable elements—all these achievements of electronics were that necessary foundation without which the formation of our ideas about the structure of matter, and consequently the successful development of all physics in those years, would have been impossible.

Later the situation changed. As the structure of the outer shell of the atom became clear, and physicists moved from questions of the structure of the atom as a whole to the study of its constituent parts, the role of electronics diminished, and now, as is well known, the problems of the atomic nucleus are at the center of attention in modern physics. Yet even in later periods of the development of physics, individual questions of electronics occupied a prominent place in the general progress of science. Thus, electron physics played an essential role in that radical breaking down of all our physical conceptions which was connected with the appearance of one of the most important and revolutionary theories of modern physics—wave mechanics. Indeed, the phenomenon of electron diffraction, discovered in 1927, remains to the present time the principal experimental confirmation of quantum mechanics. In turn, wave mechanics exerted a strong influence on the development of electronics: with its help a deeper understanding was achieved of previously established facts in the field of thermionic emission and the photoelectric effect, while the entire field of autoelectronic phenomena for the first time received a rational explanation based on a rigorous quantitative theory.

Another important moment in the development of electronics and of all physics of this period was the creation of electron optics. Having arisen in 1926 from consideration of a particular problem of the motion of electrons in an axial magnetic field, electron optics in a short time became an extensive and elaborately developed discipline, whose significance for modern experimental physics and technology continues steadily to increase (the electron microscope, cathode oscilloscope, cathode-ray tubes, high-aperture mass spectrographs, electron accelerators—all these are technical “outlets” of electron optics). The revival of interest in the long-known analogy between optics and mechanics made it possible, if only retrospectively, to impart exceptional coherence to the basic propositions of electron optics and to outline qualitatively correct ways of solving certain difficult problems in this field.

*) The experiments of Pavlov and Franck and Hertz.

It must be emphasized that, having to a certain extent lost its significance for the development of physics as a whole, electronics, beginning in the twenties of our century, found ever new and remarkable applications in technology and industry. It was precisely in these years that a full-fledged and comprehensive technical use of previously discovered phenomena unfolded. Thus, the application of thermionic emission in technology, thanks to the rapid improvement of radio tubes, led to that astonishing flourishing of radio communication of which we are witnesses. The photoelement, invented at the dawn of our knowledge in the field of photoelectricity, was transformed from an amusing laboratory toy into a valuable physical instrument; with its application and improvement there became possible the brilliant and rapid successes of such entirely new branches of technology as sound cinema and television, whose significance in the life of modern society can hardly be overestimated. Somewhat later, curious possibilities were discovered for the technical use of secondary electron emission, and the above-mentioned applications of electron optics in technology developed. In parallel with the growth of new branches of technology, the young electrovacuum industry rapidly strengthened and expanded.

The work of Soviet physicists in the field of electronics falls, in the main, precisely within this just-characterized period of its development. Considering the results achieved by Soviet physicists, we become convinced how much they have done in improving and in the deeper study of the older branches of electronics and in developing its new directions. We shall see that in some directions the work of Soviet physicists occupies a leading position; that many key questions of electronics of these years were first posed in the Soviet Union, and a number of important technical problems were also first solved in our country. The scope and variety of these works do not permit, within the limits of a short article, a sufficiently complete illumination of all the interesting questions deserving mention (let us note that the number of journal articles on electronics alone published in the Soviet Union is over 400). Below we shall consider work on the photoeffect, secondary emission, thermionic emission, surface ionization, electron optics, and electron diffraction. Many interesting works, among those devoted to more specialized questions, will not be touched upon at all; others will only be mentioned.

2. THE EXTERNAL PHOTOEFFECT

Photoelectric investigations, which developed widely in the Soviet Union, did not arise on empty ground—they stand in a line of continuity with Russian science of the past and, above all, with Stoletov’s classical works, dating from 1888–91.

To this day one cannot but marvel at the depth of Stoletov’s investigations and at the scientist’s remarkable intuition; at the then-existing level of our physical knowledge he was able to do so much for the study of this new branch of physics. To Stoletov belongs the honor of establishing the first law of photoelectricity; he proposed a perfected method for measuring photocurrents; he established the existence of a saturation current; proved the inertia-free character of the photoeffect; and discovered the phenomenon of fatigue of photocathodes. Studying photoelectric phenomena not in a vacuum, but at atmospheric and reduced pressure, Stoletov showed that the photoelectric current passes through a maximum at a definite gas pressure (the Stoletov effect), thereby laying the foundations for the later Townsend theory of gas discharge. Stoletov’s works are thus fundamental not only for the field of photoelectricity, but also for gas discharge.

Later, in 1913, an exceptionally important role in the formation of correct ideas about the nature of photoelectricity was played by A. F. Ioffe’s experiments on the “elementary” photoeffect. The statistical character of the phenomenon revealed in these excellently conceived and finely executed experiments served as weighty confirmation of the validity of the quantum character of the interaction between matter and radiation (in those years these ideas were far from generally accepted). At the same time, Ioffe’s experiments constitute one of the most immediate and direct proofs of the atomistic structure of electricity.

The first studies in the field of the photoeffect in the Soviet Union are closely connected with the name of P. I. Lukirskii, whose role in creating the Soviet school of electronics was exceptionally great. Lukirskii is to be credited not only with a number of first-rate investigations on the photoeffect, secondary and thermoelectronic emission, but in fact the majority of the work in electronics carried out by Leningrad physicists was, to one degree or another, inspired by ideas expressed by Lukirskii, or was done by his students or at his suggestion.

The first works of Lukirskii and his collaborators on the photoeffect from crystals date to 1924. In that same year, in connection with his investigations in the field of X-rays, he proposed a new method of the spherical condenser, the significance of which became fully clear in 1926, when Lukirskii and Prilezhaev began a series of works on the study of the photoelectric effect from metallic surfaces. In these investigations, for measuring the energy of photoelectrons and determining the work function of surfaces, the retarding-potential method was used. Millikan and Ives had earlier employed a similar technique, but the results they obtained by no means possessed the required degree of accuracy. Only the use of the spherical condenser proposed by Lukirskii transformed the retarding-potential method into one of the most accurate and reliable means of determining the work function. The motion of photoelectrons in the field of a spherical condenser can be predetermined

fully calculated; for given geometrical dimensions of the instrument, the maximum energy of the photoelectrons is found with a high degree of accuracy, and possible sources of error can be identified or eliminated. All this turned the spherical-condenser method into a classical method for the technique of photoelectric investigations, and subsequently it came to be widely used in work on the photoelectric effect.

The significance of the work of Lukirskii and Prilezhaev, apart from the already mentioned great methodological value, lies above all in the fact that they subjected Einstein’s equation to a rigorous quantitative test. As a result, the validity of this equation was established with such a high degree of accuracy that it may be regarded as one of the most reliably experimentally substantiated laws of physics.

In parallel with the verification of Einstein’s equation, in these same works, again with great accuracy, the numerical value of one of the fundamental physical constants—the Planck constant—was determined, and an attempt was made to determine directly, by a photoelectric method, the distribution of electrons by energies inside a metal.

From what has been said, the role played by Lukirskii’s original works in substantiating modern ideas about the external photoelectric effect becomes clear.

Several years later there appeared a theoretical work by I. E. Tamm, in which, by the methods of wave mechanics, the theory of the external photoelectric effect is developed for the case of a clean metallic surface. Considering the question of the coupling of electrons with the crystal lattice and the conditions under which absorption of a quantum by an electron of the metal is not accompanied by violation of the law of conservation of momentum, Tamm arrived at the important conclusion that there are two kinds of external photoelectric effect: of “surface” and of “volume” origin. The “surface” photoelectric effect proves to be connected here with the jump in potential energy at the boundary of the metal, while the “volume” one is connected with the potential relief inside the metal (the periodic field of the ions of the lattice). The ideas advanced by Tamm proved extraordinarily fruitful; they not only made it possible to explain, in a number of cases, the cause of the appearance of a selective maximum on the curves of the spectral distribution of the photocurrent in pure metals, but were also repeatedly invoked later in considering other cases of electron emission from a metallic surface.

The mechanism of the selective photoelectric effect for complex surfaces was studied in the works of Lukirskii and Ryzhanov, and an interpretation of spectral selectivity for pure metals, proceeding from the optical constants for these metals, was given for a number of cases in the work of Lukirskii and Khurgin. This last work is connected with an extensive series of investigations by Ives on the optics and photoelectric effect of alka-

of the alkaline metals and must be regarded as one of the important works in this field.

In the subsequent years, in the field of photoelectricity, as also in the field of thermionics, the attention of physicists was concentrated no longer on the study of the properties of pure metals, but on the investigation of complex photocathodes of various types. The invention of the oxygen–cesium photocathode dates to 1930, and up to 1937 it remains at the center of attention of the majority of works on the photoeffect. In the Soviet Union the properties of this cathode, exceptionally important for technology, are being intensively studied; its technology is being developed, and its anomalous properties and mechanism of action are being investigated.

Among a number of investigations that appeared in this direction, the works of Stalin Prize laureate Timofeev and his collaborators must first of all be noted; from this laboratory came the first Soviet highly sensitive photocells with an oxygen–cesium cathode. Timofeev and Pyatnitskii were the first to begin the study of the question of the distribution of photoelectrons by energies, which is very essential for understanding the mechanism of action of this photocathode. Later Kushnir and his collaborators successfully continued similar investigations both for the oxygen–cesium cathode and for other complex cathodes. Khlebnikov, together with Sinitsyn and Zaitsev, studied in detail the technology and properties of various modifications of the oxygen–cesium cathode, investigated its spectral characteristics, fatigue, and the distribution of sensitivity over the surface. Finally, recently, in 1944, there appeared a work by Morozov and Butslov, exceptionally rich in experimental material. In it the relation of the photoelectric and optical properties of the cathode to the thickness of the semiconducting layer is examined in detail. The experimental data obtained in this work compel one to evaluate in a new way many previously established conceptions about the action of oxygen–cesium photocathodes and, in particular, refute certain constructions developed by de Boer.

In 1936 the antimony–cesium photocathode was discovered. A year later there appeared the first work in the Soviet Union by Lukirskii and Lusheva, containing a description of the properties of photocells with cathodes of this type. It must be borne in mind that the technology of this cathode was not yet clear at that time, its mechanism of action was unknown, and the properties of the cathode had not been studied in detail. Already in this first work an unexpected and ingenious interpretation was given of a number of anomalous properties discovered in photocells with new cathodes*). Later, when

*) Thus, in particular, the absence of saturation in vacuum photocells in this case is explained by the large longitudinal resistance of the cathode and by the occurrence on it, under illumination, of a potential drop. As a result, a “sliding” photocurrent appears along the cathode, amplified by secondary emission; this current increases with voltage and is superposed on the primary current from the cathode.

The value of the new cathode becomes clear to physicists when its remarkable properties (high photosensitivity, constancy in operation, simplicity of manufacture) arouse universal interest; numerous works appear devoted to the antimony-cesium cathode, and the overwhelming majority of these works belong to Soviet scientists. The first publication on the new photocells, which appeared in the United States, dates to 1941, and there is no doubt that antimony-cesium photocells have not only been studied by us more fully than in America or England, but have also been applied in engineering earlier and more widely. It is not possible in the present article to characterize the whole aggregate of questions and works relating to this subject, and we shall note only some of them.

In 1939 Prilezhaev, in an extensive investigation carried out by the same method of the spherical condenser, studied the properties of an antimony-cesium cathode in equilibrium with cesium vapor. This work of Prilezhaev, directly adjoining Langmuir’s well-known investigations on the study of the phenomenon of adsorption of atoms on metallic surfaces, contributed much to the understanding of the process of cathode activation and the mechanism of its operation.

In an interesting and subtle investigation by Vekshinskii, carried out in the laboratory of the “Svetlana” plant in 1940, the microstructure of the antimony-cesium cathode was studied (and even earlier—that of the oxygen-cesium cathode). In these works, for investigating the photoeffect from individual elements of the surface, Vekshinskii very successfully applied a method he had developed with automatic registration of photocurrents. The results he obtained in studying the crystallization process of thin metallic films of antimony, as well as the method used, were later successfully employed by him for the metallographic investigation of alloys of various elements, i.e., far beyond the field we are considering. For these latter works, Vekshinskii was awarded the Stalin Prize in 1946. Thus, a method for obtaining and analyzing thin metallic films, which arose from photoelectric investigations, found wide application in entirely different branches of technology.

In 1939, the author of these lines determined, by a new method, the quantum sensitivity of the antimony-cesium cathode, which, as it turned out, reaches, at the point of the spectral maximum of the cathode sensitivity, the enormous value of about \(1/4\) electron per quantum. This circumstance makes it possible to regard the new cathodes as the most sensitive indicators of radiant energy with a given wavelength.

Later Khlebnikov and Melamid showed that the antimony-cesium photocathode also possesses very high sensitivity in the ultraviolet region of the spectrum. The photocells they constructed with thin-walled windows successfully solve the problem of creating sensitive instruments for recording ultraviolet radiation.

In studying the spectral characteristics of the antimony–cesium cathode, the author proposed a generalization of the well-known Fowler–DuBridge method for determining the work function to the case of a cathode of semiconductor nature.

A number of works on antimony–cesium cathodes were carried out at the Institute of Physics in Kiev and in Moscow at VEI. In particular, Morgulis and Dyatlovitskaya investigated in detail the emission properties of these cathodes, with special consideration of the influence of temperature and of the electrical conductivity of the layer on the characteristics of antimony–cesium photocells. The work of the Kiev physicists in this direction has continued in recent years. Especially promising are the recent works of Morgulis and Borzyak, in which the connection between the optical constants of the antimony–cesium cathode and its photoelectric properties is investigated. The results of these experiments may be of interest not only for the field of the photoeffect, but also for the physics of semiconductors, since from them data may be extracted on the behavior of electrons excited by light in a semiconducting medium.

An interesting physical investigation of the antimony–cesium cathode was published in 1947 by Brezhnev, who studied the influence of an electric field on the photoelectron emission of this cathode (the Schottky photoelectric effect). In the same work Brezhnev, for the first time, investigated autoelectronic emission for semiconducting surfaces using the antimony–cesium cathode as an example. Much effort toward introducing new cathodes into various fields of technology was made by Khlebnikov, who, together with Zaitsev and Sinitsyn, studied the light and electrical characteristics of antimony–cesium photocells.

In concluding the consideration of physical investigations in the field of the photoeffect, we should also point to the new viewpoint on the mechanism of emission of complex photocathodes proposed in 1945 by Khlebnikov. Relating the photoeffect from these surfaces to the energy model of a semiconductor, Khlebnikov subjects to deserved criticism many propositions of de Boer’s theory, in which the role of alkali-metal atoms adsorbed on the surface of complex cathodes is undoubtedly overestimated. Possessing great logical coherence, the ideas developed by Khlebnikov may play a positive role in the search for new photocathodes and in the investigation of existing ones.

Among the works devoted specifically to the technical applications of the photoeffect, one should especially note Braude’s work, dating from 1937. In it a new, exceptionally ingenious device intended for the television transmission of motion pictures was proposed. An essential part of this device consists of a photoelectrically sensitized metal wire, onto which a line of the transmitted frame is projected. By creating along the wire an electric field traveling with the scanning speed, and collecting on the col-

in the collector the photoelectrons emitted by the filament, we obtain in the collector circuit a photocurrent whose rate of rise will be proportional to the video signal from the element of the picture being transmitted. Braude’s television system is an entirely original solution of the problem and, as the experience of the Leningrad Television Center has shown, it has proved excellent in the transmission of motion pictures.

The technical applications of the photoelectric effect include, in essence, also those studies on the development of complex photocathodes that were discussed above. It must be specially emphasized that a great merit of Soviet physicist-engineers is the timely launching into mass production of photocells with an oxygen-cesium, and then an antimony-cesium, cathode, in the manufacture of which the Moscow Electric-Lamp Plant specialized. The parameters of the photocells manufactured by MELZ were continuously improved and are not inferior to the parameters of photocells made by foreign firms. A well-known negative aspect in this field is the insufficient use of photoelectric methods in industry—the application of photocells for purposes of automatic control, recording, and signaling still does not match the existing possibilities.

For a number of years, photocells of various special types developed by Soviet physicists have been successfully used in astronomy, in particular in the work of the Main Astronomical Observatory of the Soviet Union at Pulkovo. In the current year, astronomer Pavlov was awarded the Stalin Prize for the development of a photoelectric method of recording stellar occultations, which provides a significant increase in the accuracy of astronomical observations.

3. SECONDARY ELECTRON EMISSION

The phenomenon of secondary electron emission was discovered as early as 1898, but for quite a long time it did not attract special attention from researchers. The mechanism of the phenomenon had not been studied, and there were no technical applications. Only from the beginning of the twenties, on the one hand in connection with the negative effect that the dynatron effect has on the operation of amplifier tubes, and on the other hand in connection with studies on electron diffraction from metallic crystals, did the number of works devoted to secondary emission increase. In the Soviet Union the first study on secondary emission, which appeared in 1920, belonged to P. I. Lukirskii and N. N. Semënov; in this work they measured the secondary-emission coefficient for mercury and studied its dependence on the energy of the primary electrons. Although the numerical data obtained by Lukirskii and Semënov cannot at present be regarded as accurate, in view of the imperfection of the vacuum technology of that time,

but the qualitative explanation of the course of the observed dependence that they proposed is entirely correct.

A sharp turning point in the rate of research on the question under consideration occurred in 1934–1936. The invention in 1934 by L. A. Kubetskii of the multistage electron multiplier (an invention soon afterward repeated by Zworykin in America, Weiss and a number of others in Germany) aroused increased interest in secondary emission, and in the following years a whole stream of works on the physics and technology of secondary-electron emission appeared*). In these works the mechanism of the phenomenon is studied for the simplest case of a clean metallic surface and prerequisites are created for understanding the processes occurring in complex cathodes; new surfaces possessing a large coefficient of secondary emission are described; and numerous, ever more advanced designs of electron multipliers are proposed. The works of Soviet scientists in this field throughout this entire period developed exceptionally successfully and occupy a leading place in world science.

We shall first dwell on the physical investigations, and then on works of a technical character.

In 1938–1939 Vyatskin began the construction of a rigorous quantum-mechanical theory of the phenomenon for the case of a pure metal. Subjecting the earlier theory proposed by Fröhlich to fundamental criticism, Vyatskin, however, treated the whole problem as a purely surface one, proceeding from the Sommerfeld model of a metal. Therefore, although the theory in this form was of value and interest in the analysis of experimental data for pure alkali metals in the region of small energies of the primary electrons, it could not give a correct description of the phenomena at high energies, when ionization of electrons bound to atomic residues begins to play a considerably greater role. Later, in 1944, Vyatskin substantially developed and supplemented his theory. In his new works the secondary emission of metals is considered as a phenomenon caused by the superposition of a surface effect and two volume effects, taking place on the “free” electrons of the lattice and on atomic electrons. The volume effect of free electrons determines the fine structure of the distribution function of secondary electrons with respect to energies, whereas the surface effect gives the principal course of the distribution function. Both volume effects account for approximately 10–20% of the total secondary emission. Of interest for the theory of electron emission from a metal is the theory of absorption of electrons (primary and secondary) inside the metal, constructed by Vyatskin in these same works, due to their interaction with the electrons of the metal.

*) In the five years from 1935 to 1941, about 60 works devoted to secondary emission were published in the journals of the Soviet Union; in all the preceding years, 5–6 works.

In Kadyshevich’s theoretical works, secondary emission is considered as a volume effect, as an ionization process occurring within the bulk of the secondary cathode. The ionization theory gives, in agreement with experiment, the dependence of the secondary-emission coefficient on the energy of the electrons and, when applied to semiconductor surfaces, explains the reason for the large value of this coefficient in composite cathodes. It is true that the value of the secondary-emission coefficient is calculated in this theory only very approximately. But one should not forget that we still do not possess sufficient knowledge about the behavior of slow electrons generated in the emission process within the cathode, especially if its internal structure is as complex as is the case with modern emitters.

Experimental investigations of the question developed on a very broad front. In 1936, in the works of Afanas’eva and Timofeev, earlier than the well-known works of de Boer, an important question was clarified concerning the magnitude of the secondary-emission coefficient $\sigma$ for pure alkali metals, and it was proved that the latter have a value of $\sigma$ smaller (and not larger) than other pure metals. The method used in the experiments of Afanas’eva and Timofeev for studying secondary emission—the deposition on a metallic substrate of layers of atoms of another metal of gradually increasing thickness—proved very useful for studying the mechanism of the phenomenon and was widely applied subsequently.

Through the work of Khlebnikov and his collaborators, the role of absorbed and adsorbed gases in the secondary emission of pure metals was clarified in detail. Recent careful experiments by Morozov (1941) not only gave reliable and precise values for the secondary-emission coefficient of many pure metals, but also confirmed the conclusions of Kushnir and his collaborators concerning the independence of the secondary emission of pure metals from temperature over wide limits of its variation. In the same work, the influence on secondary emission of the passage of a metal through the melting point was studied.

In 1937 the author, together with Bernatovich, studied in due detail the dependence of the secondary-emission coefficient on the angle of incidence of the primary electrons. The increase of secondary emission under oblique incidence of the primary beam was established both for a pure metallic surface and for a composite oxygen–cesium emitter. Later this question was studied in detail in a number of works (1941–1946) by Kushnir and his collaborators; moreover, they considered not only the influence of the angle of incidence of the electron beam on the total secondary emission, but also clarified such a difficult question for experimental investigation as the influence of the angle on the function of the distribution of secondary electrons by energies. In Kushnir’s laboratory the dependence of the distribution function of secondary electrons on the angle of emergence was also studied. Investigations of the dependence of secondary emission on the angle of incidence of the primary beam

S. Yu. Lukyanov

is important in the practice of designing electron multipliers, and the interpretation of the experimental data obtained is important for understanding the mechanism of the phenomenon. In particular, certain ideas developed in the aforementioned work by the author and Bernatovich were subsequently used in constructing ionization theories of secondary emission.

The secondary emission of pure semiconductors was first investigated in the works of Afanas’eva and Timofeev and Frimer; the secondary emission of dielectrics—in the works of Vudynsky and, especially thoroughly, with the application of a new method, in the works of Kosman and his collaborators.

The study of effective emitters, i.e. surfaces possessing a high coefficient of secondary emission, is closely connected with the so-called Malter effect. This term denotes the superposition, occurring in a number of cases, of autoelectronic emission upon the true secondary emission of a surface. It is often very difficult to draw the boundary between the two phenomena—pure secondary emission and the Malter effect. This range of questions is of great significance both in principle, from the standpoint of clarifying the mechanism of secondary emission from nonmetallic surfaces, and in practice, since it is only among surfaces of this kind that we find effective emitters of secondary electrons. Naturally, this line of research aroused great interest among Soviet physicists and became the subject of lively discussion. Some investigators, above all Timofeev and his school, believe that, in general, appreciable coefficients of secondary emission of semiconducting emitters ($\sigma > 2 \div 3$) already indicate the presence of a Malter effect of a special kind, whereas other investigators (Morgulis, Vernov, Khlebnikov) believe that true secondary emission can yield values of $\sigma$ reaching 10–12.

As a result, at the present time there is an extensive series of works by Soviet physicists devoted to the study of emitters of the semiconducting type. In the course of these works the energy distribution of the emitted electrons was determined for a number of surfaces, and, in the case of typical “Malter” emitters, the existence of two groups of electrons was discovered—true secondary electrons and “autoelectronic” ones. In addition, the drop of potential in the semiconductor layer giving large values of $\sigma$ was measured, and the presence of abrupt transitions from emission of the ordinary type to Malter emission with variation of the semiconductor thickness was shown experimentally (Vernov). For many effective emitters the temperature dependence on the density of the primary current, the velocity of the primary electrons, etc., was determined.

It is essential that all these investigations, in addition to yielding data of purely physical interest, have led to the fact that we now possess an entire “arsenal” of secondary-emitting surfaces possessing a large coefficient of secondary

emission, stable in operation and permitting considerable loads. Particularly interesting in this respect are the oxygen-magnesium emitters developed by Aranovich in Timofeev’s laboratory, giving \(\sigma\) of the order of 30–50 (instead of the usual technical values 8–10) and withstanding temperatures up to \(1000^\circ\text{C}\). One should also mention the copper-sulfur-cesium emitters developed in Kubetskii’s laboratory, which proved especially suitable for use in magnetic-type photomultipliers with a cathode on glass.

Turning directly to the technical applications of secondary emission, we must again note that the pioneer in this field is L. A. Kubetskii. Although the very idea of amplifying currents through the use of secondary emission dates back to a comparatively early time, it was Kubetskii alone who not only proposed a concrete design of a multistage multiplier, but also built, in 1934, a working specimen of the device. In subsequent years, in the laboratories of the Svetlana plant, VEI, and the Institute of Telemechanics, Kubetskii, Vekshinskii, and Timofeev created numerous variants of photomultipliers and electron tubes using secondary emission and possessing great steepness. A known hindrance to the development of these tubes up to the present time was the absence of sufficiently effective and thermally stable emitters; however, in connection with the development of oxygen-magnesium and barium cathodes, this difficulty may apparently be regarded as overcome.

It would be wrong, however, to pass over in silence the fact that the initial rosy hopes for unusually broad applications of electron multipliers, and for the revolution they would bring about in amplifier technology, proved exaggerated. In the competition between two systems—the tube amplifier and the electron multiplier—the advantage, as a rule, lies on the side of the older system, but one already brought to a very high degree of perfection: the tube circuit. The multiplier itself is not free from a number of shortcomings; it is not standardized to the same extent as the electron tube, and it is less stable. Its advantages are great where enormous amplification of weak high-frequency signals is required, for example in television, since its signal-to-noise ratio proves more favorable than that of a tube circuit. In sound cinema the use of multipliers is possible, but the question of the most rational system (a tube amplifier, a photomultiplier, or a sulfur-thallium photocell with a blocking layer) still remains open.

4. THERMOELECTRONIC EMISSION

Historically, the first works here, just as in the field of the photoeffect, were purely physical studies and, moreover, theoretical in character.

As is known, in the investigations of Child and Langmuir in 1911–1913 the problem of finding the magnitude of the electron

of current in a vacuum in the presence of space charge, under the assumption of the absence of initial velocities, for the plane and cylindrical cases (Langmuir’s “three-halves law”). For the cylindrical case, however, the solution had been found only approximately, and only in 1923, simultaneously and independently of one another, in the Soviet Union by Boguslavsky and in the USA by Langmuir and Blodgett, was an exact solution obtained. Boguslavsky’s work contains a complete and rigorous investigation of the question, but, having been published after the author’s death in 1924 in a little-circulated journal, it unfortunately remained unknown to most later investigators.

Among the early works in this field one must also note the work of Pavlov and others, published in 1923, devoted to the study of the motion of electrons between two plane grids. In this work the presence of initial velocities of the electrons was taken into account (they were assumed to be identical), and for the first time a certain nonuniqueness of the solution was discovered: under certain conditions the values of the current passing through the device are not completely determined by the values of the potential at the electrodes. This work, too, proved to be forgotten, and Pavlov’s results were repeated considerably later in the works of foreign investigators.

The years 1918–1919 saw the first Soviet works on radio tubes. In these years, in the Nizhny Novgorod Radio Laboratory, under the direction of Bonch-Bruevich and Ostroumov, the foundations of the Soviet electrovacuum industry were laid, and the first experimental investigations of electronic phenomena in tubes were carried out.

Experimental investigations of thermionic emission from various surfaces, belonging to Soviet physicists, appeared later. The principal complex of these works is inseparably connected with the physical laboratory of the “Svetlana” plant (directed for a number of years by Vekshinsky and Lukirsky), which played an exceptional role in the creation of Soviet electrovacuum devices. Among these works we shall note the elegant experiments of Vekshinsky, Lukirsky, Sozina, and Tsareva (1930) on studying the influence on thermionic emission of the lining of foreign atoms adsorbed on a metal surface. These experiments, as well as the investigations of Ptitsyn, Berdennikova, Morgulis and his collaborators, Radel, Anselman, Kozlyakovskaya, served as the starting point in extensive work on the study of complex incandescent cathodes of various types, on the development and improvement of the technology first of thoriated and carburized, and then of oxide and barium cathodes.

Progress in this work ensured the high parameters of modern radio tubes and other vacuum and gas-filled devices. Touching upon technological questions, one cannot pass over that great and valu—

...of the work that was carried out in the field of electrovacuum technology by the recently deceased employee of the “Svetlana” plant, the engineer and physicist Ivanov. Very much in the creation of Soviet radio tubes was done by Vekshinsky, Shaposhnikov, and Zusmanovsky. The latter, as early as 1941, together with Kashan and Moshkovich, was awarded the Stalin Prize for the invention of low-voltage receiving-amplifying tubes.

Among investigations of a purely physical nature we should note the works of Rutkevich, Morgulis, and Dyatlovitskaya, who studied the electron emission of thoriated tungsten. Dobretsov and Morozov investigated the evaporation of barium from tungsten and determined the heat of adsorption of Ba atoms, as well as the adsorption time of Ba for various coatings and temperatures.

The Schottky effect for thermionic emission was studied in the works of Dobretsov and Morgulis. In particular, in one of the later works (1941) Dobretsov applied a thermal method to the investigation and unambiguous determination of the change in the work function under the influence of an external electric field. He made a careful measurement of the latent heat of evaporation of electrons at various external fields and showed that the change in the latent heat of evaporation corresponds to that change in the work function which is given by Schottky’s theory.

Interesting works on the investigation of the behavior of the oxide cathode in pulsed operation were recently (1944–1946) published by Andrianov, Morgulis, Kalashnikov, and others. A detailed clarification of this question is of very substantial importance in solving many problems of modern radio engineering.

There is no possibility here of systematically examining the works of Soviet physicists and technicians on the development and improvement of electron tubes and the analysis of their operation—this belongs to the field of radio engineering; but we shall nevertheless dwell on one series of investigations in which Soviet scientists did much that was new and original.

The development of UHF technology, connected with the successful development of the magnetron generator, aroused increased interest in explaining the mechanism of operation of the magnetron. In a number of articles, the first publications of which date to 1934–1935, Grinberg, together with Lukoshkov and a number of other employees of the “Svetlana” plant, calculated the fields in a split-anode magnetron and, on the basis of the graph-analytical method proposed by Grinberg, were able to construct the electron trajectories in these fields. As a result, it proved possible to explain exhaustively the previously unclear causes of the negative resistance of the magnetron, refuting the incorrect theories of this phenomenon that had existed earlier. Of substantial importance for radio engineering also was the consideration of the problem for the unsplit magnetron—the finding of the fields, the determination of the dependence of current strength on the magnetic field, etc. For the “plane”...

for the magnetron this problem was solved by Braun in 1936; for the cylindrical magnetron a complete solution of the problem was first given in 1938 in the work of Grinberg and Wolkenstein, in which, in particular, formulas were given for determining the wavelength of transit oscillations as a function of the dimensions and the applied fields.

Another group of works by the same investigators is also connected with the growth in the importance of ultra-short-wave technology. As is known, Benham laid the foundations of the theory of the passage of an electron current, when it is limited by space charge, through a plane diode at such high frequencies of the applied voltage that the period of the high-frequency field is comparable with the time of flight of the electrons through the device. In 1935 Grinberg, along with new investigations of the plane case, for the first time gave a complete fundamental solution also for the cylindrical diode. In the work of Grinberg and Bliznyuk in 1938 the corresponding calculations were carried through to the determination of the technically most interesting quantity—the complex impedance of a cylindrical diode at high frequency. At the same time, Grinberg investigated in detail the initial stages of the passage of an electron current through a diode when a voltage pulse is applied to the anode (the motion of an “electron front” and the accompanying formation of space charge).

We have noted above the significance of theoretical works on the investigation of the magnetron. The experimental development and investigation of magnetrons, besides the “Svetlana” factory, was successfully and for a long time carried out by Grekhova, at first in Moscow, and in recent years at the Gorky Physico-Technical Institute. Valuable works on electron-beam tubes also belong to her. Extensive experimental investigations on magnetrons were, moreover, performed by Slutskin, to whom there also belongs a number of theoretical calculations in this field.

5. SURFACE IONIZATION AND ION EMISSION

The phenomenon of surface ionization was discovered by Langmuir and Kingdon in 1923–1924 for the case of ionization of cesium atoms on the surface of heated tungsten. At the same time the well-known Langmuir–Saha formula was proposed, determining the temperature dependence of this phenomenon. The object of investigation itself (cesium on tungsten), however, did not permit a full verification of those propositions that formed the basis of the theory of this question, since cesium atoms undergo practically 100% ionization over the entire temperature interval convenient for investigation. Therefore only considerably later, in the works of Dobretsov and Morgulis, dating from 1934 and carried out for the case of ionization of potassium, sodium, and barium on the surfaces of tungsten, molybdenum, and tantalum, was the temperature dependence of ion emission for the first time reliably investigated,

and the applicability of the Langmuir–Saha formula was fully demonstrated. Dobrecov’s experiments were carried out with particular care; in studying the phenomenon he made use of the highly perfected technique of molecular beams. It should be emphasized that the interesting case of surface ionization of sodium atoms on tungsten (the ionization potential of Na is greater than the work function of W!) had previously not been investigated at all.

In the same years Morgulis and his collaborators studied the reverse phenomenon—the neutralization of alkali-metal ions on metallic surfaces.

The study of surface ionization on complex cathodes was begun in the Soviet Union. In 1934 Dobrecov undertook investigations of surface ionization on thoriated tungsten and subsequently analyzed in detail all aspects of this phenomenon. The interest and significance of these works went far beyond the bounds of the surface-ionization effect itself. In essence, these works were the first experimental proof of the existence of a “patch structure” in complex cathodes. Electron-optical investigations were carried out later, and the well-known considerations of Langmuir, Kingdon, and Becker concerning “thorium patches on tungsten” had, in those years, the character of hypothetical constructions. Thus, surface ionization can be used as a new method suitable for studying the structure of complex cathodes. It successfully complements thermionic investigations, in which, essentially, regions of the cathode with minimum work function are studied, whereas surface ionization occurs especially readily in regions with maximum work function.

Later, in 1936, Dobrecov, as a result of a discussion with Morgulis, showed that the influence of an electric field on surface ionization over a wide range of fields reduces to the Schottky effect for ions. In 1937–1938 Dobrecov and Konozenko, Morgulis and Dyatlovitskaya, were the first to study the influence of an electric field on surface ionization on thoriated tungsten (the “anomalous Schottky effect for ions”). This entire circle of questions was posed and fully clarified by the works of Soviet physicists.

In 1937 Ionov, at Lukirsky’s suggestion, began the study of surface ionization of atoms with the formation of negative ions. In 1940 Dukel’skii and Ionov published a work in which the formation of negative halogen ions was investigated during the interaction of alkali-halide molecules with the surface of incandescent tungsten. In these works, subsequently continued by Ionov, an attempt was made to verify the applicability of the Langmuir–Saha formula to ionization of this kind. The value of this direction of research lies in the possibility of directly measuring the electron affinities of various atoms, which can only with difficulty be determined by any other method. Similar investigations

were carried out over a number of years in Tashkent, where the experiments on the surface ionization of alkali-halide salts, begun in 1935 by Starodubtsev in Lukirskii’s laboratory, had been transferred. It was found that the study of the temperature characteristics and absolute coefficients of this type of ionization can provide a method for determining the heats of reactions on the surface of a metal. There, Shuppe and Arifov also studied the positive ionization of salts and the negative ionization of halides on thoriated tungsten.

Closely related to the range of questions under consideration are the works of Pavlov and Morozov (1935–1940) on the study of the ionic emission of various chemical compounds, and the works of Pavlov and Starodubtsev on the investigation of the interaction of slow and fast ions with metals and films of semiconductors. The large body of experimental material accumulated as a result of all these experiments requires further elaboration and systematization, since it may develop into a distinct branch of surface chemistry.

Quite recently (1946–1947), Dobretsov, Starodubtsev, and Timokhina discovered a new type of surface ionization—the ionization of metal atoms on thin films of oxides of the same metals. The observed phenomena do not fit within the framework of the usual theory and deserve further careful study.

6. ELECTRON DIFFRACTION

In comparison with other branches of electronics, the number of works on electron diffraction carried out by Soviet scientists is small, but many of them have proved essential for the development of this field.

First of all, one should note here the work of one of the pioneers of research on electron diffraction, Tartakovskii, which he conducted beginning in 1927 in Leningrad and in Tomsk. Of interest are Kollinskii’s work on the study of polycrystalline thin layers with oriented crystals, and the work of Kollinskii and Fok, in which electron diffraction from a deformed crystal was investigated theoretically and experimentally; these works were carried out at the Physics Institute of Leningrad State University. There, Alikhanian and Kosman studied the electron diffraction of relativistic electrons.

In the work of Lashkarev and his collaborators, the diffraction of slow electrons was investigated, and the dependence of the refractive index of electron waves at the boundary of a crystal on the velocity of the electrons was established. Lashkarev and Usyskin applied electron diffraction to elucidate the spatial structure of the molecule of sal ammoniac.

In addition to experimental work in this field, Soviet physicists also carried out a number of theoretical studies (Tartakovskii, Lashkarev, Kalashnikov, and others).

7. ELECTRON OPTICS

Problems of electron optics attracted the attention of Soviet physicists comparatively late—all the published works relating to this field fall within the last 7–8 years. Nevertheless, the work of Soviet scientists has substantially enriched this branch of electronics with profound theoretical investigations, new ideas, and original designs of electron-optical instruments.

It is well known what importance, in modern nuclear physics, attaches to the artificial production of ions with high energy. The best method for obtaining such ions is the cyclotron; however, the relativistic increase in the mass of the particle during its acceleration substantially limits the possibilities of this remarkable instrument. For a number of years it seemed that the only possibility of appreciable progress toward higher energies was to increase the voltage on the cyclotron dees, which required a large gap between the poles and would have increased the already enormous dimensions of powerful cyclotrons. An entirely new principle was put forward by Veksler at the beginning of 1945: he proposed making use of the method of “autophasing” discovered by him. As it turned out, by slowly changing the frequency applied to the dees, it is possible sharply to raise the limiting energy of the ions without changing the dimensions of the cyclotron. Veksler’s conclusions were soon repeated in America, and at the present time the cyclotron with modulated frequency is, apparently, one of the most advanced tools of applied nuclear physics.

Interesting work also belongs to Soviet physicists in the area of solving the problem of electron acceleration. As early as 1939, long before Kerst’s well-known work, at the Kharkov Physico-Technical Institute Kelman, Korsunsky, and Lange designed a magnetic electron mirror and began work on its application to the construction of a “quadrutron”—an apparatus for multiple acceleration of electrons. Unfortunately, these works, as well as the theoretical investigations of Terletsky (1941), who independently of Kerst subjected Wideröe’s idea of creating an electron transformer to a new examination, were interrupted by the war.

Also in Kharkov, in the work of Korsunsky, Kelman, and Petrov, a high-luminosity β-spectrograph with an inhomogeneous magnetic field was first proposed and experimentally realized, making it possible to obtain aberration-free focusing of wide-angle (up to 40°) electron beams.

Of exceptional interest are the investigations published in 1942 by Grinberg on the general theory of focusing electrons in electrostatic and magnetic fields. The significance of this work for

modern electronics consists in the fact that it establishes certain general laws of motion of charged particles under the action of electric and magnetic forces and thereby determines the conditions for focusing electron and ion beams. Some special cases of the motion of electrons in electric and magnetic fields, in which the focusing of electron rays analogous to the focusing of light rays in optical instruments took place, had long ago been considered. Theoretical electron optics has until now been concerned with the analysis of these special cases, and we noted in the introduction how varied and interesting were the practical results of these investigations. For the further development of the science, however, a substantial development of theoretical electron optics was urgently required. The need to broaden the theoretical basis of electron optics had long been recognized by those working in this field, but before the appearance of the investigations named above no one had succeeded in making substantial progress toward solving the general problem of electron focusing. The results obtained by Grinberg form the present foundation of electron optics, and now this field, in its theoretical aspect, has approached its completion.

The solution of the problem was found in the work under consideration by entirely original methods, which are of great independent interest from the point of view of theoretical mechanics. Usually in mechanics the problem of the motion of particles is posed in such a way that first the field of forces is specified, and then the motion in this field is investigated. In this case the geometrical properties of the trajectories are clarified only at the end of the calculation. In the present work the basic problem of dynamics was, if one may so express it, turned inside out. Proceeding from the practical necessity of controlling at will the form of electron trajectories, Grinberg set himself the task of ascertaining the possibility of selecting an electric or magnetic field for a beam of trajectories of a prescribed form. This new formulation of the problem fully justified itself. It proved possible not only to determine under what conditions the focusing of electron trajectories is possible, but also to give formulas for determining the fields corresponding to a prescribed form of the beam.

In 1944 Artsimovich published an important theoretical work devoted to the consideration of the electron-optical properties of emission systems. With such systems we have to do in all devices in which images are obtained of objects that emit slow electrons; examples are the emission electron microscope, the television dissector, and kine-scope electron guns. The theory of such systems, despite their great practical importance, had, before the analysis carried out by Artsimovich in the work under consideration, been developed in a completely unsatisfactory manner. Artsimovich not only found an original method for solving the corresponding differential equations of the trajectories of electron

rays for this case, but also calculated the resolving power and the principal electron-optical aberrations of these systems.

In recent years electron optics has been applied to the analysis of the trajectories of electron beams in electron multipliers (Rik, Kormakova). Electron-optical investigations of oxide cathodes were carried out by Morgulis, and of a cesium-silver photocathode by Brezhnev.

A splendid result of the extensive work in the field of experimental electron optics carried out over several years at the State Optical Institute by Academician A. A. Lebedev and his collaborators was their creation of domestic models of electron microscopes. For this work, in 1947 Lebedev, Veriper, and Zandin were awarded the Stalin Prize.

We have tried, on the preceding pages, to characterize the contribution made by Soviet physicists to electronics over the past years. There can be no doubt that in the years following the historic victory of the Soviet people, in the years of the country’s greatest economic and cultural flowering, Soviet physicists will add many more brilliant pages to this remarkable field of modern physics.