PHYSICS IN INDUSTRY

PHYSICS OF ION AND ELECTRON PROCESSES AND TECHNOLOGY

N. Kaptsov, Moscow

In physics there is hardly another field in which the problems of practical application are so closely interwoven with the problems of scientific research as the field of the physics of electron and ion processes, or, as it is sometimes called, the field of electrical phenomena in gases and vacuum.

In the article printed in the present issue, written in 1937, the American engineer-physicist Slepian points out a number of useful and harmful phenomena of gas discharge with which technology has to deal in the generation of electric current and in the distribution and transformation of electrical energy. The questions touched upon by Slepian cover only a small part of ion and electron processes and their applications. The purpose of the present note is to fill this gap somewhat. The practical applications of the physics of electron and ion processes may be divided into two large groups. The first is the use of these processes in various kinds of electrovacuum and gas-discharge devices, serving not only the field on which Slepian focused his attention, but also the problems of rectification and transformation of both weak and strong currents; further, the problems posed by all forms of wired and wireless communication, up to and including television; sound-film technology; telemechanics; and, finally, lighting engineering (gas-discharge light sources). The pressure of the gases or metallic vapors in which the discharge takes place ranges in devices of this kind from high vacuum (cathode tubes) to hundreds of atmospheres (Boyle mercury lamps). The second group consists of the direct applications of various types of gas discharge as a powerful factor for achieving definite technical aims. These include chemical reactions in a gas discharge and the processes of charging and depositing particles suspended in a gas in a corona discharge. In all these fields the practical use of ion and electron processes is interwoven with scientific research even more closely than in the cases considered by Slepian. The phenomena applied in the field of electrovacuum technology were discovered not so very long ago; their detailed investigation continues to this day and constantly yields many new essential particulars. If in certain other branches of technology in the USSR it is still necessary to introduce already prepared methods developed abroad, then in this field

the interweaving of practical developments with scientific research takes place to a very considerable degree in our Union as well. One of the vivid and clear examples of such a state of affairs can be photocells, the broad field of application of which is widely known. Only a few years ago the sensitivity of potassium photocells produced abroad and copied here did not exceed 3–4 $\mu A$ per 1 Lm, which even after amplification of the current by a discharge in the same device was still very little acceptable, for example, for use in sound motion-picture installations. Notes that appeared in the scientific literature gave occasion for research at the All-Union Electrotechnical Institute on new, so-called complex oxygen–cesium cathodes. The result was the production at the Moscow electric-lamp plant of photocells ten times more sensitive than potassium ones, which are now successfully serving sound cinema. At the present time, in the institute and factory laboratories of the Union, research and development are proceeding in parallel on antimony–cesium cathodes, which have found reflection in the pages of the Journal of Technical Physics. These cathodes are more perfect for some applications than oxygen–cesium ones. But on photocells with current amplification by means of a discharge, technical thought has not stopped either abroad or here. At present a phenomenon formerly neglected, the so-called secondary electron emission, is being intensively studied, and it is being studied not out of love for art, but for the purpose of multistage amplification of the photocurrent by means of secondary emission from special electrodes placed inside the same device. Laboratory specimens of such “electron multipliers” (VEI) have already been successfully tested in sound motion-picture installations. The same phenomena of the photoelectric effect and secondary emission are used in television devices. An engineer working in this new and important field not only cannot ignore the essence of those physical processes with which he is dealing, but must necessarily, to one degree or another, take part in their investigation. Passing from ionic and electronic processes on the surface of the electrodes of some device to processes in the volume, the totality of which in many cases can be encompassed by the term “discharge in a gas,” we see the same thing. The successful construction of such devices as current rectifiers of all power ratings, thyratrons, and other devices for current control is closely connected with the solution of research problems concerning the course of the phenomenon of the initial ignition of the discharge, repeated ignition in each period of alternating current, harmful reverse ignition in those half-periods when the current has the opposite direction, and so on. The question of the influence of various, sometimes even very small, impurities on the course of the discharge phenomenon used, the questions of deionization of the discharge gap, the influence of a magnetic field on the discharge, etc., are highly essential. Here we again see how technology cannot advance in any essential way without parallel scientific research, and again we have

close cooperation among research and factory laboratories (Moscow State University, VEI, MELZ, the Elektrosila plant, and a number of laboratories in Leningrad). Questions of constructing new economical light sources (with savings of electrical energy by \(2 \frac{1}{2}\) to 5 times) and improving their color quality require not only mastery of the electrical processes of the discharge, but also of the optical side of these phenomena, as well as the study of the phenomena of fluorescence and phosphorescence. Here again there is cooperation between scientific institutions and factories (MELZ, VEI, FIAN). Speaking of light sources, it is appropriate to mention a new type of lamp proposed by the Dutch physicist Boyle. This lamp is distinguished by extraordinarily high brightness—Boyle, in laboratory experiments with short-lived tubes, achieved a brightness greater than that of the sun. The discharge in mercury vapor in such a lamp is concentrated in a narrow (1–2 mm in diameter) quartz tube. When a certain quantity of mercury placed in the tube is completely vaporized, owing to the high temperature of the whole tube, the pressure of the mercury vapor here is on the order of several tens to a hundred atmospheres and higher. A discharge at such a high vapor density is a new phenomenon, not yet investigated. The temperature along the axis of the tube in the discharge column, measured by optical methods, proved to reach \(8{,}000\)—\(10{,}000^\circ\). Under these conditions, the ionization of mercury vapor occurs chiefly no longer through electron impacts, but through so-called thermal ionization (a phenomenon analogous to chemical dissociation when the temperature is raised). Thus we have here yet another case in which new technology in the field of electrical phenomena in gases goes hand in hand with new scientific research and with the expansion of our knowledge of physical processes. Let us take another example, distinguished by the fact that the corresponding electrovacuum device—the cathode lamp—is not new. More than ten years ago the author of this article had occasion to hear the opinion that the cathode lamp is a device with no larger number of unresolved questions than, for example, a dynamo machine, and that the cathode lamp can be calculated no less freely than the latter. However, as life has shown, the cathode lamp has become more complicated over time. Instead of one grid (the triode), three (the pentode) and more appeared. As a result of the special distribution of potential between the electrodes of such a lamp, the phenomenon of secondary emission in some of these electrodes began to play a large role and, moreover, to substantially disturb the operation of the lamp. And so the engineers, in whose hands, it seemed, there were complete theoretical data for the calculation and design of cathode lamps, were confronted with a new question: how to reduce the secondary emission undesirable in this case; and they are compelled to undertake physical research in this direction or to draw the appropriate physical laboratories into joint work. At the same time, the question of calculating fields for such a complex system of electrodes also requires a more detailed and precise solution.

Let us turn to another field. At present, electrostatic precipitators are becoming ever more widely used—apparatus installed in smokestacks to capture smoke particles for the purpose of improving the air in cities and factory settlements (in appropriate combinations with mechanical dust collectors—multicyclones). Electrostatic precipitators are also finding ever greater application in the chemical industry for capturing, in chemical production, fine solid and liquid particles of various materials suspended in gas, including valuable ones (sulfuric acid, cement, apatite dust, etc.). Without such capture these materials are lost, escaping together with the gas into the atmosphere and polluting it. The development and improvement of electrofiltration technology is again connected with the need for research in the field of electronic and ionic processes. The point is that the basic processes in electrostatic precipitators have so far been studied only in a first approximation, while many details of the phenomena occurring here, highly relevant in practice, are still entirely unclear. Practitioners working in the field of electrofiltration face the question not only of how to improve the operation of the apparatus or how to avoid one or another harmful phenomenon, but, precisely in order to obtain an answer to the question “how,” the question arises as to “why” one or another phenomenon is observed. And for this it proves necessary to study in greater detail the processes of charging particles in connection with their physical and even geometrical properties and with a number of additional conditions occurring in practice (humidity, temperature, one or another composition of the gas, etc.). It is also necessary to study the processes of deposition of charged particles on the so-called collecting electrode in connection with their properties and with the properties of the already deposited layer of particles, etc. All these are processes of a physical nature, requiring both refined experimental apparatus and a serious development of the theory of the charging, motion, and deposition of particles. In the USSR, work in this direction has been begun and is being carried out in close contact among the corresponding university, institute, and trust laboratories.

We shall not dwell in detail on the question of chemical reactions in a discharge. The application of an electric discharge here has the aim of making possible reactions that are not feasible either by ordinary methods of chemistry or with the use of catalysts. An example is the direct production of nitric acid from air by the reaction of nitrogen and oxygen. The development of sufficiently profitable practical methods here encounters the necessity of studying the connection between elementary acts in the discharge and chemical reactions, and the necessity of studying the kinetics of chemical reactions in the discharge, which again constitutes a field of electronic and ionic processes. The corresponding work is being conducted at Moscow State University.

Thus there is not a single field of practical applications of electrical phenomena in gases in which, for success, the work of engineers—

...engineering should be closely interwoven with the work of the physicist-researcher, and where such interweaving would indeed take place both abroad and in our Union.

One may say with confidence that the physics of ionic and electronic phenomena began to advance rapidly precisely when extensive and practically important fields of its application opened up, and when technology, on the one hand, set before it a series of new and bold problems and, on the other, equipped it with new apparatus that made it possible to broaden greatly the scope of research. Here we have an example of the beneficial combination of theory and practice and of the results that arise from such a combination. Such a combination is necessary in all fields. Where it is absent, neither technology nor science can develop fruitfully and at a rapid pace.