Libmonster ID: UA-12118
Author(s) of the publication: Valery SHPAK

by Valery SHPAK, RAS Corresponding Member, Director of the Institute of Electrophysics, RAS Ural Branch

Electrophysics is a young science with an extensive history. It received the current name in late 20th century when it united different specialists--from theoretical physicists to electrical engineers. But its origin goes back as far as three centuries and is closely related to the discovery of electricity, an event, which changed drastically the course of world civilization.

American enlightener, statesman and scientist Benjamin Franklin (foreign honorary member of the Petersburg Academy of Sciences from 1789), whose portrait is on a USC-note, is rightfully considered one of the first electrophysicists. He was the first to put into practice the concept of positive and negative poles in electricity, explain the role of dielectric in an electric condenser, develop electrical fuse and bifocal glasses, and suggest lightning-conductor with earthing. Georg Richmann, member of the Petersburg Academy of Sciences (from 1740) and a contemporary of Mikhail Lomonosov*, was the first Russian electrophysicist. Unfortunately, today he is remembered often as a victim of globular lightning though he is an author of

See: E. Tropp, "Along the Way to Universal Knowledge", Science in Russia, No. 5, 2011. -- Ed.

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original works on electricity and magnetism, and an inventor of the first real electrical measuring instrument--a dial electroscope.

By the way, on closer examination of the history of natural sciences one can see that every major success in any science was preceded by the birth of some new instrument having direct bearing on electrophysics. At different stages there were electron microscopes, spectrographs, tomographs, high-accuracy time and distance meters, in short, one cannot list all of them. More perfect instruments opened ways to the previously unexplored domains and helped study the events, which later on became a basis for brand new engineering. In its turn, the latter promoted development of a next generation of instruments, and everything was repeated from the start, only time between these cycles reduced and periods became shorter. However, the matter is not only in scientific instruments. Today electrophysical instruments are used in many fields of human activity, they are a substantial part of production equipment and modern weaponry, space technology and household appliances.

The scope of any science is conditional, therefore electrophysics often encroaches on fields sometimes quite far from the designated framework, although there are subjects of research, which always stir up interest in scientists. They include superstrong electric and magnetic fields, dense energy flows, but special emphasis should be placed on high-speed processes in different media and plasma. Just like any substance consists of atoms and molecules, every phenomenon is composed of short moments, and their studies open up new possibilities and allows of finding means of deeper understanding of the world.

The problems of electrophysics are dealt with at the Institute of Electrophysics in Yekaterinburg, which marked its silver jubilee in November of 2011. From the very beginning its scientists and engineers set a goal to be among the leaders in this sphere, though it is not an easy matter: in science nobody can foresee in which field the next breakthrough will take place. Therefore, research of any scientist is based on long painstaking work accompanied by search for solutions, frequent failures and rare findings. It so happens that quite new branches of science and technology emerge in the process of development of operating principles of any simple device. The author of this article chose the well-known safety fuse as such a device, as it is difficult to find a person today who has never kept it in his hands.

A fuse or, as electricians call it, a fuse link, is a glass or ceramic pipe with metal contacts and a thin wire inside. The passing current heats the wire but an increased current melts the wire, opening the circuit, thus protecting the wiring from overheating and the house from fire. It is difficult to predict even roughly how many such simple components are now in operation. There are app. a dozen of them in one car and there are more than billion of cars in the world. A fuse is a mandatory element in every power unit of TV-sets, computers and charging units for cellular phones in case of short circuit. But pragmatic Englishmen use it even in every electrical plug! And though even today short-circuit-caused fires are not rare, it often comes to light that there was no sound fuse in the right place.

It would seem that no modern physics is needed in this simple device. Indeed, back in mid-19th century physicists James Joule in Great Britain and Emil Lenz (member of the Petersburg Academy of Sciences from 1830) in Russia derived a relation between a running current and heating of conductors, today known as the Joule-Lenz law. The higher is current, the sooner melts a wire and the faster opens an electrical circuit. At extreme values of the current, the wire evaporates

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so quickly that a temperature jump excites a shock wave, the speed of which reminds a phenomenon called an explosion. Besides, all its indications are evident: a bright flare of light, a heavy sound and a shock wave, so tangible that glass mainframes of fuses break into small pieces, therefore stronger ceramics is used for high-speed units.

In the course of time the theory of exploding conductors was developed. Its main relation--the product of squared current value by wire explosion delay time is a constant value for each conducting material. Easily-melting metals and their alloys turned to be the most efficient. Thus, making a fuse from available materials, we grossly violate the said relation, and such imitation will never be a full substitute. However, despite the fact that dozens of more efficient (and certainly more expensive) reusable protection devices are available today, a simple fuse is as before a mandatory element of any electric installations. Just to be on the safe side.

One would think that the studies of exploding conductors should be completed. But, as it often happens, many points of interest were revealed in the course of studies. For example, it was Benjamin Franklin himself who suggested setting fire to a powder charge with a wire heated by an electric current. But if we speed up the current's passage through a conductor, the speed of shock wave can exceed a sound speed and pass into a range referred to as detonation in physics. This is how modern safe detonators came into being. They include a wire pressed into a pellet of a hard-to-initiate explosive substance. They withstand impacts and merely burn down even in fire. They work only in the process of passing through a wiring of a powerful current impulse from the condenser.

But what happens with a wire after explosion? It turns out that it also depends on the speed of the current, wiring material and also a medium in which such explosion takes place. The material disintegrates in the form of vapor and drops, which on cooling down become nanoparticles, with a particle size less than micron. It is perhaps the most light-weight condition of any solid substance as a liter bottle contains barely 20-30 g of such powder. We can state that the material in such condition almost has no mass but has a large surface. Hence, unusual properties of nanopowders, especially their high chemical activity. It is necessary to store metal nanopowders in a preservative liquid or inert gas as they oxidize quickly in the air. Such fine particles penetrate easily through thin gloves and even skin of hands, so, the work with them requires extreme caution. Conventional respirators are of no help here,

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therefore we can only be happy for a while that the use of the popular now prefix "nano" in the names of many goods is no more than an advertising trick. Finally, any road or construction dust always includes some nanoparticles, the so-called nanofraction.

In practice nanopowders are used as additives or a material for pressing of parts including ultra-porous ones for superfilters and membranes. It is only natural that it needs nanopowders of different materials in big quantities. Their production commercially through wire explosion is rather expensive, therefore other technologies, mainly chemical, are used. However, nanopowders with unusual properties or already their finished mixtures of different materials can be obtained by electrophysical methods. Oxides as a finished material for nanoceramic items can be produced by exploding of wires in a specific gas medium, for example, in oxygen. The process of simultaneous exploding of wires made of different materials does not involve mixing, when it is difficult to guarantee homogeneity. Lately, at our institute, powerful optical fiber lasers and high-current electron beams are successfully used for substantial increase of productivity. They help evaporate small pieces of material on a massive target, and in this case it is not necessary to make a thin wire and replace it after every explosion.

Nanopowders are pressed for production of finished items. This process is called compaction, and it turned to be not an easy matter. Widely used mechanical and hydraulic presses are unfit for this purpose as inner friction forces of the powder and the strength of press moulds did not allow of achieving desired values of pressure and temperature. Again electrophysics came to the help as mechanical properties of potent pulse magnetic fields were well known in this science long ago. Magnetic pulse presses create the so-called soft compression wave under the action time of 10-1,000 microseconds. It allows to provide the required heating of the compressed mass at the expense of powder friction and avoid destruction of rigging. Already now such presses are used for production of superhard items from aluminum nanoceramics based on aluminum oxide nanopowder. As an illustration of such items we can name thin-walled tubes made from oxide ceramic material with a submicron structure of different hardness and porosity, for example, for super-filters or electrolytes of solid oxide fuel elements, the most important element of hydrogen power engineering equipment.

Electrical engineering also made use of properties of exploding conductors as high-speed current interrupters. It is well known that current breakage in an electrical circuit with inductance creates overvoltage, which is higher, the shorter is the time of breakage. All of us saw a spark in breaking contacts of a switch or in a tram current collector. It is just a result of air breakdown during overvoltage. Such spark can pass into gas discharge called an electric arc, the same arc, which is used in electric welding or a gas-discharge lighting lamp. But it is necessary to blow out the spark as quickly as possible, otherwise the electrical circuit will not open and contacts will burn down.

It would be fair to say that current breakage in the inductance coil is long used for generation of high

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voltage pulses. In old school laboratories we can find Ruhmkorff coils, high-voltage pulse generators developed by German inventor and mechanic Heinrich Ruhmkorff. The current is broken in them by a mechanical contact system. Generators with a similar operating principle were used in ignition systems of carburetor combustion engines. In that case, a driver had to clean regularly breaker points and regulate a gap, therefore they were replaced by noncontact semiconductor systems.

The problem of any interrupter consists in speed of a current breakage as the bigger the latter, the more time is needed for its switching out. The limited breakage speed does not allow to develop powerful devices based on inductive energy accumulators, which are simpler and app. 100 times more compact than capacitance condensers. It is here that quick time of current breakage of exploding wires proved useful. They were used in developing of pulse generators of more than 1 mln volts and hundreds of thousands amperes. Peak short-time capacities of such generators reach hundreds of megawatts and are comparable with corresponding characteristics of big electric power stations. However, as their obvious and major drawback should be pointed out a necessity of wire replacement after every pulse and removal of explosion products, unsafe as shown above. As a result, as usual in modern practice, semi-conductor interrupters replaced them. However, their emergence was preceded by an enormous work. After all, in case of the current breakage, the interrupter itself should withstand a sudden voltage jump. When using a wire, no special problems arose as there was a discharge generation time or the so-called current pause. But conventional semiconductor structures were broken down and destroyed already during the first shutdown. What's to be done?

The sudden current breaking effect on switching of high-voltage semiconductor diode structures called the SOS (Semiconductor Opening Switch) effect and discovered at our institute in 1992 became a basis for creation of efficient high-voltage interrupters. The authors of the effect are Yuri Kotov, RAS Corresponding Member, Sergei Rukin and Alexander Filatov, Drs. Sc. (Tech.).

It should be noted that the developers of powerful semiconductor rectifiers fought all the time with over-voltage in electrical circuits due to sudden current breakage. The solution was found by creation of special diodes with the so-called "soft", without sudden breakage of the current, switching out. Our institute used this harmful effect, which revealed that, in case of sequential switching of a large number of semiconductor structures, reverse voltage between them is distributed evenly when a breakage of the current takes place. It allows

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practically an unlimited increase in generator output voltage without complicated and cumbersome systems of voltage compensation inevitable during sequential switching of semiconductor diodes and besides, consuming an appreciable part of energy. The revealed phenomenon was used at once for generation of powerful pulses. Now the ultimate output voltage, average and pulse power of the SOS-diode devices were limited only by the performance of cooling systems.

There are already in operation completely semiconductor generators with output capacity up to 1 mega-volt and pulse-repetition frequency more than 1,000 pulses per second. High service life and performance reliability in any conditions are among their advantages. Such generators are used as "ozone factory" for air cleaning from toxic admixtures. The group of staff members of the institute was awarded the Russian Federation Prize for 2004 for a set of research and development of a new type of devices based on the SOS-effect. A leading role in this work belongs to the RAS Corresponding Member Yuri Kotov, who possessed a rare talent to bring all his works to practical application. Among his achievements we can name explosive electric detonators, nanopowder plants and powerful inductive accumulator generators.

If we do not plan to explode a whole wire, such form of an interelectrode gap can be suggested in principle when a cathode is made in the form of a spike, and we shall explode only its point. In this case, a circuit may close through an electron flow from the spike provided the voltage between a cathode and an anode is sufficiently high so that an electric field on the spike exceeds the value of 107 V/cm. Such field can over-

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come a force retaining electrons in solids under usual temperature, and it is sufficient for the so-called auto-electronic (cold) emission. The currents of this emission are small, just microamperes, but sections of microspikes, through which they pass, are also small. Therefore, density of the current passing through them reaches giant values, up to 108 A/cm2, and it is already quite enough for microspike explosion for a time of about 1 ns (billionth fraction of a second). By comparison, during this time light passes in the air a distance of no more than 30 cm.

How can such fields be obtained? It is well known that any electrode surface heterogeneity or roughness intensifies sharply an electric field. High-voltage electrodes are even subjected to polishing to raise insulation strength. But there are no perfectly smooth surfaces, and the related microphotographs made by electronic microscope show that even after optical polishing scores of microheterogeneities remain. Apart from them, the same role of "field intensifiers" can be played also by sharp edges of crystals, edges of thin contamination films, microdefects of materials or residues of abrasive powders used at metal working. That is why electric insulation strength of gas and vacuum gaps depends greatly on electrode condition.

It should be noted that it is not at all necessary to create heterogeneities on a cathode after every pulse. It is clear that exploded heterogeneities disappear, and the cathode surface is polished, thus increasing electric strength of insulation. Such phenomenon is called training and is widely used in creation of high-voltage vacuum and gas-discharge devices. But the opposite phenomenon was also revealed: if the current or time of its passing is increased during training, the cathode surface becomes more rough, and new microheterogeneities appear on it. This allows creation of vacuum and gas dischargers with stable characteristics and a long service life.

One more thing. In case of microspike explosion there forms a plasma bead (flame) on a cathode electrode. It scatters at a high (over 10 km/s) speed into a vacuum or gas gap between two electrodes, cathode and anode. This plasma is dense and hot (around 50,000°C), it not only shines brightly but is also a powerful source of charged particles, electrons. Such effect is called an explosive electron emission and was registered in 1976 as a discovery by a group of Siberian and Leningrad scientists headed by Academician Gennady Mesyats, founder, first director and research supervisor of the Institute of Electrophysics of the RAS Ural Branch, the present RAS Vice-President and director of the Lebedev Physics Institute.

The studies of the explosive electron emission is under way. It is just this effect that is considered finally responsible for emergence of electric puncture of gas and vacuum insulation, therefore it cannot be forgotten during creation of any high-voltage instruments. The plasma jet from a cathode flame has also found application. Its supersonic speed is an important indicator of a jet engine. After all, the higher jet velocity, the less fuel consumption at equal thrust--it is a direct way to increase a spacecraft life. For the time being, such engines exist in the form of mockups but specialists from the Research Institute of Machine-

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Building (Nizhnyaya Salda, Sverdlovsk Region), whose low-thrust jet engines work successfully at the International Space Station, are rather optimistic.

At present explosive emission is a basis of cathode work used in high-current accelerators. They are the so-called direct-action accelerators, i.e. a short voltage pulse from dozens of kilovolts to several dozens of megavolts is supplied to a gap between anode and cathode. This voltage corresponds to the maximal energy of accelerated electrons, which is usually measured in kiloelectronvolts or keV. Already under 100 keV electron velocity becomes close to the light speed. Thus, in principle, such accelerator needs a high-voltage source, adequate vacuum insulation of the accelerating gap and an emitter providing the required electron current from hundreds to several thousands of amperes. Naturally, apart from explosive plasma cathodes, no other emitters can provide such currents. By comparison, the most common hot oxide emitting cathodes (their action is based on the ther-moelectronic emission effect), known to us through electron tubes and kinescopes of old TV-sets, can provide current density only up to 100 A/cm2, besides, they work at the temperature of around 800°C and are very particular about vacuum conditions.

The question naturally arises: what are such powerful accelerators needed for? They are usually used as an instrument for action on different materials or generate electromagnetic radiation by powerful electron beams. It suffices to note a great discovery made by Nobel Prize-Winner Wilhelm Roentgen in 1895, as X-rays called by his name caused a revolution in medicine and not only in it. This phenomenon emerges at interaction of accelerated electrons with an electron shell of anode atoms, therefore such radiation is often called braking. The X-ray tube is one of the most-used direct-action accelerators. It is natural that the greater beam current, the more powerful radiation. Today medical and industrial X-ray tubes make use of "good old" hot cathodes. However, rapid radiography needs short but powerful pulses, and there explosive emission cathodes are indispensable and require no heating. Pulse X-ray apparatuses using cold cathodes have appreciably smaller sizes, consume less energy and can work from storage batteries. They are indispensable in field conditions, for example, in X-ray examination of oil and gas pipelines. Mobile or the so-called ward pulse X-ray apparatuses are already used for medical hospitals.

A number of different purpose accelerators were created at the Institute of Electrophysics of the RAS Ural

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Branch. The biggest of them are installed in special rooms equipped with high-power protection of personnel from harmful radiation effect. But there are also such accelerators, which can be easily placed in a small suitcase. First of all, high-current accelerators--an efficient device allowing research in different fields. In fact, short pulses possess high power plus relatively low energy, therefore, they allow to see a result, which cannot be obtained during a lasting action as the material is destroyed under thermal effect.

However, an electron beam is accelerated only in vacuum, and not every beam can be placed into a vacuum chamber. For this purpose there are electron tubes to take accelerated electrons out of vacuum to the air (their character to penetrate into materials is used). If electrons are accelerated to the energy of 150 keV, about a half of them will pass to the air through a 50 mem aluminum foil. The foil will be a reliable barrier for the air, which will not impede the work of the tube.

A pulse cathodoluminescent analyzer became one of the unusual methods of application of such accelerator and was developed at our institute under guidance of Vladimir Solomonov, Dr. Sc. (Phys. & Math.). In this facility, under the action of a high-power electron beam, all nonmetal materials, including exotic ones, in particular, diamonds, shine and luminesce. Each material has its inimitable luminescence spectrum, by which it is possible to determine not only the composition, for example, of the mineral, but also deposit, admixtures, etc. Luminescence is registered and analyzed by a spectrometer and passes to the computer, which compares the received spectrum with a database in its memory. It goes without saying that such analyses do not need highly skilled mineralogists. More than a dozen of such instruments already operate in Russian laboratories.

Among many high-power pulse instruments created at our institute there are X-ray devices for industry and medicine, accelerators for rapid surface sterilization of medical instruments and materials, and electromagnetic pulse sources. They successfully function at research centers and universities of 15 countries of the world. In 1998, a group of staff members of our institute was awarded State Prize for the development of the above instruments.

That is only a small amount of works carried out by the Institute of Electrophysics. The research is under way, and it is difficult even to foresee what findings are ahead for us. But everything began with a simple safety fuse.

The research was supported by grants of the Russian Fund of Fundamental Research: 09-08-00101; 09-08-198; 10-08-000814; 10-08-00517.


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