There is Only a Blink

Since childhood we are used to electricity in our life and benefit from it without giving it much thought. Though we do not notice it, electric power runs along wires, puts in motion electric engines, revives electric circuits, becomes released in the form of heat in electric ovens and tea-kettles, makes our rooms cozy, and allows us to be sustainable and self-confident.

Clicking a switch, we do not think of what is happening there at that moment, but we are annoyed if there are sparks in the switch or a bulb burns down. Taking off a sweater or caressing a cat, we can feel and hear sparks and think: this is static electricity.

In fact, it is only half-true. The term “static” means motionless and unchanged. The electric charge accumulated on the sweater or on the cat’s hair is static. Crackling of the sparks, on the opposite, is a fast and unsteady phenomenon. Both a small flash at the moment when a bulb or a fuse box burns down and spark discharges between electrified objects are processes in the form of short pulses. As a result of these pulses, the electric circuit usually transforms from one state to another: from the absence of current in the circuit to its flow or, vice versa, from a difference of potentials to their equalization. Such phenomena have a special name: transition phenomena. If the transition process is finalized in a short time, it is called a pulsed process.

Uncontrolled transition and pulsed processes in the systems of power production and consumption are hazardous. Meanwhile, there are many devices whose operation is based on the pulsed principle. Let us recall piezoelectric lighters, flash lamps, and car ignition systems.

An example of natural powerful pulsed processes is a thunderstorm.

In our everyday life, we are little worried about the duration of the transition processes when we switch on a bulb or by the duration of the spark “generated” by a cat or a sweater. We rarely think about the details of the thunderstorm discharge: seeing a lightning, most people feel the presence of an uncontrolled force.

Yet, there is a branch of science and engineering that studies all effects related to powerful electric pulses. Researchers have studied a wide range of pulsed electrophysical processes in detail and have learned to predict and control them. In addition, researchers have found out how to obtain electric pulses of a tremendous power.

Nanoseconds, megaamperes, and terawatts

Pulsed power deals with problems of generation and conversion of short powerful pulses of electric energy. The high-current electronics deals with high currents, as it follows from its name. Both disciplines supplement each other.

In conventional power engineering, electric energy is continuously generated and consumed. Pulsed power operates with pulses whose duration is several nanoseconds or microseconds (billionth or millionth fractions of a second). Electric energy is accumulated for a comparatively long time and then is released in a short pulse of a high voltage and current. The power of the largest pulse generators approaches 10¹⁴ W or 100 TW (1 TW = 10¹² W). For comparison, the total power of all electric power plants in the world is approximately 2 TW.

Currents generated by pulse generators reach tens of millions of amperes, and voltages can reach several million volts. The energy of a single pulse can reach tens of megajoules (for comparison, a body with a mass of 1 ton falling from a height of 100 m acquires a kinetic energy of 1 MJ).

To imagine the duration of pulses formed by high-current generators, we can add that light covers only 30 cm in vacuum during one nanosecond and 300 m during one microsecond, while microsecond pulses generated by high-current generators are considered in this field of science to be “long” pulses!

Up the stages of energy compression

All pulse generators have the following elements: energy storage devices, electric switches (devices for current initiation or interruption), devices for current or voltage conversion, lines for pulse transfer, and finally, load.

Electric energy is first stored in capacitors. As compared with usual accumulators, capacitors can ensure a much faster energy release, but the stored energy density is lower: in the best case, it does not exceed 100 kJ/m³. To have several megajoules of energy rapidly released, it is necessary to have a rather complicated capacitor several cubic meters in volume. The largest modern pulse generators are national-level facilities, and their creation requires drawing on intellectual and material resources at the national level.

High voltages are obtained by various methods. For instance, it is possible to take n capacitors and charge each capacitor to a comparatively low voltage. After that, all capacitors are connected in series with the use of special switches. As a result, the voltage increases by a factor of n. This is the operation principle of the Marx generators. Another option is to use pulse transformers.

A commutator is essentially a switch, more exactly, a closing switch (in pulsed power engineering, principally different devices are often used to close and open the electric circuit). In a usual switch, we simply close metallic contacts. In this case, however, we can obtain a microsecond switching time in the best case. It is impossible to ensure mechanical contact of massive high-current electrodes within nanoseconds. Therefore, the discharge in powerful switches is organized between motionless electrodes. Advanced dischargers can reliably ensure megavolt voltages and megaampere currents.

Two energy storage devices connected to each other by means of a switch and a transformer are called the stage of compression of electromagnetic energy. At each next stage of compression, the electric pulse duration decreases, and its power increases. Different stages of energy compression in a pulse generator can operate on the basis of different physical principles. It is possible, for instance, to discharge a set of capacitors and “pump up” the current in the circuit possessing a certain inductance, and then rapidly break this circuit. In accordance with the electromagnetic induction law, a high-voltage pulse is generated at the breakdown point.

And now, how can the formed pulse be delivered to the point of its application (load)? This procedure becomes a problem at super-high powers. Ordinary wires are not suitable for this purpose: in the case of short pulses, they become sources of electromagnetic radiation, energy losses, and extremely strong interferences. Powerful pulses are transported along closed-type transmission lines. These lines have to endure, without breakdown, pulse voltages up to several megavolts. To understand their structure, we can imagine a coaxial TV cable with its cross section increased by a hundred times.

In transporting a short electric pulse, it is important to retain both its energy and its shape. Therefore, the medium insulating the line, on the one hand, should be strong in the electrical aspect and, on the other hand, should not have high dispersion, i.e. dependence of electromagnetic wave velocity on their frequency. Substances possessing low dispersion in the nanosecond range are liquid dielectrics, such as transformer oil. The minimum dispersion is observed in low-density media (gas and vacuum). Gas, however, is a good insulator only when it is compressed to high pressures, while vacuum possesses excellent insulating properties. In addition, the use of vacuum in large-volume facilities ensures better safety and is often technically simpler. Therefore, transmission lines with vacuum insulation have enjoyed wide application in pulsed power.

Vacuum, however, also has its limit of electrical strength! Experiments performed by the mid–1960s on breakdown of vacuum gaps clearly demonstrated that the properties of electrodes bounding this gap play an important role in this phenomenon. Nevertheless, the mechanism of electric breakdown in vacuum has been a puzzle for a long time. This issue will be discussed later.

Looking for “extreme”

A high-current pulse is a desired tool for a researcher involved in studying matter under conditions of extremely high densities of input energy. The knowledge of properties of a matter under high-energy actions has become especially important owing to the development of nuclear power engineering and research in controlled nuclear fusion as well as for creating new types of weapons.

What is the simplest way to impact the energy of a powerful electric pulse into matter? Let us close the gap between two electrodes with a thin-walled metallic cylinder or a set of thin wires or just inject a gas. When a high-current pulse is applied to the gap, the substance evaporates or the gas is ionized. The so-called plasma liner is formed, and the current further flows through it. A strong magnetic field affects the charged particles moving in the plasma and compresses the liner toward its axis. As a result, the plasma layers collide at the axis, and their kinetic energy transforms to thermal energy. This phenomenon is called the Z-pinch (from the English word pinch, which means squeeze, and the letter Z usually indicates the axial direction in cylindrical symmetry problems).

If a wide hollow cylinder is replaced by a narrow solid metal cylinder, its compression can generate pressures up to tens of millions of atmospheres! At such pressures, the matter density is three to four times greater than the density of the initial solid (which is considered practically incompressible in the course of school physics). The theory predicts that this state of matter, which has not been yet given a name, can have fairly unexpected properties. We can add that the only place where the matter is in a similar state, apart from laboratory experiments, is the interior of a nuclear explosion and the cores of some stars.

Another area of application of Z-pinches is pulsed radiography. At the moment of the most intense compression, the liner substance generates a powerful pulse of soft X-rays. The use of such X-radiation flows offers a unique opportunity to peep inside dense short-lived physical objects (e. g., inside the nuclear explosion). One more application of powerful X-ray pulses is the radiation tests of various devices and equipment.

From the very beginning, the study of Z-pinches was aimed at solving the most important problem of the humankind: obtaining thermonuclear energy. It is known that there are two methods for realizing controlled (or, at least, “dosed”) “thermonuke.” The first method implies heating and confinement of deuterium-tritium plasma during a considerable time – tens of seconds. This is the basic operation principle, for instance, of a tokamak – a toroidal facility for magnetic confinement of plasma.*

The second approach to obtaining thermonuclear plasma is the pulsed procedure. It implies energy input into a millimeter-sized deuterium-tritium target within such a short time that the thermonuclear reaction occurs before the substance heated to tremendous temperatures becomes scattered. The short time here means a period of the order of 10 ns. For the energy released in the reaction to be substantially greater than the input energy, the energy input to the target should be several hundreds of kilojoules. In addition, the energy input and target compression should be spherically symmetrical.

The best option is to heat the thermonuclear fuel by a powerful pulse of X-ray radiation inside a special cavity also having a millimeter size. In turn, there are two effective methods of heating the cavity walls to “X-ray” temperatures.

First, it is possible to use powerful laser pulses with megajoule energy. There are several research laser-based thermonuclear facilities under development in the world. The two most powerful facilities are NIF in the USA (which is already at the stage of commissioning) and LMJ in France. The Institute of High-Current Electronics SB RAS participated in the development of pilot pulsed sources for optical pumping of lasers of the LMJ facility.

The other method of X-ray heating of the target is based on using the Z-pinch. In this case, the target is placed inside the liner. Energy utilization in this approach is much more efficient than in the laser-based approach. Its implementation, however, requires a large-scale electrophysical facility as well. The estimated current through the Z-pinch should be 60—70 million amperes. A project of such a super-generator is developed at the Sandia National Laboratories (USA) with active involvement of the Institute of High-Current Electronics SB RAS. IHCE is responsible for the development of LTD-stages, which are basic modules for linear pulse transformers. Hundreds of these transformers will be used in a facility with a power of 1 petawatt (1 PW—10¹⁵ W).

Born in a microexplosion

Let us now consider a physical object that gave its name to the second part of “high-current electronics,” namely, an electron. Immediately after creation of the first powerful pulse generators, attempts were made to use high-current high-voltage pulses to form electron beams.

It is not difficult to form such a beam. It suffices to apply a high-voltage pulse to the vacuum gap between two electrodes, with one of them (cathode) emitting electrons, to obtain a flux of accelerated electrons on the anode; the kinetic energy of these electrons corresponds to the voltage applied. A nontrivial problem, however, was the development of effective sources of electrons: high-current emitters. Many as they are, none of the known types of electron emission could provide the value of current comparable with that produced by pulse generators (4 to 5 orders higher).

The method of obtaining powerful electron beams was suggested by nature, and it happened in the field where the researchers of pulse generators encountered a serious problem: breakdown of vacuum insulation. This method was developed in the mid-1960s after the team of researchers headed by G. A. Mesyats performed unique experiments and proved the mechanism of electric breakdown in vacuum. The new mechanism of emission of electrons, which was called the explosive emission, was officially registered in 1976 as a scientific discovery.

The essence of the explosive emission phenomenon is the thermal explosive destruction of microscopic nonuniformities of a metal within nanosecond-scale times under the action of a strong electric field generated near the metal surface in vacuum. These microexplosions form a dense plasma, from which electrons are extracted under the action of the electric field. Plasma is the most perfect emitter created by nature. The current density of explosive emitted electrons can be extremely high.

Cathodes with explosive emission had made it possible to generate electron beams with the current strength that could not be reached previously and is completely consistent with the capabilities of pulse generators. Powerful pulse lasers, X-ray tubes, and accelerators of charged particles were created on the basis of generators with such cathodes.

High-current beams of accelerated electrons can be used to generate a powerful hard X-ray radiation. Technically, this is rather simple: it is sufficient to decelerate such a beam on a target made of a dense material. The thus-generated electromagnetic radiation is called the bremsstrahlung. The greater the charge of the nucleus of the target atoms and the higher the energy of electrons in the beam, the higher the efficiency of bremsstrahlung generation.

High-current generators with explosive emission cathodes were used to create a family of pulsed X-ray sources featuring a large variety of power and size: from portable devices to “monster” stationary devices that can be used for radiation tests of large engineering objects.

The most extensive area of application of all electron beams is the generation of electromagnetic oscillations in the radio-frequency and microwave ranges. The higher the electron energy, the higher frequencies can be reached owing to relativistic effects. The use of high-current beams in microwave electronics made it possible to increase the peak power of radiation by several orders and to pass from the metric radio waves to the range of centimetric and millimetric wavelengths. A new research field was formed: relativistic high-frequency electronics.

The power of modern microwave generators reaches several gigawatts. This is greater than the power of household microwave ovens by a factor of several million and greater than the power of continuous-operation microwave generators used in TV and radio broadcasting by a factor of tens of thousands. Nevertheless, it is not that easy to bake a chicken using a repetitively-pulsed relativistic generator: its mean power rarely exceeds one kilowatt.

One of the basic applications of repetitively-pulsed microwave generators is radar devices. A small pulse duration combined with a high peak power allows the distance to the target with a detection range of 100—200 km to be determined with an accuracy to one meter, and the high pulse repetition frequency allows effective distinction between small moving objects and large motionless objects.

Another area of application of powerful microwave generators is testing of electronic equipment. Rarely can an electronic device maintain its workability after irradiation by an electromagnetic wave easily causing breakdown in the air!

The high-current electron beam was helpful in solving one more problem. There was a breakthrough in the field of laser engineering in the 1970s: powerful lasers based on a volume electric discharge in a pressurized gas were developed. What is the principal difficulty in creating such lasers? To ensure laser radiation, one needs a nonequilibrium medium. Such a medium could be produced by organizing volumetric conduction of current in the gas discharge. Alas! This shape of discharge was stable only in long tubes with low pressures of the gas. All attempts to increase the pressure failed: the spatial form of current passage turned to the channeled form, where generation of laser radiation is impossible.

Accelerated electrons can effectively ionize the medium through which they pass, including high-pressure gases. The higher the electron beam intensity, the higher the volume conductivity of the resultant gas plasma and the higher energy can be imparted to the active medium of the laser. Long-term studies made it possible to construct systems with record-beating values of energy and power of laser radiation.

Finally, let us briefly consider technological applications of powerful pulsed devices. The number of electron-ion-plasma technologies is increasing now in an avalanche manner. Production of nanostructural materials occupies an important place here.

A promising field is industrial application of technologies of electron-beam modification of metal surfaces. Irradiating the metal by a short powerful beam, it is possible to ensure instantaneous high-quality polishing of the surface having an extremely complicated shape, which is next to impossible to perform by mechanical methods. Melting under the action of the beam and instantaneously cooling down, a thin (several microns) layer of metal is cleaned from impurities and acquires a nanocrystalline structure possessing high hardness and corrosion and wear resistance. A pulse electron beam can cope even with brittle super-hard alloys difficult to process. Using combined beam-plasma methods, one can create surface alloys whose composition and properties seem to be impossible from the viewpoint of conventional metallurgy.

Based on long-term research of the low-pressure gas discharge, the Institute of High-Current Electronics SB RAS developed effective sources of plasma of both gases and many metals. Using such sources, it is possible to ensure rapid modification of the surface layer of metallic articles (e.g. nitride hardening) and apply thin coatings onto the surface for the latter to obtain required functional properties. Moreover, it has turned out that thin strong films can be applied even onto glass and plastic.

High-current pulse electron beams demonstrated their efficiency in technological processes of solidification and modification of varnish coatings and rolled polymers, sterilization of medical equipment and powder materials, in plasmochemistry, and in emission mitigation. Based on luminescence under the electron beam, it is possible to identify many jobbing and precious stones. The electric explosion of conductors makes it possible to obtain nano-sized powders.

References
Bugaev S. P., Kreindel Yu. E., Shchanin P. M. Large-Section Electron Beams.– Energoatomizdat, Moscow, 1984.
Korolev Yu. D., Mesyats G. A. Physics of Pulse Breakdown in Gases. – Nauka, Moscow, 1991.
Kremnev V. V., Mesyats G. A. Methods of Multiplication and Transformation of Pulses in High-Current Electronics. – Nauka, Novosibirsk, 1987.
Mesyats G. A. Pulsed Power and Electronics. – Nauka, Moscow, 2004.
Mesyats G. A. Cathode Phenomena in a Vacuum Discharge: The Breakdown, the Spark and the Arc. – Nauka, Moscow, 2000.
Mesyats G. A., Ivanov S. A., Komyak I., Peliks E. A. Powerful Nanosecond X-Ray Pulses. – Energoatomizdat, Moscow, 1983.
Mesyats G. A., Osipov V. V., Tarasenko V. F. Pulsed Gas Lasers. – Bellingham: SPIE Optical Engineering Press, 1995.
Thus, we see that high-current pulses and electron beams are not only interesting research objects and tools for basic research, and they not only assist in solving problems of defense and peaceful “thermonuke” for the distant future, but also are ready to work for the benefit of national economy: invisibly, hard, and effectively.

* SCIENCE First Hand, No. 2, 2005 (Rus., Engl.), “Star Reactors” by E. P. Kruglyakov