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Author(s) of the publication: Yevgeny SUVOROV

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by Yevgeny SUVOROV, Dr. Sc. (Phys. & Math.), Director of the Plasma Physics and High-Power Electronics Division, RAS Institute of Applied Physics (IAP), IAP Deputy Director for Research

The Institute of Applied Physics (IAP) of the Russian Academy of Sciences (RAS) is one of the world's largest laboratories involved with high-power sources of microwave radiation for practical uses in radiolocation (radar), plasma and nuclear physics, and in the industrial technologies for the production of new materials.

IAP is also concerned with ways of reception and evaluation of microwave radiation for remote diagnostics of the atmosphere, radioastronomical experiments and the like.

The IAP Plasma Physics and High-Power Electronics Division is the focal center of this work.

VACUUM RF ELECTRONICS

One dominant area of this work involves gyroresonance setups (superhigh-power generators of electromagnetic radiation on the millimeter and submillimeter wave-bands), now known widely by the name of cyclotron resonance masers (CRM)*. These use the


* Masers-quantum generators and RF amplifiers; cyelotron resonance-selective absorption (or emission) of electromagnetic energy by charge carriers within a magnetic field at frequencies equal to or multiple of cyclotron frequency, when a dramatic increase in electrical conductance is observed. - Ed.

стр. 38


A diagram demonstrating how a complex mode generated in the megawatt gyrotron resonator (bottom right) gives rise to a quality quasioptical wave beam (top right) by means of a system of profiled mirrors.

induced radiation of electrons spinning in a static magnetic field. In contrast to other sources of microwave RF radiation, CRM have an electron flow interacting with a high-frequency field in electrodynamic systems, their characteristic dimensions being high above the generated wavelength. A unique possibility of mode* selection allows to realize high-performance single-mode generation techniques.

The mechanism of radiation underlying CRM was discovered in the late 1950s by physicists of different countries almost simultaneously. Yet only one team led by Acad. Andrei Gaponov-Grekhov, one of the pioneers in this field, got down to the practical implementation of the conceptual idea in high-power electronics. Young radiophysicists Mikhail Petelin and Valery Yulpatov made a key contribution to basic research, while Drs Arkady Goldenberg, Valery Flyagin, Vladimir Zapevalov and Grigori Denisov did much in the way of practical application of high-performance devices.


* Mode - in the general case, harmonic oscillations in a system. The number of modes in an electrodynamic system is infinitely high, while the density of their frequency spectrum increases with a frequency increase (which is typical of superdimensional systems). - Ed.

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Cyclotron resonance masers (CRM) are now manufactured in different modifications: either as generators (gyrotrons and gyro-BWO, i. e. backward wave oscillators) or else as amplifiers (gyroclystron and gyro-TWT, i. e. the traveling wave tube). An impressive practical example is offered by the use of high-power gyrotrons on the short-wave millimeter band in systems of electron-cyclotron heating effected by controlled nuclear fusion (CNF) setups realizing magnetic confinement of the plasma. Such setups are capable of generating radiation of up to 1 MW at 40 to 50 percent efficiency in pulses of hundreds of seconds at frequencies ranging from 30 to 170 G Hz. The superhigh-power units are equipped with diamond output windows and recuperation (reuse) systems for utilizing electron beam residual energy.

Early in the 1990s a R&D enterprise, GYKOM, was founded at Nizhni Novgorod for industrial output of gyrodevices (GYKOM, the Gyro-Manufacturing Company). Working in close cooperation with IAP, it has emerged as one of the world's leading manufacturers of gyrotrons and fixings for magnetic confinement CNF setups: tokamaks* (including an international tokamak type reactor, ITER, slated for construction in France soon) and stellarators**. At present our physicists are working to upgrade hydrotrons in power and efficiency as well as on developing devices for staggered frequency shifting over a wide range, which will make it possible to expand sizeably their application domain in controlled fusion reactors.

High-energy amplifiers-35 and 94 GHz gyroclystrons (designed by Drs Yevgeny Sokolov and Yevgeny Zasypkin) have been developed for radar systems; these gyroclystrons provide for output power of hundreds of kilowatts in pulses of up to 100 microseconds at gain factors to 30 dB. But in gyro-TWT modified by Dr. Grigori Denisov the amplification band is significantly wider than it is in gyroclystrons. Proceeding from the results of initial experiments, upgraded models of gyro-BWO and gyro-TWT are being developed for commercial production.

Another dominant area in vacuum microwave electronics: the use of high-current relativistic electron beams for generation of superhigh-power RF radiation. This possibility was realized in the 1960s with the development of amplifiers where output megavolt electron beams had kiloampere currents.

The world's first microwave generator of coherent radiation excited by a high-current electronic amplifier was developed on the initiative of Drs Andrei Gaponov-Grekhov and Matvei Rabinovich during a joint experiment carried out in 1972 by the Radiophysics Research Institute in the city of Gorky (now Nizhni Novgorod) and the Moscow-based Lebedev Institute of the USSR Academy of Sciences. In power this relativistic version of the backward wave tube topped dozens of times corresponding facilities of this range then in use. That success story stimulated further work on microwave relativistic electronics at many laboratories the world over. At present devices of this class supply power 10 -10' W over the range of 1 - 100 GHz at 1 - 100 ns pulses. Our IAP Division is now on the cutting edge of this research, too.

Senior researchers of our High-Power Electronics Division (Dr. Mikhail Petelin and his pupils-Drs Vladimir Bratman, Naum Ginzburg, Grigori Denisov, Nikolai Kovalev and others) have analyzed the peculiar


See: V. Glukhikh et al, "On the Brink of Thermonuclear Era", Science in Russia, No. 3, 2003; L. Golubchikov, "Tokamak - International Challenge", Science in Russia, No. 1, 2004. - Ed.

** Stellarator - a closed magnetic trap for confining (containing) high-temperature plasma: unlike tokamaks, stellarators induce a magnetic field only by means of external coils. - Ed.

стр. 40


characteristics of stimulated (induced) radiation at relativists velocities of electrons. They have suggested a number of microwave generators and amplifiers subsequently realized at IAP jointly with other research centers and laboratories in Russia and abroad. For instance, backward wave oscillators have been developed in cooperation with RAS Institutes - Lebedev Physics Institute, Prokhorov Institute of General Physics, High-Current Electronics Institute (Siberian Branch of the Russian Academy of Sciences, Tomsk), Minz Moscow Radio-technical Institute, and Thomson Company (Italy-France). Line-focus electron beam generators now at the gestation stage hold out good promise since they will make it possible to up significantly the output power. The same is true of relativistic generators with subnanosecond pulses (setups based on the superradiation of extended electron bunches).

ELECTROMAGNETIC RADIATION AND PLASMA

Problems of the interaction of electromagnetic radiation with plasma, a perennial area of our work at IAP, hark back to the late 1950s and early 1960s. At that time Drs Gaponov-Grekhov and Mikhail Miller pioneered in the averaged description of charged particles moving in high-frequency electromagnetic fields (the concept "Miller force" has made its way into the literature here in Russia and elsewhere). Subsequently the concept was validated mathematically by means of averaged equations for field and plasma, thus allowing to address a broad class of processes of self-action and interaction of waves in plasma.

Acad. Alexander Litvak and his pupils (Drs Vyacheslav Mironov, Gennady Freiman, Alexander Sergeyev and others) prioritized in developing a theory of wave self-focusing in isotropic and magnetoactive plasma; they also pioneered in the study of penetration of hard radiation into dense (supercritical) plasma and of thermal parametric instabilities and self-action effects of relativistically strong electromagnetic waves. Dr. Litvak and coworkers (Drs Boris Yeremin, Yuri Brodsky, Alexander Kostrov et al.) conducted pioneering experiments on the self-focusing of electromagnetic wave beams in plasma, and they were the

стр. 41


Scheme of an electron-cyclotron resonance source of multiply charged ions (SMIS-37):

1 - discharge vacuum chamber,

2 - magnetic coils,

3 - input quarts RF window,

4 - diagnostic vacuum chamber,

5 - pulse valve of gas breeding-in,

6 - plasma electrode,

7 - multiply charged ions extractor,

8 - high-voltage insulators.

first to observe the effect of self-trapping of such waves in dense plasma.

This and other effects also determine to a great extent the nature of interaction of superhigh-power laser pulses and plasma, which is one of the most intriguing areas of present-day physics. Associated areas include laser-driven nuclear fusion, search for supercollider acceleration methods and for new sources of optical and X-radiation.

We attach particular attention to studies into the electron-cyclotron heating of plasma in toroidal systems. Already the first experiments that we carried out on tokamak T-10 jointly with the Russian "Kurchatov Institute" involving high-power millimeter gyrotrons (which we made at our IAP) demonstrated good prospects of this method of heating for larger setups of similar type.

Side by side with all that we are studying the physics of electron-cyclotron interaction of electromagnetic waves with thermonuclear plasma. An important milestone in this work was our system of the first harmonic electron-cyclotron plasma heating making possible radiation inputs into toroidal setups (Drs Litvak, Suvorov and Anatoly Freiman). Today this system of plasma heating and current-drive is employed in most CNF toroidal setups. Energy input profiles are calculated by a handy and effective ray-optics method with the use of geometrical optics in the three-dimensional inhomogeneous magnetoactive plasma adopted in all leading nuclear research centers of the world.

Years ago Dr. Mikhail Tokman and the author of the present article made a theoretical prediction concerning the stark nonlinear modes of electron-cyclotron heating when electrons can build up their energy considerably in one electron transit in the area of RF field localization. This hypothesis was confirmed in the 1980s by experiments at L-2 setups (Prokhorov Institute) and Alcator-C (Livermore, USA).

The higher reliability of gyrotrons coupled with a considerable increase in the time of generated pulses brought new achievements in nuclear fusion research, also in cooperative work with our partners in other countries. Thus in the mid-1990s, cooperating with the Institute of Plasma Physics in Germany, at the W7-AS stellarator, we could first register the H-mode of plasma confinement in the second harmonic electron-cyclotron heating. And earlier in the decade an impressive series of experiments was carried out at the ASDEX-U tokamak (Max Planck Institute, FRG) in suppressing magnetohydrodynamic (MHD) instabilities through the strictly localized heating of the electron component or current-drive. And in experiments at the Japanese LHD stellarator the first hour-long plasma discharges were obtained (2005); the discharges were sustained by high-power gyrotron radiation. All these results are important for attaining the rated parameters in the international tokamak reactor ITER. We are getting ready by developing continuous MW gyrotrons.

WHAT HAPPENS IN A GAS DISCHARGE?

A freely localized high-frequency discharge triggered by high-power focused electromagnetic radia-

стр. 42


tion is a novel research area in nonlinear physics. In fact, back in the 1970s and 1980s Drs Alexander Litvak and Vladimir Hildenburg conducted theoretical and experimental research for addressing general problems pertaining to the nonlinear dynamics of MW discharges; they and coworkers actually laid a groundwork for this new research trend. Carrying on this work in their experiments, research teams under Drs Anatoly Vikharev and Sergei Golubev determined the principal types of mechanisms responsible for discharge propagation and of structures thus formed depending on gas pressure and intensity of RF radiation. On this basis a theory of ionized instabilities of a discharge was evolved, instabilities responsible for its small-scale structure; also, fresh evidence was obtained on the nonlinear self-confinement of plasma density in it. This allowed to calculate the parameters of global discharge structures (Drs Vladimir Hildenburg, Vladimir Semenov, and Arkady Kim). Our IAP was the first to predict the effect of an adiabatic increase in the frequency of a high-power RF pulse which, propagating, ionizes the environment. The results of these studies have been used for the interpretation of experiments conducted at many laboratories in Russia and elsewhere. By now certain concepts and ideas deduced from research into discharge phenomena in RF fields have been translated with much success into microwave optics to be used in the description of the nonlinear dynamics of high-power laser-driven pulses.

NEW TECHNOLOGIES

Over the past few years radiation on the centimeter and millimeter wavelengths has been adapted for the production of new materials. We have made significant headway in understanding processes involved in the sintering of ultra-disperse ceramic powders and composites; in growing diamond films from the gaseous phase in plasma sustained by electromagnetic radiation; and in the formation of intensive plasma flows and high-current ion beams under the conditions of an electron-cyclotron discharges in magnetic traps.

Yuri Bykov's laboratory has developed and built specialized gyrotron complexes for technological uses such as high-rate sintering of composite and ceramic materials, express annealing of semiconductors, "welding" of heterogeneous dielectrics, and so forth). These 3 to 15 kW complexes operating at 24 - 30 GHz use "warm" solenoids or permanent magnets. Unique in their functional possibilities, such setups have been installed at leading research centers in China, Japan, France, Germany and the United States.

Using microwave sintering, fine-grained ceramic materials have been obtained. Their specimens based on metal oxide nanodimensional powders exhibit improved mechanical properties. For example, the crack and wear resistance of ceramics made of aluminum oxides with additives of oxides of magnesium and zirconium and stabilized by yttrium oxide is 30 to 50 percent higher than that of ceramics produced from microne and submicrone powders.

In these past ten years and so Dr. Anatoly Vikharev and coworkers have developed a technology of chemical vapor deposition of diamond films and disks. The polycrystal plates grown this way are remarkable for optical transparency and low dielectric losses; they are also characterized by high mechanical strength and high heat conductivity. Diamond disks, 75 mm across and up to 2 mm thick, are used for making output windows for radiation generated by high-power gyrotrons developed for plasma heating in nuclear fusion setups. A laboratory reactor version has already been designed for the deposition of diamond films in plasma driven by millimeter-wave radiation. Thereby we can increase the size of the grown plates and ensure a higher (5 to 7 fold) rate of diamond growth compared with that available in state-of-the-art setups.

The dense, highly nonequilibrium plasma produced in magnetic traps by RF radiation under the conditions of electron-cyclotron resonance in heavy gases at high specific energy inputs-formerly an obscure object of plasma physics-is of practical interest, above all as a source of soft X-radiation for high-resolution projection lithography and also as a source of beams of multiply charged ions for heavy particle accelerators.

In the mid-1990s Drs Sergei Golubev and Vladimir Zorin studied a new type of ion sources which they called pulse-driven "gasodynamic" ones, where beams of multiply charged ions with a unique combination of high current and brightness were obtained. Their results spurred to develop a new generation of electron-cyclotron resonance sources on the basis of higher-frequency radiation provided by state-of-the-art gyrotrons. We at IAP are also working on a short-pulse source of multiply charged ions of costly radioactive

стр. 43


Diagram of a flowing cyclotron maser in the earth's magnetosphere showing the formation of pulsed polar lights. Broad arrows show magnetic drift of electrons with anisotropic distribution of velocities (red, before generation, blue - after).

isotopes for the use in the international program of oscillations (spontaneous conversions) of neutrino (Beta Beam Project, CERN, Switzerland).

FROM EXTRATERRESTRIAL TO TERRESTRIAL PROCESSES

The start-out objective for research at the Astrophysics and Phisics of Cosmic Plasma Division (supervised by Acad. Vladimir Zheleznyakov) concerns interaction of plasma with electromagnetic waves. Its data are used in the theory postulating the origins of different components of electromagnetic solar radiation (Drs Valery Zaitsev, Yelena Zlotnik) and also for elucidating the phenomenon of solar flares, for analyzing the structure and dynamics of the solar corona, including coronal magnetic loops, electromagnetic processes in solar prominences, and solar wind.

Dr. Viktor Trachtengertz has done pioneering works on the cyclotron instabilities of the earth's van Allen (radiation) belts. Good progress has been made in furthering the theory of cosmic cyclotron masers determining the dynamics of radiation belts in the magnetospheres of solar system planets and their self-radiation. These objects have much in common with laboratory analogs, the electronic devices of the MCR type. The data thus obtained on classical cosmic plasma, besides purely astrophysical applications, are used in attacking practical problems of classical and quantum electronics as well as in semiconductor and liquid crystal optics.

Superstrong magnetic and gravitational fields as well as powerful hard radiation effect substantial changes in plasma characteristics near compact astrophysical objects - white dwarfs, neutron stars and "black holes". Electron motion is quantified there, vacuum magnetized, and radiation impacts the micro- and macrodynamics of particles. Therefore astrophysicists devote most attention to the interpretation of observable spectral and temporal characteristics of radiation emitted by these sources and diagnostics of physical conditions around compact objects (the domain of Drs Vitaly Kocharovsky and Vladimir Kocharovsky, who is RAS corresponding member).

High-sensitivity spectral receivers on millimeter and submillimeter frequency bands opened up possibilities for studying the ambient medium by methods of remote sounding, for this very range contains lines of rotation spectra of the principal gaseous components and of a large number of impurities, those of anthropogenic origin, too. Thus, right from the earth's surface we can determine the concentration of ozone, carbon and chlorine oxides, and other impurities at altitudes of 20 to 70 km. Our measuring complexes have made it possible to realize a large-scale program of studies on temporal and spatial variations of the ozonosphere of our planet in different geographical zones. For example, a mobile ozonometer designed by Dr. Yuri Kulikov's and Dr. Alexander Feigin's team has been tested with much success during the launch of the Proton rocket from the Baikonur spacedrome (2004), in the monitoring of the ozone layer (aboard the Captain Dranitsyn ice-breaker, 2005) in high latitudes; this ozonometer was also tested in railway car laboratory and in the Troika-9 expedition

стр. 44


Radio image of a mass star formation region in lines N2H+(1 - 0) (color) and CS (5 - 4) (contours). The significant difference in the distribution of these molecules may reflect chemical differentiation effects in the protostar (Δα and Δδ - relative coordinates in right ascension and declination).

in 2005 for monitoring the ozone layer in the middle latitudes, and also high in the mountains (Kislovodsk High-Mountain Laboratory of the RAS Obukhov Institute of Physics of the Atmosphere, 2006).

PEERING INTO THE MORROW

In conclusion we might as well name a few promising areas of research that we have begun but recently.

The terahertz (1012) range, which is intermediate between the radio and optical frequency range, is one of the least mastered today. It is in these frequencies that the "characteristic" lines of absorption of many complex organic molecules, DNA including, lie. This frequency range will enable us to identify their structural changes and exert controlling effects on them.

Another area deals with atmospheric electricity (the laboratory headed by Dr. Yevgeny Mareyev). It involves theoretical search, field experiments and modeling. Particular attention is attached to the monitoring of thunderstorm activity and methods of controlling it.

A team led by Dr. Renat Akhmedzhanov is studying interference effects arising in the interaction of multiple frequency resonance radiation with multilevel systems: in such practical applications as high-precision magnetometry, optical and in future quantum data management, an area that will produce on-line memory devices of record-high storage time.

A setup is being developed for laser-driven trapping and cooling of the neutral atoms gas to temperatures of the micro- and nanokelvin range. Experiments involving cryogenic gases are of interest in several areas of physics, for they can model processes in other systems out of reach for experimentalists: neutron stars, quark gas in "black holes", quark-gluon plasma in the earliest stages of the evolution of the universe. Apart from contributions to basic science, such studies may yield practical, hands-on applications, for one, with respect to compact and superhigh-precision time standards, quantum memory cells and computation algorithms for performing certain operations billions of times faster than present-day electronic computers do.

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Yevgeny SUVOROV, PLASMA PHYSICS AND HIGH-POWER ELECTRONICS // London: Libmonster (LIBMONSTER.COM). Updated: 25.09.2018. URL: https://libmonster.com/m/articles/view/PLASMA-PHYSICS-AND-HIGH-POWER-ELECTRONICS (date of access: 06.12.2021).

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