Libmonster ID: RU-17233
Author(s) of the publication: Vitaly MIKHAILIN

by Vitaly MIKHAILIN, Dr. Sc. (Phys. & Math.), head of the department of physical problems of quantum electronics, Skobeltsyn Research Institute of Nuclear Physics, head of the optics and spectroscopy chair of the physics department, Lomonosov Moscow State University

At present synchrotron radiation is widely used in all domains of science, which deal with radiation-substance interaction. This subject is studied intensively by a research team of the department of physical problems of quantum electronics at the Skobeltsyn Research Institute of Nuclear Physics of the Lomonosov Moscow State University. But this research started more than 50 years ago.

BLUE LIGHT

Discovery of synchrotron radiation was predicted in the spring of 1944 by Dmitry Ivanenko and Isaak Pomeranchuk (Academician from 1964), both Drs Sc. (Phys. & Math.). Addressing a theoretical physics seminar at the Lomonosov Moscow State University (LMSU), which discussed the maximum energy of electron acceleration in betatron (cyclic induction accelerator*), they contemplated that limitation of this characteristic was caused by gyrosynchrotron radiation with a capacity proportional to a biquadrate of accelerated particle energy. Though

* Cyclic accelerator is an accelerator of charged particles, where they move along orbits close to circular or spiral ones passing many times through one and the same accelerating electrodes. -- Ed.

стр. 36

not all those present agreed to such reasoning, the authors of that hypothesis sent articles with presentation of their ideas for printing, which were published in the same year in the magazine Proceedings of the USSR Academy of Sciences and Physical Review (USA).

But soon the American physicist John Blewitt showed that the theoretical prediction by Ivanenko and Pomeranchuk was confirmed: on reaching of the maximal energy by electron ~100 MeV radiation reduction of its orbit radius is observed, and a particle falls on an internal target. The attempts by Blewitt to record visually this radiation and also to identify it in a microwave band proved to be unsuccessful. But his young colleague and a staff member of the same laboratory, the engineer Floyd Haber turned to be more lucky. In April of 1947, when he carried out a check-up of a 80 MeV synchrotron glass chamber*, he removed a part of opaque metallized coating. But when the accelerator was switched on again, the glaring blue light emitted by electrons escaped from the chamber to the laboratory.

In such a way radiation of relativistic (close to light speed) electrons called synchrotron radiation was observed for the first time. A year later Dmitry Ivanenko

* Synchrotron is one of the cyclic resonance accelerators. Its main characteristic is that in the process of acceleration of particles, a beam orbit remains with a fixed radius, while the leading magnetic field of rotating magnets, which define this process, increases. Besides, frequency of an accelerating electric field remains fixed. -- Ed.

стр. 37

and Arseny Sokolov, Dr. Sc. (Phys. & Math.), the then head of the LMSU physics department published in the Proceedings of the USSR Academy of Sciences the paper "On the 'Luminous' Electron Theory", containing calculations of related angular and spectral response curves. The follow-up research of our theoreticians laid a foundation for a school of the LMSU physics department in this sphere. As a result, a bibliographical list of works by LMSU physicists on the study of synchrotron radiation and its application numbers at present more than 1,200 publications, dozens of doctoral and hundreds of Ph.D. theses.

The very first experimental validation of "the luminous electron theory" was carried out in our country in 1956 by Yuri Ado (later Dr. Sc. (Phys. & Math.) and Pavel Cherenkov, Dr. Sc. (Phys. & Math.), the 1958 Nobel Prize-Winner, Academician from 1970). Later on, studies of synchrotron radiation properties was conducted by LMSU theoreticians and experimentalists (Igor Ternov, Dr. Sc. (Phys. & Math.), Oleg Kulikov and Alexei Yarov, both Cands Sc. (Phys. & Math.) et al. together with the laboratory of high energy electrons of the Lebedev Physical Institute (FIAN): Mikhail Yakimenko, Dr. Sc. (Phys. & Math.), Yuri Alexandrov, Cand. Sc. (Phys. & Math.) et al.). These studies were supported by Academician Dmitry Skobeltsyn, FIAN director. In 1967, due to his help, the first in the country spectroscopic channel was constructed on the 680 MeV synchrotron.

Let's turn back to the 1940s. In 1948, Alexander Prokhorov, Cand. Sc. (Phys. & Math.), one of the founders of quantum electronics, the 1964 Nobel Prize-Winner, Academician from 1966, carried out a series of successful experiments involving studies of coherent properties of radiation of relativistic electrons, moving in a uniform magnetic field. He proved that synchrotron radiation could be used as a source of coherent radiation in a centimeter band, identified characteristics and a level of a source capacity. This research was a subject of his doctoral thesis, which he defended in 1951. It is pertinent to note here that with the aid of Academician Skobeltsyn, Prokhorov together with a group of young employees from the FIAN laboratory of oscillations created a national school of radiospectroscopy in a short period of time. This group included also a graduate of the Moscow Engineering Physics Institute Nikolai Basov (together with Alexander Prokhorov he was one of the founders of quantum electronics, 1964 Nobel Prize-Winner, Academician from 1966). But we shall discuss this research

стр. 38

Circularorbit accelerator diagram.

1--injector,

2--vacuum chamber,

3--accelerating gaps,

4--magnet quadrants,

5--bundle of electrons.

work later on and now describe properties of synchrotron radiation.

Its operating principle is emission of electromagnetic waves using an accelerated charge. In a cyclic accelerator it moves almost at the light speed around a circle in a magnetic field, practically uniform along a particle trajectory. In such conditions a relativistic electron becomes a powerful source of electromagnetic radiation.

Let us consider briefly a design of such synchrotron. From the injector (as a rule, it is a linear electron accelerator or microtron) preliminarily accelerated particles get to a circular orbit. Their "capture" into a synchrotron acceleration regime is possible upon achieving by them of relativistic velocities. They are kept in a circular orbit by a field of rotating magnets, which increases as electron energy rises (its limit in betatron is approximately 300 MeV).

Synchrotron, like betatron, relates to cyclic accelerators but has a number of advantages in comparison with betatron. Usually its circular chamber is divided into 4 parts (quadrants), and rectilinear spaces are formed between them. In one of them a resonator with a variable electric field is installed, where electrons are accelerated almost to the light speed. Synchrotron advantages are essential as magnets, in contradistinction to betatron, are installed only in curvilinear sections of the trajectory, and synchrotron radiation energy loss is compensated. Therefore, the limit of attainable energy is determined by linear dimensions of the accelerator, magnetic fields and synchrotron radiation losses.

The main capacity of synchrotron radiation is concentrated in a hard part of the spectrum, i.e. vacuum ultraviolet and X-rays, and just this wavelength range is important for practical application of synchrotron radiation. In addition, two characteristics of the latter are essential--an aperture angle and wavelength of the maximum value.

The afore-mentioned Oleg Kulikov and his coauthors were pioneers in studies of polarization angular characteristics of synchrotron radiation. Using the 680 MeV synchrotron at FIAN they obtained photos of angular distribution of synchrotron radiation intensity in components of linear radiation polarization* for 250 MeV electron energy. The experiment in line with the theory has proved that a component of linear polarization with an electric vector, perpendicular to an orbit plane, has a typical angular distribution with a minimum in the plane of the latter. A component with an electric vector, parallel to an orbit plane, has maximum in its plane. Directly in it, radiation is almost fully linearly polarized. By "cutting out" radiation in the orbit plane, we can obtain linear polarization, reaching 98 percent. The degree of linear polarization averaged over all angles and wavelength reaches 75 percent. These factors are very important for studies of synchrotron radiation characteristics.

The time structure of synchrotron radiation is connected with a machine type being its source. Acceleration cycle on synchrotron, as well as pulse packets of synchrotron radiation, repeats, as a rule, with frequency

*Linear polarization is a state of a propagating electromagnetic wave (for example, light wave), in which its electric vector, in every point of space occupied by the wave, when oscillating remains all the time in the same plane passing through a direction of wave propagation. -- Ed.

стр. 39

Angular pattern of relativistic electron radiation in a circular orbit.

1--orbit, 2--radiation direction, D--observation point.

Angular pattern of accelerating electron radiation.

a--nonrelativistic electron,

b-longitudinally accelerating relativistic electron,

c-transversely accelerating relativistic electron.

of 50 Hz. Electron bunch length in orbit determines duration of this minimum pulse, it reaches hundreds of picoseconds.

Storage units are next after synchrotrons but they are already a specialized source of the said radiation. In them electrons exist in orbit for hours. It is important here to take into account a bunch length reaching several centimeters (duration up to 100 picoseconds), the number of bunches in orbit and electron inversion frequency.

Storage units as sources of synchrotron radiation have important advantages over synchrotrons. They provide for a long-time utilization of monoenergetic, i.e. possessing equal kinetic energy, electrons, accumulation of a great number of particles in orbit, a lesser section of an electron beam, a higher vacuum (10-9 torr), a lower radiation background around the plant, which allows to place research equipment in close proximity to a synchrotron radiation source, etc. Synchrotrons as sources of synchrotron radiation played a key role in the studies of the latter as they were used for its detection, studies of its main characteristics and conducting (up to now) of experiments for its utilization. But tomorrow is, of course, with storage units as it is just on their basis that specialized sources of synchrotron radiation are created and being developed.

UNDULATOR RADIATION

In 1947 the future Academician Vitaly Ginzburg, 2003 Nobel Prize-Winner, when studying the problem of creating rather powerful and reliable microwave band oscillators, turned attention to possible radiation by relativistic electrons during their movement in systems with a periodic alternating magnetic field. The problem considered by him turned out to be a very successful mock-up of future electromagnetic radiation generators called undulators. This term is first met in the works by British physicist Henry Motz who put forward in 1951 an idea of magnetic undulator, in which electrons pass a train of magnetic fields of different polarity. The first experiments carried out in 1953 using an instrument created in the USA involved observations of undulator radiation in the microwave range and the region of visible light. The radiation was generated at passing of relativistic electrons through the instrument with electrons accelerated by a linear accelerator to energy of 120 MeV. This is how a new macroscopic light generator radiating visible light came into use.

стр. 40

Estimated emitting powder of synchrotron radiation (W) depending on the wavelength at different electron energy.

At first undulatory radiation, similar to synchrotron one, drew no special attention. It was largely attributed to the fact, that the instrument used for this purpose was considered mainly as a radiation source in a millimeter-wave range. Nevertheless, the first successful experiments encouraged further research. The theoretical research in this field was carried out in the USSR by scientists of the LMSU physics department, the Institute of Nuclear Physics of the Siberian Branch of the USSR Academy of Sciences (Novosibirsk), the Lebedev Physical Institute, the Tomsk State University and also the Yerevan Physical Institute, and resulted in development of a rather complete theory of the problem at hand.

The first observations of radiation produced by the undulator built in a cyclic accelerator chamber were carried out in our country. In 1977 an employee group from FIAN headed by Academician Pavel Cherenkov and the LMSU physics department revealed radiation using the FIAN Pakhra synchrotron, which accelerated particles to energy of 1.2 GeV. They also obtained the first photos of a new radiation type and studied its spectral and angular characteristics and the so-called quasi-monochromatic effect. The latter implies that undulatory radiation contrary to synchrotron radiation has a different wavelength depending on the angle. The history of visual observation repeats itself: like synchrotron radiation revealed first in 1947 undulatory light broke out from an accelerating chamber window and declared its existence.

A notification of the achievement by the Soviet physicists at the International Conference on synchrotron radiation in Orsay (France, 1977) aroused a keen interest: new opportunities opened up for experimental studies of thes new radiation properties and ways of its application were laid down (alongside with synchrotron radiation) in physics, chemistry, biology and engineering processes. Undulators won steadily recognition as instruments necessary in all sources of synchrotron radiation, which expanded their efficiency and capabilities of experiment. In recent years they assume great and independent importance owing to implementation of the coherent radiation oscillator program based on free electrons. The new stage implies actually "the second" of undulators, in this regard the synchrotron radiation using rotating magnets is relegated to the background.

стр. 41

Flat undulator and its radiation.

Free-electron lasers represent physics of the future. Amplification (or generation) of coherent electromagnetic radiation by free (not bound in atom or molecule) relativistic electrons takes place in these macroscopic instruments. One method to implement such laser is induced radiation of electrons in the undulator, in which case the amplifying wave propagates in the direction of forward motion of electrons moving at a relativistic velocity. Perspectiveness of free-electron lasers is stressed by an ability of smooth tuning of instrument frequency in a broad range by a simple change of characteristics such as particle energy and magnetic field intensity and also by a rather simple control of electromagnetic wave polarization.

Thus, the properties of undulator radiation proved to be so attractive that the instrument proper comes to the forefront as a new independent radiation source and a special tool of physical research. In this context, the functions of both storage unit and accelerator also change--now they play a supporting role as sources of fast electrons necessary for undulator operation.

CREATION OF QUANTUM ELECTRONICS

The Research Institute of Nuclear Physics (RINP) of the Lomonosov Moscow State University was one of the first scientific centers in our country where development of quantum electronics as a new territory of physics started in the second half of the 1950s. In 1954, on the suggestion of Academician Skobeltsyn, who was at one time director of FIAN and RINP, Alexander Prokhorov, head of the oscillations laboratory at FIAN, set up at RINP a new scientific unit dealing with some problems of nuclear physics by radiospectroscopy methods. It was planned to apply the method of electronic paramagnetic resonance for studies of compounds enriched with special stable and radioactive isotopes to identify yet unknown nuclear constants.

As early as 1955 in the newly established laboratory experiments started on the equipment created by its first workers and students. Such quick development of work was explained by close and fruitful cooperation with the FIAN oscillations laboratory where by that time considerable methodical and technical experience in radiospectroscopy research was accumulated. It should be noted that later this cooperation led to the continued up to now interaction with a number of scientific units originated from the oscillations laboratory of the Institute of General Physics of the USSR Academy of Sciences (later RAS) headed since its establishing by Academician Prokhorov.

Setting up of the spectroscopy laboratory at RINP was simultaneous with the birth of quantum electronics. It was connected with experimental development of molecular oscillators (masers) with an ammonia molecule beam as a working medium at the Columbia University (USA) and FIAN in 1954-1955. The next step in development of this new territory of physics was marked by creation of quantum paramagnetic microwave amplifiers, i.e. solid-state masers with an extremely low level of their own noise and therefore record sensitivity. Their operating principle is based on electronic paramagnetic resonance and needs low (helium) temperatures. As the spectroscopy laboratory had actually all things needed at that time, it was there that, under command of Prokhorov and in cooperation with the FIAN oscillations laboratory, the first in our country and one of the first in the world operating laboratory models of quantum paramagnetic amplifier was created in 1958.

A prominent place among active materials on whose base quantum amplifiers and oscillators of radio-frequency and optical ranges (masers and lasers) is held

стр. 42

by the so-called "impurity crystals" containing isomorphous (i.e. "built-in" in place of some ground atoms of matrix) impurity ions of transition groups of Mendeleev's periodic law. Study of impurity crystal properties and their intrinsic physical processes by methods of radio-frequency and optical spectroscopy allows identifying not only their suitability in principle as active materials but also predicting basic characteristics of quantum-electronic instruments thereon and finding ways of improving them.

Beginning from 1958 over a period of almost ten years the RINP spectroscopy laboratory in cooperation with FIAN and a number of other institutes participated in research and development, which resulted in creation of a series of quantum paramagnetic amplifiers operated on a different wavelength range. They are utilized in deep-space communication systems and radar astronomy and gained high recognition. Besides, the corporate authors represented by workers of RINP (including Georgiy Zverev and Leonid Kornienko, both Drs Sc. (Phys. & Math.)), FIAN and other organizations were conferred USSR state prize in the field of science and technology in 1976.

The basic research of crystals with impurity ions of transition groups of iron and rare earths carried out at the RINP laboratory in the 1950s-1960s was a noticeable contribution to solid-state physics, radio spectroscopy and basic physics of quantum electronics.

Research in laser physics started also at that time, when the first ruby laser was launched at the laboratory in 1962. It was used in creation of pulsed paramagnetic millimeter-wave (up to 1 mm) oscillators with optical pumping. Besides, the technique of strong (up to 10 Tesla) pulsed magnetic fields was also mastered.

Next followed a series of research in laser oscillation on a number of the first synthesized crystals. Among them there is a laser emitter on Nddoped lithium niobate crystal (LiNbO3:Nd3+). Lithium niobate crystals (pure) were used extensively in quantum electronics for getting of harmonic curve and laser beam modulation. It is obvious that laser on such crystal with good electro-optic characteristics was of interest for many practical applications.

Early in the 1970s the leading role in the laboratory research passed from radio spectroscopy to optical methods. As a result, a common laboratory dealing with physical problems of quantum electronics was set up and later reorganized to a division. Studies of solid-state laser oscillation dynamics, in particular, those lasers with a resonator optical delay line, occupied a prominent place in the laboratory activity. The said subject allows substantial increase of its effective length and its change over a wide range thus controlling solid-state characteristics of laser in different modes of operation. This work of our division left behind similar research in other scientific centers of our country and abroad.

For some time past synchrotron radiation used for testing and study of scintillators* designed for the Large Hadron Collider (European Organization for Nuclear Research--CERN, Switzerland), contributed to production of a specimen with a short (nanosecond) scintillation time for its detectors. Eventually, all 150 t of this substance (lead tungstate) were produced at the Russian enterprises. The division workers developed also high performance scintillators for high energy physics and medicine (positron-emission tomography).

* Scintillators are substances capable of radiating light at absorption of ionizing radiation (gammaquanta, electrons, etc.). They are mainly used in scintillation nuclear radiation detectors.--Ed.


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