PROTON ACCELERATOR AND ITS MISSION

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Опубликовано в библиотеке: 2021-08-31
Источник: Science in Russia, №3, 2010, C.61-67

by Nikolai TYURIN, Dr. Sc. (Phys. & Math.), director of the Institute of High-Energy Physics, ROSATOM State Corporation, Moscow, Russia; Sergei IVANOV, Dr. Sc. (Phys. & Math.), deputy director of the Institute of High-Energy Physics

 

Russia's largest U-70 proton accelerator (uskoritel) is in operation 70 miles south of Moscow, at the border-crossings of the Moscow, Kaluga and Tula administrative regions. Commissioned on October 14, 1967, it is still a major experimental base in elementary-particle physics and applied problem solving geared to high-tech technologies of the nation's science and engineering.

 

TRAVELING INTO THE MICROWORLD

 

This accelerator is designed for studying the funda-mental properties of matter. The objects of such studies lie within the small-scale spectrum of the rapidly con-tracting spatial sequence "molecule -> atomic nucleus -> nucleons (protons and neutrons) -> elementary par-ticles", where the characteristic dimensions contract 1010 fold to 1018 m. In bare outlines this setup could be visualized as the regular television picture tube, or kinescope (cathode-ray tube, CRT). One of its ele-ments, the cathode, heated by the filament, gives off electrons. Flying into a narrow aperture in the acceler-ating electric field, they gain additional energy under the effect of the potential difference between the elec-trodes, an energy measured in electron-volts (eV). In the TV picture tube this field is generated by static volt-age ~20 kV. To get the phosphor-coated screen "glow-ing", a 20 keVenergy momentum will suffice. Abeam of electrons will activate the phosphor. The CRT evolves as the simplest linear accelerator that rides up electrons to a definite kinetic energy level.

 

Thus speeded up, electron beams prove to be quite good for "scanning" microobjects, their assortment and

 
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Shorthand of charged particle accelerators within the U-70 complex.

 

characteristics. But a much higher energy input is needed for that-of dozens, hundreds and even thousands ofgiga-electron-volts (GeV). At first natural radioactive sub-stances were used as sources of high-energy radiation. However, this involved many constraints. In the 1930s nuclear physicists began work on setups that could gener-ate more powerful and controlled beams. The well-known "Livingstone diagram"*. The diagram shows that the power of such machines has been increasing more than 30fold every ten years since the 1930s. Today they can produce beams of charged particles with energies ranging from thousands to trillions (thousand billions) eV. Understandably, the higher energy that we want to impart, the longer our linear accelerator should be-dozens, perhaps even hundreds of meters long. But this is hardly feasible in practice. It would be better to "roll" a linear accelerator into a ring, getting elementary particles to pass, back and forth, the sites where an accel-erating electric field is in force. To keep beams in orbit,

 

 

* Named so after the American physicist Stanley Livingstone who, together with Ernest Lawrence, built the first circular-orbit accelerator in 1932-a 1.2 MeV cyclotron that actually was a desk device.-Ed.

 
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we are using strong rotary and focusing magnets provid-ing for greater accuracy of fields and their stability in time. It is the dimension and cost of an accelerator's magnetic structure that often limits the finite energy.

 

What happens next? A bunch of particles thus form-ed (usually, electrons or heavier protons) is directed to a fixed target; colliding with it, the particles produ-ce new, secondary particles registered by detectors. Simultaneously, their mass, electric charge, velocity and other characteristics are determined. Thereupon, following computer-aided data management, which is a very sophisticated process, the properties of subnuclear particles and their interactions are determined. New knowledge, the be-all and end-all of research activities, is gained this way.

 

A colliding beam line can be used instead of the fixed target. One such setup, the Large Hadron Collider (LHC), is at work in the European Center for Nuclear Research (CERN, Geneva).* This circular-orbit accel-erator and accumulator of colliding proton beam lines (with the rated capacity of each, 7 TeV) commissioned in 2009 is the world's largest.

 

Our country, which has been building accelerators of her own since the mid-1940s, has been for many years a world's leader in high-energy physics. The Dubna syn-chrophasotron commissioned in 1956 (proton energy, 10 GeV; orbit length, 200 m; annular magnet weight, 40,000 tn)**, was the worldbeater. Then came our proton uskoritel synchrotron U-70, its orbit just a little below 1.5 km; jogging a beam to 70 GeV, it had been in the lead for nearly five years, up until the Fermi National Accelerator Laboratory (Fermilab) in the United States built a 200 GeV giant in 1972.

 

CHIPPING IN

 

U-70 is the main setup of the Institute of High-Energy Physics established in November 1963 for research into fundamental properties of matter. Its first director was

 

 

See: L. Smirnova, "The 21st Century Megaproject", Science in Russia, No. 5, 2009.-Ed.

** See: A. Sissakian, "Dubna's Wforldwide Glory", Science in Russia, No. 2, 2006.-Ed.

 
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Anatoly Logunov (elected to the National Academy of Sciences in 1972), a theoretical physicist who did so much for getting this research center off to a good start. True, the government decision on U-70 had been adopt-ed back in March 1958. The physical part of the project realized under the guidance of Vassily Vladimirsky, cor-responding member of the national Academy of Sciences, envisaged a 50 GeV synchrotron. But, as it came out, a strong-focusing setup like that was capable of overcoming a "critical energy" for a beam, and so the basic parameter of the synchrotron could be upgraded to 70 GeV.

 

Yet another essential modification involved directing the accelerated particles at external targets. For this pur-pose the two rectilinear gaps between each of the twelve elements of the magnetic structure (regions where the accelerating electric field is generated) were upped to 4.8 meters via a small decrease in the length of some magnetic blocks and distances in between.

 

Chosen as the injector-a "pump" for injecting charged particles-was an Alvarez linear accelerator.* Its out-put energy of 100 MeV was high enough for the assigned intensity of a beam needed for the experiments- 1012 protons per cycle.

 

In 1960 Protvino was a beehive all astir as a giant con-struction project was launched there to build a new setup, U-70, as well as laboratory and technological shops equipped with the latest facilities. Although this sent the overall building costs nearly threefold, the pro-ject was still carried on.

 

Each and everyone "chipped in" on the project: work-ing there were as many as a thousand (!) research centers and enterprises-designers, builders, installers, engi-neers... Major Moscow research institutes were pitching in-the Institute of Radio Engineering and Electronics (Academy of Sciences of the USSR); the State Specialized Design Institute and the All-Russia Research Institute Tyazhpromelektroproyekt (heavy industry and electric engineering) named after F. Yakubovsky. Leningrad-based centers were also busy at Protvino: the Yefremov Research Institute of Electrophysical Engineering, and the Institute of High-Power Radio Engineering. They did the job: on October 14, 1967, the U-70 uskoritel jogged a proton beam to a finite energy of 76 GeV and 3 • 109 intensity per cycle, the world's high indicator then. In 1970 a group of U-70 developers were awarded Lenin and State Prizes in science and engineering.

 

The newly established Research and Coordination Board took charge. It included top experts from the Dubna Joint Institute for Nuclear Research (JINR), the Lebedev Physics Institute of the Academy of Sciences of the USSR, the Alikhanov Institute of Theoretical and Experimental Physics, the Kurchatov Atomic Energy Institute, Lomonosov Moscow State University, and the Moscow Institute of Physics and Engineering.

 

 

* Named so after Luis Walter Alvarez, an American physicist awarded a Nobel Prize in 1968; as good as all linear proton accelerators were built to his design in those years.-Ed.

 
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The giant accelerator complex turned into a multiuser center right away. Dozens of research teams of this and other countries were experimenting there, including experimentalists from the United States, France, Italy and Japan. Six joint research programs were carried out together with the European Center for Nuclear Research alone. Western physicists were developing unique equipment and systems, and bringing all that for experiments on the world' largest proton accelerator. A great many projects were carried out jointly with experts of socialist community countries working at JINR. Engaged shoulder to shoulder with experts of our research center (whose worktime input approached 30 percent) were experimentalists from all research centers and laboratories of this country involved with elemen-tary particles.

 

PROBING IN-DEPTH

 

As a matter of fact, all prior proton accelerators, both here in this country and abroad, were ahead of experi-mental programs, and thus ran idle for a while for lack of adequate programs. A program like that was devel-oped well in advance for U-70 (the Research and Coordination Board saw to it, too!), and so experimen-talists could get busy right away as soon as the first pro-ton beams were in.

 

Our U-70 setup was "just" 2.5 fold as powerful as those onstream in the United States and Western Europe, and some experimentalists were not happy about that. The setup should be ten times as powerful! the pessimists lamented. However, the U-70 energy output made it pos-sible to study physical phenomena at distances down to 2 • 10-15 cm, or dozens of times as short as a nucleon's dimensions and correlating with the quark level of the hadron structure. So it came off well.

 

Already in the first three years the machine produced wonderful results that, promulgated at the international conference at Lund, Sweden, in 1969, were called "Russian" in the Western press.

 

Experiments on antimatter nuclei begun soon after resulted in the detection of antihelium-3 (3He)-the sec-ond element in the Mendeleev Periodic Table-in 1970. The nucleus of this element is composed of two antipro-tons and one antineutron. This finding confirmed the theory on antimatter postulated by the British physicist Paul Dirac, one of the creators of quantum mechanics, way back in 1931. The U-70 data were registered as dis-coveries. Later on, in 1973, physicists detected and identified nuclei of antitritium containing 1 antiproton and 2 antineutrons. Those were the first artificial- "real"!-nuclei of antimatter (unlike the antideuteron with its low bonding energy).

 

Experimenting in the energy spectrum above 20 GeV, our research physicists discovered a heretofore unknown phenomenon-not expected "sluggish" drop in total cross-sections of hadron interactions (important values

 
стр. 65

 

 

Elements of the U-70 accelerator complex. Schematic overview.

 

for the course of reaction between two colliding parti-cles) in keeping with the then dominant idea but, rather, their sudden growth and higher energy output. This dis-covery registered in 1973 and dubbed a "Serpukhov effect" was a sensation and caused to revise the theory explaining particle interactions at high energies. More than 200 publications on this subject appeared in the following three years. Confirmed by subsequent mea-surements at the proton circular-orbit accelerator at the Fermilab and at CERN, this phenomenon is yet to be interpreted theoretically.

 

U-70 made it possible to conduct experiments on what is known as inclusive reactions of hadron collisions in the range of energies inaccessible before that. A new law-that of large-scale invariance in hadronic interac-tions-was thus discovered; it became one of the first bits of experimental evidence that argues in favor of the quark structure of nucleons. This discovery has an important practical side to it in a variety of fields, name-ly in designing large superhigh-energy setups, and in biological protection against radiation.

 

Mention should be also made of the most significant achievement scored over the last few decades in the physics and technology of charged particles-the discov-ery of the phenomenon of radio-frequency quadrupole focusing (from the Latin quadrum, or quadrangle, and Greek pólos, or pole). This phenomenon is now known internationally as RFQ (Radio-Frequency Quadrupole). It was discovered by Vassily Vladimirsky, corresponding member of the Academy of Sciences of the USSR, Dr. Ilya Kapchinsky of the Institute of Theoretical and Experimental Physics and Dr. Vladimir Teplyakov of our research institute. They suggested using this method for preliminary acceleration of protons. In 1977 our institute designed and built the world's first full-scale RFQ setup. Today this major technological system, good for any pro-ton or ion accelerator, is in use worldwide.

 

Overall, more than 200 experiments have been con-ducted at Protvino, and eight discoveries made. Its researchers have merited 14 top awards, the State Prizes of the USSR and Russian Federation.

 

MEGAINSTRUMENT FOR BIG SCIENCE

 

In the narrow sense of the name, U-70 means a 1.5 km proton synchrotron. But experts would rather see it in a broader sense relative to the accelerating, experimental and engineering-technical complex of our institute. This is a multicomponent and large-scale object that comprises four proton (ion) resonance accelerators (U-70 proper, a string of injectors-auxiliary setups getting a beam ready for the main accelerator: I-100, Ural-30, the fast-cyclic intermediate booster U-1.5) and the branchings; plus an extensive network of channels for the transportation and formation of the extracted beams of different sorts and energies. In addition, we have a pool of major physical plants geared to experiment. Each has a name of its own, seemingly rather odd: OKA, VES, SVD, FODS, SPASCHARM, SPIN, HYPERON and all that. Thus, OKA stands for experiments with K-mesons*, and VES codes for a Vertex spectrometer. This complex is supported by a special-purpose engineer-ing infrastructure supplied with powerful cryogenic equip-ment, electric power systems, demineralized water cool-

 

 

* K-mesons-a bunch of four elementary particles, two charged and two neutral.-Ed.

 
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ing, and radiation control safeguards. Combined into one technological cascade of sets, all these components make up one charged particle accelerator, Russia's largest.

 

Once it ran four working sessions a year, and now two, 800 to 1,500 hours each. The "24/7" schedule is on dur-ing these sessions, that is both the personnel and hard-ware are supposed to work 24 hours nonstop seven days a week. One such session may employ as many as nine experimental units, with seven producing a beam simul-taneously in each acceleration cycle.

 

The U-70 complex is above all a beam "perveyor" catering to experimentalists. It is also one-of-a-kind test rig for practical research into various aspects of charged particles and particle accelerators. Therefore in each session we devote about a week of machine time to task management bearing on the accelerator complex prop-er. This will open up new avenues for theoretical and applied research involving high-energy beams.

 

OUTLOOK FOR TOMORROW

 

The U-70 uskoritel is still among the world's ten largest. Such accelerators are not written off as outdated with the advance of time-quite the contrary, they are upgraded all along to tackle new research problems.

 

For instance, today we are acting on a program for acceleration of light ions, in particular, carbon nuclei. Not only physicists but also physicians need that. A research team under Yuri Antonov, our laboratory head, has joined hands with colleagues of the Obninsk Nuclear Medicine Center (Russian Academy of Medi-cal Sciences) on radiation therapy in oncology. Ion radi-ation treatment is now the in-trend in clinical practice. Unlike X-ray, photon and electron radiation, this tech-nique provides for better dosage distribution and enhances the tumor-damaging effect, without affecting sound tissues. Since the mid-1990s it has been used with much success in the world's leading clinics. Our High-Energy Physics Institute has every condition for the hands-on application of this advanced technology.

 

We have traveled a long stretch of the road. On the eve of 2007 we accomplished another first by feeding pro-tons into a new beam transportation channel linking the 100-meter ring of the intermediate booster U-1.5 to the I-100 setup. Early in 2008 we succeeded in boosting the energy level to 450 MeV per nucleon, and could extract beams to an external deuteron absorber, the heavy hydrogen isotope nuclei. Next, in 2009 we switched deuterons from U-1.5 into the principal ring, thus obtaining longtime circulation. Since in physical terms deuterons are full analogs of carbon nuclei, there should be no technical snags in getting carbon beams. Now we should get busy with practical aspects of light ion accu-mulation in the circular-orbit accelerator U-70 and slow extraction of light ions into an experimental room equipped with medical treatment booths.

 

In real terms the costs of this healthcare center will amount to only 20 percent of outlays had we begun from scratch now, and this owing to the available infrastruc-ture. Jointly with other centers operating in Moscow, Dubna, Obninsk, Troitsk and St. Petersburg, ours will be forwarding high technologies into practical medicine to cater to the vital needs of the public health service.

 

Our institute is working on pulse neutron generators on U-70 beams, and on a new experimental zone of intermediate energy beams extracted to users right from the U-1.5 booster, without further acceleration in the main synchrotrone. These lines of diversification will be instrumental in a dramatic rise in the share of innova-tional projects, with the costs involved in such ventures recouping rather fast. Our longterm plans provide for construction of superconducting proton accelerator (energy, ~ 100 GeV, intensity, ≤ 1014 particles per momen-tum). This will open up new opportunities for ele-mentary particle physics on the domestic experimental base.

 

Circular-orbit accelerators with a fixed target are per-forming well in the 50 to 70 GeV energy range, as shown by world experience. They are catering both to basic research and to innovative technological and nuclear power projects of the future. That is why the United States has developed BNL AGS and FNAL MI proton synchrotrons; Japan is winding up construction of her singular accelerator complex J-PARC, while Germany is starting to built SIS-100 and SIS-300 setups. All this demonstrates that machines of such a class and potential are needed. Like our proton synchrotron U-70.

 

 


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