NUCLOTRON: FIRST BEAMS AND EXPERIMENTS AT THE SUPERCONDUCTING SYNCHROTRON IN DUBNA


A.D.Kovalenko
Laboratory of High Energies,
Joint Institute for Nuclear Research,
141980 Dubna, Russia


ABSTRACT

The first superconducting synchrotron named Nuclotron based on a miniature iron- shaped field SC-magnets was designed, constructed and put into operation at the LHE. Five runs of a total duration of about 1400 hours have been provided by the present time. Unique experimental results on the cryomagnetic system of a novel type as well as new data on nuclear collisions using internal target have been obtained. Successful experience of the Nuclotron building gives no doubt in further applications of such a low cost reliable technology for future accelerators both as for superhigh energies or high intensity beams production.

Table of contents
1. Introduction
2. The LHE accelerator complex
3. Nuclotron design and construction
4. First beams and experiments at Nuclotron
5. Conclusion

References

1. INTRODUCTION

It is my great privilage to present this talk at the Anniversary symposium devoted to the discovery of the phase stability principle. Fifty years were passing after V.I.Veksler's famous paper was published in 1944 [1]. Revolutionary progress of high energy nuclear and elementary particle physics as well as the progress of high technologies in different directions of human activity are connected with the development of accelerators based on this fundamental principle.
Since 1957 the major research facility of the LHE JINR is 10-GeV proton Synchrophasotron built under the leadership of V.I.Veksler. The proposal of the Nuclotron construction as well as the synchrophasotron upgrade program were unitiated by A.M.Baldin in the beginning of 70s [2,3]. The ideas of relativistic nuclear physics aimed at the study of colour degrees of freedom in nuclei, QCD of large distances and fundamental properties of highly exited nuclear matter defined the prospects for the LHE accelerator complex development as relativistic nuclei and polarized beam facility. In accordance with this concept the following main problems were solved during the past years:


A large scale R&D program was carried out at LHE on superconducting magnet technology, cryostat systems and its cooling before the final option of the Nuclotron has been choosen.
The superconducting relativistic heavy ion accelerator--Nuclotron was put into commissioning the last week of March 1993. It was the first test run of a new superconducting synchrotron constructed at the Laboratory over the period of 1987-1992. The Nuclotron is intended to accelerate nuclei and multicharged ions including the heaviest ones (uranium) up to an energy of 6 GeV/u.
It took only five years to construct new facility for multipupose physics researches based on advanced technologies. Nontraditional concept of a low- field iron dominated SC-magnets made it possible to fabricate practically all equipment of the SC-magnetic and cryostat systems without recourse to "big" industry. At present time both accelerators are under operation.

2. THE LHE ACCELERATOR COMPLEX

A general composition of the accelerators and experimental areas at the Laboratory of High Energies is shown in Fig.1.

Fig.1. Accelerator Centre of the Laboratory of High Energies.


( Also, sensitive map of LHE Accelerator Complex is available. )


The Synchrophasotron provides beams of protons, light ions (including sulphur), polarized and aligned deuterons as well as secondary beams (neutron, pion, 3H and others). About 500 physicists of 120 institutions from many countries are involved in the research program.Main aspects of which are the following:

The investigations of relativistic nuclear collisions have begun at the Synchrophasotron since 1970. The first experiment performed with 4.5 GeV/u deuterons was aimed at verifying the hypothesis of cumulative particle production in 1971 [4].
The possibilities for obtaining a wide range of relativistic beams of nuclei as well as its intensities are predeterminated by the parameters of the injector complex. The main components of the complex are: ion sources, forinjector tube supplied by 600 kV - pulse transformer, and the Alvaretz-type linear accelerator LU-20. Both as acceleration of protons up to the energy of 20 MeV and acceleration of nuclei or multicharged ions with charge-to-mass ratio Z/A 0,33 up to 5 MeV/u are provided by the LU-20 operation modes. Four types of ion sources are used: duoplasmatron, electron beam ionizer - KRION (named EBIS in western publications), laser drive source - LDS and polarized deuteron source - Polaris. The specification of the particle beams which are available at the injector complex is presented in Table 1.

Table 1
Accelerated particles Intensity charge/pulse Pulse duration Type of the source Comments
p 1.5·1014 500µs Duoplasmatron E=20 MeV
d 1.0·1014 500 Duoplasmatron E=5 MeV/u
4He2 1.0·1013 500 Duoplasmatron -
3He2 3.5·1011 500 Duoplasmatron -
7Li3 5·1010 15 Laser E=5 MeV/u
6Li3 3·109 15 Laser -
12C6 6.5·1010 25 Laser -
16O8 6·109 10 Laser -
19F9 2.5·109 6 Laser -
22Ne10 2·107 40 KRION -
24Mg12 2·108 25 Laser -
28Si14 1·108 25 Laser -
32S16 4·106 80 KRION-S -
40Ar20 2.5·106 80 KRION-S -
84Kr29 1·106 80 KRION-S -
d 2.5·1010 100 Polaris Polarization
index and value Pzz=0.54±0.08
Pz= 0.47±0.04
Pz= 0.60±0.08


The solid-state Nd-glass laser was first used to obtain relativistic carbon nuclei at our accelerator complex in 1976 [5]. The LDS based on a CO2 laser was developed and put into operation several years later [6].
The electron-beam ionizer - KRION was suggested by Donets in 1967 [7]. The highest value of the ionization factor is provided by the KRION-type source. Since 1977, the sources of this type is used at the LHE accelerator complex. The heaviest nuclei accelerated at the synchrophasotron was sulphur [8]. This experiment was carried out in accordance with the request of the users to be interested in the mostly complete experimental data on sulphur nuclei interactions over energy range of 1-200 GeV. But acceleration of the ions even as sulphur at the synchrophasotron is inefficient due to a big losses of the beam intensity caused mainly by unsufficient vacuum, low energy of injection and low accelerating r.f. voltage. The progress of the LHE relativistic heavy ion program is connected with the Nuclotron operation and proposed development of the injector complex. The EBIS-type ion sources are considered as one of the basic devices in the frames of this program.
Beams of polarized deuterons were obtained in 1981 after a special cryogenic source "Polaris" was developed and put into operation [9]. The maximal momentum and intensity of accelerated beam are: p=4.5 GeV/c and N=5·109part./pulse. Investigations of spin phenomena using polarized beams and the development of experimental facilities for this researches are the subject of particular interest.
There are two directions (labeled MV-1 and MV-2 in Fig.1) of beam extraction from the Synchrophasotron. Along the MV-1 the beams are extractred slowly (Textr 500 ms) and transported to the main experimental area - hall 205 which has been built during the end of the 70s. Efficiency of slow extraction at maximal beam energy of about 95%. Along the MV-2 both as slow or fast (Textr 1 ms) beam extraction modes are available.
Eight beam transfer lines, namely the main one VP-1 and seven lateral 1V7V can provide ten physics setups with beams in hall 205. During the past few years the MV-2 direction of beam extraction was operated less intensively but the using of an "old" experimental area - hall 1A is included into the full scale prospect of the Nuclotron userôs policy. More detailed description of the synchrophasotron external beams was presented for example in paper [10]. Notice, that using of existing experimental areas was one of the conditions under the Nuclotron design.
Normally the total running time of the synchrophasotron up to 1991 was about 4000 hours per year. Later, disbalance between budget funding and prices for electric power led to catastrofic decreasing of the accelerator running time. Only about 1000 hours per year were available for the users in 1991 and in 1992. Starting from 1993 the Nuclotron is under operation. It seems realistic to support total running time (Synchrophasotron + Nuclotron) at the level of about 2000- 2500 hours per year during several next years. The synchrophasotron will be used mainly to obtain polarized beams. The efforts should be concentrated also at the problem of beam extraction from the Nuclotron.

3. NUCLOTRON DESIGN AND CONSTRUCTION

The first conceptual design proposal of development of the LHE accelerator centre was published in 1973. The "Nuclotron", a superconducting strong focusing accelerator of relativistic nuclei, was considered as a three-stage accelerating facility, consisting of: 10 MeV/u linac, 750 MeV/u booster ring (both conventional and superconducting ones were considered) and 2025 GeV/u main ring [11]. Pulsed superconducting dipoles of a cos - type with a peak magnetic field of 6 T were suggested to be used for the Nuclotron main ring. However, after the first tests of cos - type high field SC magnets had been performed, further R& D works were reoriented at the investigation of a miniature iron-shaped field SC-magnets. It was the only feasible way of constructing a new accelerator at LHE because of very limited funs allocated to the relativistic nuclear physics program. The final option of the Nuclotron was developed to the 80s [12],and the project: "Reconstruction of the synchrophasotron magnetic system to the superconducting one - Nuclotron" was approved in December 1986.
The proposal of the using a pulse 2-2.5 T magnets based on a window- frame type iron yoke and SC-coils for a relatively small synchrotrons was made by I.Shelaev. Five modifications of such type magnets with the SC-coils made of plane SC-cable a immerse type of cooling were fabricated and tested at LHE up to 1978 [13]. New magnets had very attractive parameters such as: low cost, low stored energy, high quality of the magnetic field over the aperture, the lowest specific weight (only 30 kg/m). The question was: "Is the peak beam energy of 6 GeV/u instead of 20 GeV/u enough for the problems to be solved in the frames of basic physics program at Nuclotron?". According to general representation developed by A.M.Baldin [14] - relative 4-velocity between colliding particles (hadrons, nuclei) should be bI,II >> 1. In this case the nucleons cannot considered as quasi-particles of nuclear matter and the influence of quark-gluon degrees of freedom in interactions of hadrons and or nuclei should be observed. The basic quantitives bik which the probability distribution in the process of multiple particle production

I + II 1 + 2 + 3 + ...



depend on are dimension less positive and relativistic invariant:

where pi , pk - 4-momenta of partciles i and k ; mi , mk - their masses; ui , uk - 4- velocities.
Moreover, a set of data on cumulative particle production at the synchrophasotron was obtained by that time. The experimental data showed that limiting fragmentation of nuclei is observed at the energy of E 3.5 - 4.0 GeV/u, and respectively for bI II 5.0. Being constructed at the magnets with peak field of 2T the Nuclotron would provide the upper limit of bI II 10 and hence, it would be appopriate accelerator for relativistic nuclear physics. Notice, that for total set of above mentioned physics problems it is important both as bik > 5 (asymptotic regime) and 0,1 < bik < 5 (transition regime).
So, the concept of a low-field iron-dominated SC-magnets satisfied all criteria.
Later, the model 1.5 GeV superconducting synchrotron SPIN based on the SC-magnets proposed by I.Shelaev was constructed and tested. It was was nesessary stage to obtain needed technological and operational experience.
Special types of superconducting magnets were proposed by A.Smirnov were designed and investigated for the Nuclotron. These are fast cycling iron- shaped magnetic field magnets with a winding of hollow composite superconductor and a circulatory refrigeration system [15]. One of the main feature of the Nuclotron structural SC-dipoles and quadrupoles is low inductance of the windings: 1.1 and 0.44 mH only. Due to this fact the magnetic field rise up to several T/s can be achieved. The design parameters of the dipoles are: B=2.2 T and dB/dt = 2 4 T/s. Nominal current amplitudes are: up to 6.3 kA and 6 kA for the dipoles and quadrupoles respectively. There are 96 dipole, 64 quadrupole and 32 correcting SC-magnets in the Nuclotron ring with circumference of 251.5 m. Averaged specific weight of the magnetic system is only 0.32 t/m.

Fig.2. The Nuclotron dipole magnet inside its cryostat near the synchrophasotron.


General view of the Nuclotron dipole and quadrupole magnet are presented in Fig.2 and Fig.3. The magnets is fastened to a vacuum shell of the cryostat 540 mm by 8 suspension parts of stainless steel. A nitrogen shield 490 mm covered with 20 lagers of superinsulation is placed in the vacuum space between the magnet and the vacuum shell. The dipole magnet has a window-frame type iron yoke with the sizes of window of 11055 mm2. The quadrupole lens has the iron yoke with hyperbolic poles. The SC-cable was manufactured of a 5 mm in diameter cupro-nikel tube with a wall thickness of 0.5 mm and 31 connected in parallel multifilament strands of 0.5 mm in diameter covering an outer surface of the tube. The strand consist of 1045 NbTi filaments 10 mkm in diameter stabilized by copper.

The nuclotron ring is installed in the tunnel around the synchrophasotron. This tunnel was originally built for cable communications and the equipment of the synchrophasotron vacuum system. The Nuclotron median plane is at 3.7 m below the synchrophasotron one. The picture of the Nuclotron in the tunnel is presented in the talk by A.M.Baldin.
The Nuclotron lattice is typical for a strong-focusing separated function synchrotrons. It contains 8 superperiods and 8 stright sections, one of which is "warm". Different targets for the experiments at internal beam can be installed more easy using this place.



Fig.3. The Nuclotron quadrupole lens.


General scheme of the Nuclotron cryogenics is presented in Fig.4. More detailed it was described in [16]. All the magnets are connected in parallel with supply and return helium headers. The internal diameters of the headers are 36 mm and 52 mm respectively. The cooling of the magnets is performed by two- phase helium flow. The mass/vapour content varies from 0 at the inlet of the magnet to 0.9 at its outlet. The temperature sensors are places at the helium inlet and outlet of the winding and also at the helium outlet of the iron yoke of each magnet. Totally the temperature measuring system uncludes about 600 points. The Nuclotron operational temperature is 4.5-4.7 K.

Fig.4. General scheme of the Nuclotron cryogenics. 1 - vacuum shell; 2 - heat shield; 3 - suuply header;
4 - return header; 5 - dipole magnet; 6 - quadrupole magnet; 7 - subcooler; 8 - separator; 9 - refrigerator;
10 - gas bag; 11 - storage vessel; 12,14,15,17 - compressors; 13,16 - purifiers.



The cryogenic supply system is based on three industrial helium refrigerator/liquifiers with a total capacity of 4.8 kW at 4.5 K. A fast cycling mode of the Nuclotron operation (up to about one pulse per second) can be actieved in the case of a nominal heat load.
The Nuclotron cryogenic supply complex, was used for large scale production of liquid helium in the frames of the Russian-American contract [17].

4. FIRST BEAMS AND EXPERIMENTS AT NUCLOTRON


During the first test run (March 17th-26th, 1993) the Nuclotron ring was cooled down to 4.5 K (it took about 110 hours), the dipoles and quadrupoles were supplied by dc-current and a 5 MeV/u deutron beam was injected. Soon after tuning the levels of magnetic field and gradient, we observed the first turns of particles in the vacuum chamber. This result was reported at the CERN PS Seminar and caused very big interest [18]. Deuteron beam acceleration up to an energy of 0.2 GeV/u and internal target irradiation were performed during the second run in June 26th -July 6th [9,10]. The intensity of the accelerated deuteron beam was up to 2·109 per cycle. Beam dynamics was stable enough, and a maximum beam energy was limited by the run program. Additional adjustment of the magnets power supply sources and quench protection system should be provided before the next step.
New results on acceleration of heavier ions were obtained at Nuclotron in December 1993 run. The cryogenic electron beam ion source "KRION-S" was installed at the linac. Beams of argon and krypton ions were obtained and accelerated to an energy of 5 MeV/u at the linac and the krypton beam was injected into the Nuclotron ring.
But we were forced to interupt this run due to failure the cathode of the electron gun at the KRION. The run was continued after the KRION was replaced by the laser drive source. An accelerated beam of carbon ions with the intensity of ~109 per cycle was obtained at Nuclotron using the LDS. So, operation of LDS with the Nuclotron system was tested [19].
The Nuclotron is excelent tool for the experiments at internal target (Fig.5) due to the possibility of a beam energy variation starting from injection (5 MeV/u) up to maximum value (6 GeV/u) with very fine steps. The step of guiding magnetic field rise is 1 G. So, the energy variation of 10-4 can be provided for the experiments over energy range of (16) GeV/u.
The first physics experiments with relativistic deuterons were carried out during the fourth Nuclotron run in March 17-29. The run began just after the completion of a Synchrophasotron polarized deuteron run. In accordance with the program, a polarized deuteron beam injection and acceleration (up to 100 MeV/u) were provided. After that the polarized deuteron source "Polaris" was replaced by a duoplasmatron and the Nuclotron continued to operate for physics program.The parameters of the magnetic field cycles were 6, 8.5 and 10 kGs with a rise of 6 kGs/s. The beam dynamics was stable and no particle losses were observed during the acceleration time after 50 ms. Thus, the maximum momentum of deuterons of 3.5 A GeV/c at beam intensity of 2·109 per cycle has been reached.

Fig.5. The fragment of the Nuclotron ring and "warm" streight section where the box with internal target inside.



A beam of deuterons accelerated up to a momentum of 3.7 GeV/c was used for data taken by three teams of experimentalists in the frames of SFERA Collaboration . As target foils 1.57 µm CH2 and 1.72 mm Au were used. Despite of not so long exposition time (about 8 hours) a lot of interesting results were obtained [20]. A set of measurements on the yields of -quanta, , K mesons and nuclear fragments - from protons to helium and lithium isotopes was performed. For instance, about 50 rare events of K+ -meson production in a region below a kinematical limit of nucleon-nucleon collisions. Technical aspects and background conditions of application of various types of detectors including lead glass electromagnetic calorimeters in the ring tunnel were carefully investigated.

The example of beam-target interaction is shown in Fig.6. Observable time of interaction is about 400 ms. It is easy to show that the luminosity level of about 3.1033 cm-2.s-1 for dC- interactions was obtained in that experiment.



Fig.6. Beam-target interaction at the Nuclotron internal beam.



Ultraviolet and X-ray radiation produced in interactions of beam particles with target material has been applied to study the radiofrequency structure of accelerated beam.
The accelerated beam microstructure for different conditions is shown in Fig.7. The upper diagram shows a time structure of accelerated beam just before the beginning of the flat top (plato) of the magnetic field cycle. The middle one is correspond to the moment when r.f. voltage has been switch off. A typical bunch structure of the beam is observed in this case. The last diagram demonstrate longitudinal beam density in the process of the particles circulation at the plato of magnetic field after the r.f. voltage was switch off. There are no bunches and longitudinal beam density is quite uniform.This mode of operation is the most attractive for data taken.



Fig.7. Microstructure of accelerated beam.



The first results obtained at Nuclotron are very promizing. Reliability of cryogenic system is better than 98 percent. The cooling process of the magnets takes 90-100 hours. Vacuum inside a beam pipe is about 10-9 - 10-10 Torr. This is sufficient for acceleration not only heaviest nuclei but also multi-charged heavy ions. The next steps are completion of the beam slow extraction system and the beam energy increasing up to the maximum design value. The Nuclotron will provide more possibilities for the users. Regular physics experiments have started.

5. CONCLUSION

Successful operation and development of the synchrophasotron, design and construction of the new superconducting accelerator Nuclotron would not be possible without great efforts of many people. I feel my duty to express gratitudes to L.P.Zinoviev, L.G.Makarov, I.N.Semenyushkin, I.B.Issinsky, Yu.D.Beznogikh, S.A.Averichev, A.A.Smirnov, K.V.Chekhlov, A.I.Mikhailov, A.P.Tsarenkov, V.A.Monchinsky, V.I.Volkov, B.D.Omelchenko, N.N.Agapov, H.G.Khodgibagiyan, M.A.Voevodin, V.A.Mikhailov, A.I.Malakhov, V.M.Slepnev, O.I.Brovko and all engineering and operational stuff of the LHE Accelerator complex.


REFERENCES

1. V.I.Veksler. Doklady Akad.Nauk SSSR, v.43, 346 (1944) and v.44, 393,(1944).
2. A.M.Baldin. In: Nuklotron i Relativistskaya Yadernaya Fizika, JINR, 8309, p.7-16, Dubna, (1974).
3. A.M.Baldin. In: Proc. VI Int.Conf. on High Energy Physics and Nuclear Structure, Santa Fe, USA, June 1975; see also: JINR, E2-9138 Dubna (1975).
4. A.M.Baldin et al. Proc.Rochester Meeting APS/DPF, Rochester, USA, (1971), p.131-137; see also: JINR, P1-5819, Dubna, (1971).
5. A.B.Anan'in et al., Kvantovaya Electronika (Moscow), v.7, 1547 (1977).
6. Yu.D.Beznogikh et al. JINR, R9-84-246, Dubna (1984).
7. E.D.Donets, USSR Inventorôs Sertificat No.248860 (March 15, 1967), OIPOTZ No.23,65 (1969).
8. A.D.Kovalenko et al. JINR Rapid Comm. No. 2[59]-93, JINR, Dubna, (1993).
9. A.A.Belushkina et al. In: High Energy Physics with Polarized Beams and Polarized Targets, Basel, p.429, (1981).
10. I.B.Issinsky et al. Acta Physica Polonica B. vol.25 (1994), no.3-4, p.673-680.
11. V.P.Alexeev et al. JINR, 9-7148, Dubna 1973 (in Russian).
12. A.M.Baldin et al. IEEE Trans. on Nucl.Sci., NS-30, p.3247-49, March 1983.
13. S.A.Averichev et al. JINR, P8-11700, Dubna, 1978 (in Russian).
14. A.M.Baldin. Doklady Akad.Nauk SSSR, 222, No.5, p.1064, (197).
15. A.A.Smirnov et al. Journal de Physique, Col. C1, no.1, vol.45, C1-(279-282), (1984).
16. A.M.Baldin et al. Adv. Cryog.Eng.39, 501-508, (1994); see also: JINR, E9-93-273, Dubna, (1993).
17. A.M.Baldin, N.N.Agapov, A.D.Kovalenko. JINR, E8-95-66, Dubna, (1995).
18. A.M.Baldin and A.D.Kovalenko. CERN bulletin, 14/93, no.4, (1993).
19. A.D.Kovalenko. In: Proc. EPAC-94, London, June 1994, v.1, p.161-164 (1995).
20. A.M.Baldin et al.Nucl.Phys. A583, p.637-640, (1995).


NUCLOTRON: FIRST BEAMS AND EXPERIMENTS AT THE SUPERCONDUCTING SYNCHROTRON IN DUBNA