NUCLOTRON: MAIN RESULTS AND PERSPECTIVES
N.N. Agapov, A.D. Kovalenko, A.I.Malakhov (JINR, Dubna)
The Nuclotron is a basic JINR facility aimed at obtaining multicharged ions (nuclei) with the energy till 6GeV per nucleon, proton beams as well as polarized deuteron beams.
The accelerator Nuclotron was constructed during 1987-1992 on the unique technology of superconducting magnets which was proposed and developed at the Laboratory of High Energies named after Academicians V.I.Veksler and A.M.Baldin. Design developments,testing and assembling of the Nuclotron elements were totally fulfilled by the collective of the Laboratory. Production of the structural elements was carried out in the JINR Central Workshop. Results of the first tests of the accelerator are given in [1,2].
All the design parameters of the machine have been reached in general. Practice has demonstrated high reliability of the liquid helium supply system and of the magnet-cryostat system operating at the temperature of 4.5 K. Physical experiments were started at the inner target in 1994. The slow extraction system of the beams from the Nuclotron had been completed and tested by 1999. First physical experiments on the external beam were carried out in March 2000.
2. MAIN RESULTS
Since March 1993 the Nuclotron has fulfilled 23 runs having operated totally for 7133 hours, the volume of operation was mainly limited by the funding available. It is necessary to emphasize that upgrading of the cryogenic system held in 1999 allowed one to increase significantly the duration of uninterrupted operation of the accelerator and technical data. As a result 45% of the operation time, i.e. 3202 hours, refer to the last two years. At this stage the share of the start- up periods of the accelerator (cooling of the magnets till helium temperatures) has been reduced upto 16%, and 54% of the total operation time refer to the work with physical experiments. The time spent for studying the beam dynamics and development of the machine itself, is equal to 30%.
The main parameters of the accelerator are given in Table 1. Additionally it is necessary to stress that the accelerator is characterized by excellent flexibility in control when changing the energy of the accelerated particles as well as while regulating the duration of the magnetic field "table". Obtaining the maximal design energy of particles is still limited by the necessity of completing the upgrade in the systems of searching for the normal zone and energy extraction from the structural superconducting magnets. The repetition rate of the operating cycles of the Nuclotron is determined mainly by the capabilities of its injector Lu-20 and,in particular, of the high voltage (upto 800 kV) pulse transformer of the accelerating tube of the for-injector. To rise the velocity of increasing the field in the accelerator is of no principle difficulty since all the structural elements (dipoles, quadra-poles) have been already tested for the exploitation regime: B=2 T, dB/dt=4 T/c, f=1 Hz. The existing limitation on the velocity of field increasing in the ring is related only to unsufficient operating voltage of the supply source for the dipole magnets contur.
Table 1: The Nuclotron main parameters.
Beam extraction is realized by excitation of the radial betatron oscillations resonance Qx=20/3. The system of resonance swinging includes two pairs of sextapole lenses determining the amplitude and phase of the resonance frequency, and four specialized quadra-pole lenses shifting the frequency of the radial betatron oscillations from the working point till the extraction resonance. The particles captured in the resonance are crossing the working volume of the electrostatic septum, after that they enter the Lambertson magnet. The electrostatic septum (ES) deviates the beam into the horisontal planes by the angle of 5 mrad, and the Lambertson magnet provides the beam deviation in the vertical direction by the angle of 96 mrad. The last parameter was chosen to provide both - the necessary deviation of the beam to circle the body of the structural quadrapole lens installed after the Lambertson magnet, and the coincidence of the extracted beam with the existing transport lines in the experimental zone.
The high voltage part of the electrostatic septum is similar to those in other laboratories but there are peculiarities of the construction. They are related with the passing through the device of the superconducting electric shinas of the structural dipoles and quadrapoles supply. These shinas are going through the vacuum coil unified with the electrostatic septum. Besides, first experiments on the beam extractions have shown that in our case when the energy of the beam being injected into the accelerator is small (Einj=5 MeV/n), it is necessary to use the pulse high voltage supply of the electrostatic septum. The Lambertson magnet consists of two sections 1.5 m long each. The same as in the Nuclotron the iron yoke of the magnet is at the helium temperature, and its superconducting coil is made of the tubed superconductor. Thus, the transverse cross-section of the Lambertson magnet is almost the same as in the structural elements of the accelerator. The sections of the Lambertson magnet are supplied in turn with the structural dipoles.
For precise correction of the extraction angle in the vertical direction in the limit of ±6 mrad, two specialized supply sources and additional current guides are foreseen in the first section of the Lambertson magnet.
The beam extraction system and the results of its tuning are described in  more detailed. Some of the main parameters are given in the Table 2. The part of the beam extraction on the Nuclotron beam and the time structure of the extracted deuteron beam are shown in Figs. 1 and 2, correspondingly.
Table 2: Nuclotron beam slow extraction.
Fig. 1. The beam extraction area of the Nuclotron ring
Fig. 2. Structure of the extracted beam
Many technical and technological problems have been fixed while constructing the extraction system. In particular, an opportunity to transport the Lambertson magnet as well as the electrostatic septum in the horisontal plane upto ±20 mm, was provided. The operations on this kind of shifting were also carried out several times at the helium temperature in the process of optimization of the MLand ES elements position. Naturally, some improvements in the extraction systems are planned, for example, the increase of the operational voltage of the electrostatic septum, but even at the moment it is possible to conclude that the slow extraction system being unique on technology has successfully passed all the tests and provides all the given parameters of the beams extracted from the accelerator.
2.3 Ion sources
In June 2002 for the first time the Nuclotron run carried out the acceleration and the extraction of the argon ion beam. The ion source KRION-2 operating in the regime of the electron string, and all the necessary equipmentwere mounted on the high voltage terminal of the Nuclotron injector, were tested and used to obtain the beams of high charged ions of Ar16 and to further accelerate them in the linear accelerator Lu-20 upto 5 MeV/n. Then the Nuclotron accelerated the beams upto the energy of 1 GeV/n and 1.5 GeV/n.
The extracted beams of 1 GeV/n were used in the test experiments. In that first run of the operation at the accelerator the source produced upto 150 mcA ions of Ar16 in the duration pulse of 8 mc that corresponds to the one-turn injection to the synchrotron. It was shown that all the beam obtained without losses was transported through the aperture of the linear accelerator 15 mm in the diameter. The integral instability of the current of the Ar16 ions has not exceeded 7% during 96 hours of operation of the complex KRION - LU-20 - NUCLOTRON. The run required only one scheduled interval to pour liquid helium into the cryostat of the ion source.
The same source has successfully passed the test in the regime of obtaining and accelerating the N7 beams in the Lu-20. The pulse current of N7 reached 360 mcA, and the capture time of ions was 200 mc.
2.4 Cryogenic supply
The upgrading of the cryogenic helium facilities KGU1600/4.5 has been performed. Preliminary cooling of the compressed helium flow by means f the liquid nytrogen was substituted by the adiabatic expansion in the additionally installed turbodetanders. As a result, not only the cost of one hour of the accelerator operation has been reduced substantially, but the limitations on the run duration due to the liquid nytrogen deficit, have been cancelled: its necessary volume for the Nuclotron has become less than the JINR nytrogen production capacities. The cryocapacity of the facilities KGU-1600/4.5 by the present moment is higher than 2 KWt and the unit enrgy expenses to obtain the cold at the helium temperatures reached 290 Wt/Wt, that corresponds to the world level of efficiency .
3.1 Acceleration of polarized deuterons
Investigations of polarisation phenomena are an important part of the research programme of the LHE, JINR. Earlier these studies were performed on the Synchrophasotron (4.5 GeV/n) by using the set-up POLARIS - a polarised deuteron source D+ at the intensity upto (3-5)*109 d/cycle. To continue investigations in the field of spin physics at a new accelerator - the Nuclotreon, it is required to upgrade the existing source POLARIS and the injection area on the accelerator ring.
Since the Nuclotron project foresees only one-turn-injection of the positive ions, to incresae the intensity of the accelerated polarised beams till the level of (0.7-1)*1010 d/pulse, it is planned to realize a multi-turn (20-30 turns) recharging injection of the negative ions D. This programme includes:
These issues are considered in  in detail.
3.2 Superconducting channels
Already at the present moment about 50% of the energy expenses while operation of the acceleration complex of the LHE are related with the transportation system of the beams to the experimental facilities (Fig. 3). And it is clear that here comes again the task to substitute the existing magnet system for the superconducting one. In this case the power supply will reduce from 8-13 MWt to 0.2-0.4 MWt, and the annual saved resources will be at least 500 thous. $US if the acceleration complex is operating 3000 hours per year.
Fig. 3. The beam distribution system in the experimental hall. Build. 205 of LHE.
The general schedule of work can be as follows:
The construction of the booster will give an opportunity to increase the intensity of the accelerated beams by 10-15 times (see the Table 3). The cost of the project, basing on the traditional scheme of"the thermal" synchrotron is estimated at the level of 3 mln US$. By using the technology on the superconducting magnets, developed at the Nuclotron, this cost can be reduced by not less than 2 times. The concept of the superconducting booster has already been developed rather sufficiently (Grant of the RF).
Table 3: The Nuclotron beams.
The period of time to prepare the technical design and construction of the machine itself, will, probably, take about 3 years after approval. The location of the booster of the Nuclotron is shown in Fig.4. To minimize the beam emmitance, the electron cooling is considered to be applied. The design repetition rate of the cycles - 1-2 Hz. A more detailed description of the structure and of the superconducting magnetic booster of the Nuclotron is given in .
Fig. 4. The Nuclotron booster ring location
A new technology of superconducting magnets has been probed at the Nuclotron and positive results of their application in the accelerator have been obtained. These results will be rather helpful to design new fast cycling superconducting synchrotrons for different applications.
 A.D. Kovalenko. "Status of the Nuclotron", EPAC'94, London, June 1994. Proceedings, v.1, p.p. 161-164, (1995).
 A.M.Baldin et al. "Cryogenic System of the Nuclotron - a New Superconducting Synchrotron", Advances in Cryogenic Engineering, v.39, p.501-508, New York, 1994.
 V.A.Vasilishin et al. "Slow Beam Extraction from the Nuclotron: First Results, Future Development". JINR E1,2-2001-76, Dubna, 2001, p.280-283.
 N.N.Agapov et al. "More effective wet turboexpander for the Nuclotron helium refrigerators". Advances in Cryogenic Engineering", New York, 2002, v.47. p. 280-287.
 V.Angelov, V.P.Ershov, V.V.Fimushkin et al. "Polarization at the Nuclotron", Relativistic Nuclear Physics: from Hundred of MeV to TeV, p. 84-88, Dubna, 2001.
 N.N.Agapov, A.V.Butenko et al. "Rapid Cycling Superconducting Booster Synchrotron", EPAC 2000, Vienna, Austria, vol.1 p.560-562.
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