A. G. Kobets

Study of nonneutral plasma storage in a magnetic trap with a rotating electric field at the lepta facility

Results from experimental studies of plasma storage in a Penning-Malmberg trap at the LEPTA facility are presented. The number of stored particles is found to increase substantially when using the so-called “rotating wall” method, in which a transverse rotating electric field generated by a cylindrical segmented electrode cut into four pairs is applied to the plasma storage region. The conditions of transverse compression of the plasma bunch under the action of the rotating field and buffer gas are studied. The optimal storage parameters are determined for these experimental conditions. Mechanisms of the action of the rotating field and buffer gas on the process of plasma storage are discussed.

On the development of an ion-beam stochastic cooling system for the nuclotron superconducting accelerator complex

The Joint Institute for Nuclear Research (JINR) initiated the creation of a unique heavy-ion collider, the Nuclotron-based ion collider facility (NICA), which is planned to be put into commission in 2016. According to the calculation data, the collider luminosity, which should be kept at a record high level of 1027 cm−2 s−1, will gradually decrease, mainly due to intrabeam scattering. To maintain luminosity at a high level, it is necessary to include a cooling system in the base project of the accelerator. Among the two cooling methods (electron and stochastic) most frequently used for heavy ion beams, stochastic cooling seems more attractive. However, there has been a lack of experience in the development and commissioning of such systems in Russia. For this reason, an experiment on stochastic cooling on the Nuclotron accelerator is being prepared to explore the technology and possibilities of this method. In this work, the method of stochastic cooling, the technique for calculating the cooling dynamics, and the experimental setup under development are briefly described.

Development of the Low Energy Particle Toroidal Accumulator project

The Low Energy Particle Toroidal Accumulator (LEPTA), a positron storage ring with electron cooling, was constructed and put in operation at the Joint Institute for Nuclear Research (Dubna). The storage ring is a generator of directed beams of ortho-positronium (o-Ps) produced upon the recombination of the beam of positrons circulating in the storage ring with a single-pass electron beam. In 2004 the storage ring was put in operation with the circulating electron beam. The source of positrons of the positron injector was tested with a new radioactive source delivered from South Africa. The positron trap was put in operation for electrons. The electron cooling system was tested with a pulsed electron beam. The progress in commissioning LEPTA is described in this paper.

Storage ring with longitudinal magnetic field LEPTA (Stellatron)

The project of the Low Energy Particle Toroidal Accumulator (LEPTA) is dedicated to the construction of a positron storage ring with electron cooling of positrons circulating in the ring. Such a specific feature of LEPTA automatically enables it to be a generator of positronium (Ps) atoms, which appear in the recombination of positrons with cooling electrons inside the cooling section of the ring. The project has the following goals: particle dynamics in the modified betatron, electron cooling with a circulating beam, positronium generation in flight, positronium physics, and feasibility of antihydrogen generation in flight. All key elements of the ring—the kicker, electron beam injection system, helical quadrupole, septum magnet—have been tested, and the expected design parameters have been achieved for these elements. The construction of LEPTA has been completed, and the circulating electron beam has been achieved. This paper discusses the issues of particle dynamics in such an accelerator, the results of numerical simulation and experimental findings of the research into beam dynamics, measurement of betatron number and beam lifetime.

Positron injector for LEPTA

The low energy positron injector for the Low Energy Particle Toroidal Accumulator (LEPTA) accumulator was assembled at the Joint Institute for Nuclear Research (JINR). Key elements of the injector have been tested. The cryogenic source of slow positrons was tested with a test isotope 22Na of the initial activity of 0.8 MBk. A continuous slow positron beam intensity of 5.8 × 103 particle per second with an average energy of 1.2 eV and a spectrum width of 1 eV has been obtained. The achieved moderator efficiency is about 1%. The accumulation process in the positron trap was investigated with electron flux. The lifetime of the electrons in the trap, τlife ≥ 80 s and capture efficiency ɛ ∼ 0.4, were obtained. The maximum number of accumulated particles was N exper = 2 × 108 at the initial flux of 5 × 106 electrons s−1.

The pulse injector of the low energy positrons

The pulsed injector of the low energy positrons for positron accumulator LEPTA has been constructed and tested at JINR. The injector is based on 22Na radioactive source. Positrons from the source are moderated in the solid neon and injected into positron trap, where they are accumulated during about 80 seconds. For injection the positrons are extracted by the pulsed electric field and accelerated up to the required energy. The injector will generate positrons of the energy of up to 10 keV at relative energy spread of 2•10-3, intensity of 108 - 109 particles per pulse and at injection pulse duration of 300 nsec. The cryogenic source of slow positrons has been tested with a test isotope 22Na of the initial activity of 80 MBk. The continuous slow positron beam with average energy spread of 1.2 eV, width of a spectrum 1 eV has been obtained. The achieved moderator efficiency is about 1%. The accumulation process in the positron trap was investigated with electron flux. The life time of the electrons in the trap, τlife ≥ 80 s and capture efficiency ε ~ 0.4 have been obtained. The maximum number of the accumulated particle was Nexper = 2*108 at the initial flux of 5•106 electrons per sec. The dynamics of slow positrons (electrons) in the injector and physics of the particle storage process are discussed in the report. The work is supported by RFBR grant No. 05-02-16320, the president of Russia Federation grant (MK-3948.2007.2) for supporting of young scientists and leading sceintific schools.

The LEPTA diagnostics and control system

The Low Energy Particle Toroidal Accumulator (LEPTA) is expected to provide experiments with intense flux of positronium. The diagnostics and control system of LEPTA consists of PCs and electronic equipment grouped in accordance with their applications. Devices in CAMAC standard, instrumentation and specially designed control cards imbedded in the LEPTA equipment are used.

A positron injector for the LEPTA accumulator

An injector of monochromatic positrons for the low-energy positron accumulator (LEPTA) is being tested at the Joint Institute for Nuclear Research. The source of positrons is the radioactive source 22Na. At the output of the source, positrons are slowed down in a solid target. Frozen neon is used as a moderator. For this purpose, a system of cryocooling of the source and the neon supply line have been assembled. A method of detection of slow positrons has been developed and tuned. The first experiments with the frozen moderator have been performed. A continuous beam of slow positrons with an average energy of 1.2 eV and spectrum width of 1 eV has been obtained.

Dynamics of charge particles in the stellatron type focusing system

The Low Energy Particle Toroidal Accumulator (LEPTA) was commissioned in September 2004 at JINR. The facility is dedicated to studies of particle beam dynamics in a storage ring with longitudinal magnetic field focusing (so called stellatron), application of circulating electron beam to electron cooling of antiprotons and ions in adjoining storage ring, electron cooling of positrons, and positronium in-flight generation. The last modes of the ring operation enables setting of numerous experiments with positronium in-flight and generation of directed and monoenergetic flux of antihydrogen. The positronium (Ps) atoms appear in recombination of positrons with cooling electrons inside the cooling section of the ring. An assembling of the storage ring LEPTA was completed during year 2004. Peculiarity of the storage ring is focusing of circulating particles with longitudinal magnetic field which covers whole orbit. As result, the particle motion in the ring is coupled in transverse plane. First results of the experimental study of the particle dynamics in the ring performed with circulating electron beam are presented. The beam lifetime was achieved above 20 ms at electron energy of 4 keV and vacuum pressure of 30 mTorr. The limitations of the beam lifetime and the possibility of its enhancement are discussed in the report.

Perspectives of LEPTA

The Low Particle Toroidal Accumulator (LEPTA) project is under development at JINR. The ring assembling is completed. The test experiments with circulating electron beam are in progress and the results are presented. Assembling and test of the positron injector is started. The program of experiment on directed o‐Ps flux generated at LEPTA is listed in the report. The first experiment that is under preparation presently has the goal to measure p‐Ps life time by interference of ortho‐ and para‐ states of Ps in magnetic field.