From 5 February to 4 April 2018, the 55th run of the Nuclotron accelerator complex was held in the Veksler and Baldin Laboratory of High Energy Physics. In course of this run, ions of carbon, argon and krypton were accelerated. The intensive research program, which included two experiments on the newly created BM@N detector (baryonic matter at the Nuclotron) and experiments on radiation biology, was fully carried out.

A special significance of the run was presupposed by its role in the strategy of fulfillment of the research programme at the NICA accelerator complex. It was scheduled to launch BM@N operation with heavy ions before creation of a new injection chain of the Nuclotron (that would have a new linear accelerator, a booster and necessary distribution channels). Furthermore, the injection of ions into the Nuclotron was carried out from the existing linear accelerator LU-20 that had been significantly upgraded in recent years. It was decided to use heavy ion source KRION-6T of Electron String Ion Source for heavy ion generation. Its major task is to work out production technologies and operation modes of the gold ions source of high intensity to operate then as a part of the injection chain of the NICA booster. However, energy of electrons in a string of this source allows obtaining heavy ions as well and they can be accelerated in the LU-20 facility. The first run of the Nuclotron with the CRION-6T source was held in June 2014. At that time, the beam of argon ions was accelerated and extracted from the Nuclotron with the intensity of 100 thousands particles per one cycle.

The most complicated task of the 55th run was accelerating argon and krypton ions with the intensity necessary for the optimal operation of the BM@N detector systems. Increase of beams’ intensity demanded to reduce particle losses at all stages of formation, accelerating and extraction.

Nuclotron control room. The last shift of the 55th run

In order to reduce losses, a buncher developed by ITEP and produced in Chernogolovka was installed in the linear accelerator between the preinjector and the LU-20 in June 2017. The buncher and the entire channel leading to the Nuclotron were adjusted for about 3 months at the ion beam of the laser source. At the same time, since April, optimization of the KRYON-6T source operation modes for Ar16+ and Kr26+ ions generation was carried out at the stand. In October, the source was transported to the accelerator complex and, before the start of cooling the magnetic system of the Nuclotron, work on providing the maximum beam intensity at injection continued almost without interruption.

To improve capture in the Nuclotron the adiabatic capture mode was worked out in the course of the previous 54th session, which required improvement of the control system of the high-frequency accelerating system of the Nuclotron. Special actions were taken to increase the stability of the magnetic field on the “table” of injection.

Adjustment of the accelerating mode was held when most of the standard diagnostics devices had not got sufficient sensitivity. An ionization profile monitor based on microchannel plates, designed just for such working conditions, provided invaluable assistance.

An equally important task was to achieve the efficient extraction of the accelerated beam and to obtain the required homogeneity of it. Complexity of this task will be clear if we say that at intensities typical for heavy ions, one particle is extracted, on average, in ten turns of the beam in the Nuclotron. The microstructure of the beam acquired by it in the process of capture and acceleration is revealed. It is possible to smooth the inhomogeneity of the beam density by a broadband noise. The first experiment on beam extraction using noise exposure by a diagnostic kicker magnet of the system of measuring frequencies of betatron oscillations was carried out at the Nuclotron in the 54th run. In the 55th run, this technique was used in the normal mode. In the language of electrical engineering, the Nuclotron was used in the run as a current source with a level of 100 femtoamperes (10-13) with instability of less than one percent.

These efforts resulted in a hundredfold increase of intensity of the argon ion beam in comparison with the previous run. The intensity of the accelerated and extracted beam of krypton ions (an element with a large atomic number and a larger ion charge, so the task of obtaining an intense beam is much more difficult) reached about 100 thousand. Work with ions of all grades made it possible to provide intensity, duration and quality of extraction required for optimal loading of the detector systems.

Dependence of current of the extracted krypton ions beam on time. Output pulse duration 2.5 seconds

Preparation to the run demanded a lot of efforts on development of technological systems as well. In the shortest time before start of the run a significant part of equipment of the cryogenic helium facility was upgraded. For several years modernization of the vacuum systems of the beam distribution channels in the experimental building was held. Several new power sources for the components of the magnetic optics of the channel were brought into operation before the launch of the run. Testing and adjustment of the BM@N detector’s systems were performed to fulfill the research programme of three previous runs. A modern synchronizing system based on the White Rabbit technology was installed at the detector.

ITEP specialists took part in the 55th run. As far as the research programme was intense, work on accelerator physics was carried out in the background. In spite of it, new significant results were achieved. For example, the transfer functions of the beam at all three grades of ions were measured, information necessary for making up a system of stochastic cooling of heavy ion beams in the collider was obtained.

At the end of the run, work on creation of the NICA injection complex was resumed, and construction work on creation of the tunnel of the head part of the beam transfer channel from the Nuclotron to the Collider was launched. In September, it is planned to start the assembly of the magnet-cryostatic booster system.

Anatoly Sidorin, JINR Weekly Newspaper
Photo by Nikolai Topilin