FAIR storage ring CRYRING ready for use
The first FAIR storage ring, the so-called CRYRING, is ready to conduct experiments for the scientific research community. CRYRING is an extremely successful ion storage ring that has enabled key research contributions in atomic and molecular physics in Stockholm for many years. It was brought to GSI and FAIR in Darmstadt as a Swedish in-kind contribution. Here it was modernized, adapted to FAIR standards and connected to the experimental storage ring ESR at GSI under the project name "CRYRING@ESR". Along with the already existing trapping facility HITRAP, the ESR and the future high-energy storage ring HESR, CRYRING will be part of the unique portfolio of trapping and storage facilities for heavy ions of FAIR.
CRYRING is a facility for storing highly charged heavy ions at low beam energy. With its high vacuum, the storage ring is particularly suitable for storing and cooling the ions at low energies. With low-energy ions, experiments of the highest precision, but also in a previously unattained regime of slow, adiabatic collisions are possible, allowing us to gain new insights into atomic, astrophysical and nuclear physics processes.
The recommissioning of CRYRING was started in 2015. In 2017, ion beams could be stored for the first time, initially from a local source. In recent years, work has been carried out on the optimization of the ring operation, beam cooling and diagnosis, the new FAIR control system and the experimental infrastructure. Commissioning is now nearing completion and has progressed so far that the facility is ready to serve scientific experiments.
Various experiments by scientific users with international participation were already planned for this spring at the CRYRING. However, due to the corona pandemic they had to be cancelled for the time being and postponed to future operating periods. Nevertheless, work continued on the full commissioning of the facility. This involved not only transporting highly charged, heavy lead ions (beryllium-like Pb78+ and later also Pb82+ with completely removed electron shell) from the GSI linear accelerator UNILAC via the ring accelerator SIS-18 and the experimental storage ring ESR to CRYRING, but also storing, cooling and using them for tests of the experimental infrastructure. The lifetimes and electron cooling of the stored beams were in line with previous estimates.
In the first tests, X-ray detectors at the so-called electron cooler registered the characteristic X-ray spectrum of the highly charged heavy ions, which is of particular interest for the fundamental understanding of the electromagnetic force in extremely strong fields (quantum electrodynamics). Ions stored in the CRYRING fly through a cold, dense cloud of electrons of the same speed in the electron cooler, which is primarily used for beam cooling. A secondary side effect is that also a small fraction of the ions capture an electron from the cloud and release the energy gained in this process as X-rays. Researchers can use this radiation to study quantum electrodynamics.
Due to the twelve-sided geometry of the CRYRING, the X-ray detectors can be placed in an ideal position exactly on the axis in front of and behind the ion beam and yet very close to the interaction area with the cooling electrons. This largely eliminates uncertainties in the observation angle caused by the Doppler shift. The low beam energy of the stored ions already helps to reduce this shift per se, so that X-ray spectra can be recorded with unprecedented precision and clarity.
For the future, it is planned to reschedule the experiments that could currently not be conducted due to COVID-19 as soon as possible. Furthermore, a completion of the commissioning is planned. For this purpose, the extraction process will be implemented, which will allow the cooled, slowed-down ions to be removed from the ring and enable material and biophysical experiments with solid targets. (CP)